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Phytoconstituents—Active and Inert Constituents, Metabolic Pathways, Chemistry and Application of Phytoconstituents, Primary Metabolic Products, and Bioactive Compounds of Primary Metabolic Origin

  • A. N. M. Alamgir
Chapter
Part of the Progress in Drug Research book series (PDR, volume 74)

Abstract

Phytoconstituents are non-nutrient active plant chemical compounds or bioactive compounds and are responsible for protecting the plant against infections, infestations, or predation by microbes, pests, pathogens, or predators. Some are responsible for color, aroma, and other organoleptic properties. Phytoconstituents are synthesized in plants through primary and secondary metabolic pathways and many of them may be grouped as active drug constituents and inert nondrug constituents. A wide range of active components has been discovered and they have been divided into 16 main or more groups and the most important of them are alkaloids, terpenoids, phenols and phenolic glycosides, coumarins and their glycosides, anthraquinones and their glycosides, flavones and flavonoid glycosides or heterosides, mucilage and gums, tannins, volatile oils, saponins, cardioactive glycosides, cyanogenic glycosides, etc. Other relevant active constituents in plants, such as vitamins, minerals, amino acids, carbohydrates and fibers, some sugars, organic acids, lipids, and antibiotics, are essential nutrients. In addition to other functions, secondary metabolites produced in plants are used for communication as signal compounds to attract different pollinating agents including insects (honey bees, bumble bees, moths), birds, lizards, bats, etc. Classification of phytochemicals may be made based on their elemental constituents such as C & H; C, H & O; C, H, O, N, S & P containing compounds, O/N containing heterocyclic compounds, and other miscellaneous compounds. Some of these may be grouped as primary and others as secondary metabolites. Primary metabolic products consisting of C & H; C, H & O; N, S & P elements include hydrocarbons, carbohydrates, lipids, amino acids, proteins, nucleic acids, organic acids, etc. Genetic effects and environmental factors exert both qualitative and quantitative alterations of the active constituents in medicinal plants.

Keywords

Phytoconstituents Active and inert constituents Metabolic pathways Primary and secondary metabolites Factors affecting the metabolic pathways Classification of elemental constituents 

2.1 Phytoconstituents

Plant or phytoconstituents are non-nutrient plant chemical compounds or bioactive compounds and are responsible for protecting the plant against infections, infestations or predation by microbes, pests, pathogens, or predators. Phytoconstituents are active chemical compounds that occur naturally in plants but not established as essential nutrients. Some are responsible for color, aroma, and other organoleptic properties. The term is generally referred to as biologically significant chemicals of secondary metabolic origin with therapeutic importance. Phytochemistry determines the biological activities of phytoconstituents including their qualitative and quantitative assessment as well as analysis of their beneficial and harmful effects on human health. Phytoconstituents are synthesized in plants through primary and secondary metabolic pathways and many of them may be grouped as active drug constituents and inert nondrug constituents.

2.1.1 Active Drug Constituents

The active constituents in plants are the chemicals that have a medicinal effect on the body. Pharmacological activity in herbal drugs is due to these chemical compounds which are called active principles, drug principles, or components. A wide range of active components has been discovered. Phytochemistry deals with methods of obtaining these active ingredients and their classification according to the functional organic chemical group which it bears and studies the analytical methods to verify its quality. Active components are found in different parts and organs of plants or plant exudates. They have been divided into 16 main or more groups of which the most important are alkaloids, terpenoids, phenols and phenolic glycosides, coumarins and their glycosides, anthraquinones and their glycosides, flavones and flavonoid glycosides or heterosides, mucilage and gums, tannins,volatile oils, saponins, cardioactive glycosides, cyanogenic glycosides, etc. The main chemical groups of active drug components under broader heads are heterosides (e.g., anthraquinones, cardiac glycosides, cyanogenics, coumarins, phenols, flavonics, ranunculosides, saponosides, sulfurides), polyphenols (e.g., phenolic acids, cumarins, flavonoids, lignans, tannins, quinones), terpenoids (e.g., essential oils, iridoids, lactones, diterpones, saponins), and alkaloids (atropine, cocaine, daturin, hiosciamin, lysergic acid, nicotine, quinine). Mucilage and gums are heterogeneous polysaccharides, formed by different sugars, in general, they contain uronic acids. Other relevant active constituents in plants, such as vitamins, minerals, amino acids, carbohydrates and fibers, some sugars, organic acids, lipids, and antibiotics, are essential nutrients.

Alkanoids are nitrogen-bearing molecules that make them particularly effective as medicines (e.g., deadly nightshade, aconite, cinchona, etc.), alkanoid, though poisonous, are valuable as medicines, e.g., curarine is a powerful muscle relaxant, atropine is used to dilate the pupils of the eyes, and physostigmine is specific for certain muscular diseases; narcotic alkaloids morphine and codeine are used for the relief of pain and cocaine as a local anesthetic; quinine, caffeine, nicotine, strychnine, serotonin, and lysergic acid diethylamide (LSD) are examples of some other common alkaloids; anthocyanins maintain blood vessel health (e.g., blackberries); anthraquinones stimulate the large intestine, causing contractions and bowel movement (e.g., aloe, rhubarb, cascara, senna, frangula); bitter principles are recognized by their disagreeable, astringent, or acrid taste (e.g., gentian, chirata, picrorrhiza, quassia). The active ingredient stimulates the flow of saliva and gastric juices, thereby improving appetite and digestive function (e.g., wormwood and devil’s claw); saponins are glycosides with foaming characters (e.g., soapwort, saoproot, soapbark, soapberry; commercial source: Yucca schidigera and Quillaja saponaria) and many saponins have beneficial effects on blood cholesterol levels, cancer, bone health and stimulation of the immune system, and detergent properties of saponins have led to their use in shampoos, facial cleansers and cosmetic creams; cardiac glycosides (e.g., digitalis, squill, strophanthus, ouabain, theveti) have a strong direct action on the heart and support and strengthen the rate of contraction. They are significantly diuretic and these plants help lower blood pressure (e.g., foxgloves), aglycone of glycosides may be; phenolics are widely spread throughout the plant kingdom (fruits and vegetables), they are important for their potential protective role against oxidative damage diseases (coronary heart disease, stroke, and cancers); coumarins (e.g., visnaga, ammi, psoralea) as multitasking constituents thin the blood, relax smooth muscle and can act as a sunscreen all at once (e.g., celery); cyanogenic glycosides have a sedative and relaxing effect on the heart and muscles (e.g., elder plants); flavonoids are anti-inflammatory, and also maintain healthy circulation (e.g., lemons); glucosilinates increase the blood flow to the painful joint area and this aids in healing as it helps remove the build-up of waste products (e.g., radish are applied as a soft, moist mass onto painful joints); phenols are antiseptic and can reduce inflammation when taken internally, but if used externally on the skin can have an irritant effect (e.g., arbutin, slicin thyme); saponins (steroidal saponins and triterpenoid saponins) are expectorants, i.e., increase bronchial secretions and facilitate their expulsion through coughing, spitting or sneezing, also aid in nutrient absorption and have a marked effect on hormonal activity (e.g., liquorice); tannins (gallotannins, ellagitannins, complex tannins) are widely distributed in many plant species of plants and protect from predation and pesticides, and function in plant growth regulation. Tannins can contract the skin’s tissue and thereby improving the skin’s resistance to infection (e.g., oak tree); and volatile oils have many therapeutic effects and are used in perfumes, food flavorings, and aromatherapy (e.g., chamomile). Minerals can act as mineral supplements (e.g., Dandelion); mucilage soothes inflammation and stops irritation and acidity, by lining the mucous membranes of the digestive tract (e.g., slippery elm), vitamins function as structural material as well as coenzymes (dog rose contains enough vitamins to contribute to one’s daily intake). More than 600 carotenoids provide yellow, orange, and red colors in fruits and vegetables, act as antioxidants in the body, and scavenge harmful free radicals, alpha-carotene, beta-carotene, and beta-cryptoxanthin (pumpkins and carrots) which are provitamins necessary to keep immune system active and eye healthy. Lycopene (e.g., tomatoes, watermelon, pink grapefruit) has been linked to a lower risk of prostate cancer. Lutein and zeaxanthin (e.g., spinach, kale, collards) protect from eye problems like cataracts and age-related macular degeneration. Ellagic acid of different berries (e.g., strawberries, raspberries, pomegranates) protects against cancer in several different ways.

Medicinal and aromatic plants (MAPs) are the richest bio‐resource of crude drugs for traditional systems of medicine, modern medicines, folk medicines, nutraceuticals, food supplements, fragrances, flavors, cosmeceuticals, health beverages, pharmaceutical intermediates, and chemical entities for synthetic drugs. The medicinal value of a crude drug depends on the presence of one or more active chemical constituents (e.g., alkaloids, glycosides, resins, enzymes, etc.). A vegetable drug is composed of many tissues such as cells, fibers, vessels, and other structures. The cell walls may consist of cellulose, lignins, tannins, etc. The aromatic drugs like cinnamon and coriander contain volatile oils in specialized cells or glands. The glycosides and alkaloids may occur in solution in the cell sap and deposit in the cells later. The total contents of the cells do not carry physiological importance, e.g., calcium oxalate occurs as a crystalline deposit and protein may occur as solid aleurone grains. Both these components are rejected in the preparation of a tincture or extract of the drug.

2.1.2 Inert Nondrug Constituents

The chemical constituents present in plant (or animal) kingdom, which do not possess any definite therapeutic values as such but are useful as an adjunct either in the formulation of a drug or in surgery are collectively known as inert constituents. The inert components of a drug product do not increase or affect the therapeutic action of the active ingredient (active drug constituent). The inert nondrug constituents are added during the manufacturing process of pharmaceutical products, e.g., tablets, capsules, suppositories, injections, etc. The inert nondrug constituents or inactive ingredients may also be referred to as excipients including binding materials, dyes, preservatives, and flavoring agents. Agents that combine with active ingredients to facilitate drug transport in the body or palatability are also considered inactive or inert nondrug constituents. The inert ingredients or excipients in the medication do not exert the intended effect for taking it, and do not cause the side effects. The inert constituents are the cellulose, wood, and other structural parts of the drug, and in some instances starch, albumen, etc. Inactive ingredients are used in the manufacturing process and/or are present in the final medication product. They fulfill a variety of purposes, from delivering the active ingredient to making the pill look and taste good, along with other tasks such as coatings for easier ingestion, and timed or targeted release, flavors, and colors to make swallowing medicine more pleasant. Different types of nanoparticles are being used to encapsulate the active ingredients of drugs, to reduce required doses, and improve organ and tissue specificity.

The FDA approves inactive ingredients that are included in pharmaceutical products. However, not all inactive ingredients are always inactive, e.g., alcohol as an ingredient that may be active or inactive depending on the specific formulation of the medication. Patients may have allergic reactions or other adverse effects to inactive ingredients, but a patient may have an allergic reaction to this inactive ingredient. Inactive ingredients like sulfites, benzoates, aspartame, saccharin, oleic acid, benzyl alcohol, lactose, soya lecithin, propylene glycol, and sorbitan trioleate may cause reactions in some patients as per the previous report. Patients who have allergic or adverse reactions to certain inactive ingredients may be able to use products that are color or preservative free.

The inert constituents, both plant and animal origin, that are invariably present in natural drugs include starch, cellulose, lignin, suberin, cutin, albulin, coloring matters, etc., (plant origin); and keratin, chitin, etc., (animal origin). Microcrystalline forms of cellulose are used as combination binder disintegrants in tabletting. Colloidal cellulose particles aid in stabilization and emulsification of liquid; lignin is used to precipitate proteins and to stabilize asphalt emulsions; suberins are esters of higher monohydric alcohols and fatty acids as cutin does; starch as pharmaceutic aid functions as tablet filler, binder, and disintegrant; albumin from soybean (albumins) functions as emulsifiers; coloring matters like cochineal are used for coloring food products and pharmaceuticals; keratin is used for coating enteric pills that remains unaffected in the stomach but undergoes disintegration (dissolved) by the alkaline into intestinal secretions; chitin as deacylated chitin (chitosan) is used for treatment of water but sulfated chitin functions as anticoagulant in laboratory animals. It has been observed that the very presence of “Inert Constituents” either act towards modifying or check the absorbance and the therapeutic index of the active constituents.

2.2 Metabolic Pathways and the Origin of Primary and Secondary Metabolites Chemistry of Plant Constituents and Their Application

Metabolites are intermediates and products of metabolism. Physiologically active plant chemical constituents are usually classified in groups according to their metabolic origin, chemical structure, and function. Metabolism in plants may be classified into primary and secondary metabolic pathways.

2.2.1 Primary Metabolic Pathways and Primary Metabolites

Primary metabolic pathway produces primary metabolites, usually referred to as central metabolites, which are directly involved in normal growth, development, and reproduction; produced in generous quantities and can easily be extracted from the plant; part of the basic molecular structure of the cell; perform physiological functions in the organism (i.e., an intrinsic function); distribution is ubiquitous to all organisms or cells (Fig. 2.1).
Fig. 2.1

Primary metabolism and primary metabolites in plants

In microorganisms, primary metabolites are typically formed during the growth phase as a result of energy metabolism, and are deemed essential for proper growth, e.g., alcohol (ethanol), organic acids (acetic acid, lactic acid, citric acid), nucleotides (5′guanylic acid), antioxidants (isoascorbic acid), certain amino acids (aspartic acid, l-glutamate and l-lysine), vitamins (B2), and polyols (glycerol). Compounds, such as phytosterols, acyl lipids, nucleotides, amino acids, and organic acids, are found in all plants and perform metabolic roles that are essential and evident. Products resulting from primary metabolism include glucides, lipids, amino acid derivations, etc. Many of these metabolites can be used in industrial microbiology to obtain amino acids, develop vaccines and antibiotics, and isolate chemicals necessary for organic synthesis.

2.2.2 Secondary Metabolic Pathways and Secondary Metabolites

Secondary metabolic pathway produces secondary metabolites that are typically organic compounds and are produced through the modification of primary metabolites (Fig. 2.2).
Fig. 2.2

Secondary metabolism and secondary metabolites in plants

Secondary metabolites are not directly involved in growth and developmental processes, are not part of the basic molecular structure of the cell, are produced in small quantities and their extraction from the plant is difficult and usually performs important ecological function (i.e., a relational function), i.e., important adaptive significance in protection against herbivory and microbial infection, as attractants (pigments or scents) for pollinators and seed-dispersing animals, and as allelopathic agents (allelochemicals that influence competition among plant species); secondary metabolites seem to be important primarily in ecological interactions with other species and between the plant and its environment; distribution is not ubiquitous to all organisms or cells, i.e., restricted to a limited taxonomic groups or set of organisms or cells of plants, fungi, bacteria, etc. They include some broad groups of metabolites such as alkaloids (strychnine, nicotine, caffeine, cocaine, capsaicin), phenolics (flavonoids, tannins, lignin, salicylic acid), terpenoids (aromatic oils, resins, waxes, steroids, rubber, carotenoids), and some other miscellaneous products (glycosides, antibiotics, peptides and growth factors). These are the most important active components of herbal drugs. Secondary metabolites are typically formed during the end or near the stationary phase of growth. Many of the identified secondary metabolites have a role in ecological function, including defense mechanism(s), by serving as antibiotics and by producing pigments (carotenoids, anthocyanin). Examples of secondary metabolites with importance in industrial microbiology include atropine and antibiotics such as penicillin, erythromycin, and bacitracin. It is, however, difficult to distinguish between primary and secondary metabolites by either structure, biochemistry, or function. Plant growth regulators may be classified as both primary and secondary metabolites due to their role in plant growth and development. Some of them are intermediates between primary and secondary metabolism.

Metabolic pathways are a linked series of chemical reactions that take place within a cell, tissue, or organism. The reactants, products, and intermediates of an enzymatic reaction are collectively known as metabolites and in a metabolic pathway, the product of one enzyme acts as the substrate for the next. For convenience, different metabolic pathways function in different cell compartments or organelles. In a eukaryotic cell, the photosynthetic CO2 fixation Calvin cycle, H2O photolysis, cyclic and noncyclic electron transport, generation of energy (photophosphorylation) and reductive pool such as nicotinamide adenine dinucleotide phosphate-NADPH, etc., take place within chloroplast; the citric acid cycle, electron transport chain, and oxidative phosphorylation all take place in the mitochondrial membrane. In contrast, glycolysis, pentose phosphate pathway, and fatty acid biosynthesis all occur in the cytosol of a cell. Anabolic (synthetic) and catabolic (degratative) pathways are two basic metabolic pathways which are characterized by their ability to either synthesize molecules with the utilization of energy or break down of complex molecules by releasing energy in the process. The two pathways complement each other in that the energy released from one is used up by the other, i.e., the degradative process or catabolic pathway provides the energy required to conduct a biosynthesis of an anabolic pathway (coupling processes). In addition to these, there exists another pathway, the amphibolic pathway, which can share the products of both catabolic and anabolic processes. Metabolites that are produced in different processes are arbitrarily grouped as primary and secondary metabolites and their pathways are so named as primary (including photosynthesis, glycolysis, TCA cycle, photo- and oxidative phosphorylation, pentose phosphate pathway, fatty acid synthesis, amino acids and protein synthesis, nucleotides and nucleic acid synthetic pathways, etc.) and secondary metabolic pathways (shikimic acid pathway, malonic acid pathway, mevalonate pathway, MEP-methylerythritol phosphate pathway, etc.). Products of the primary metabolic pathways are the source materials for secondary metabolic pathways. Cells have evolved to use feedback inhibition (when a reaction product is used to regulate its own further production, i.e., the products inhibit further enzyme activity) to regulate enzyme activity in metabolism.

The primary metabolism consists of chemical reactions that allow the plant to live. The secondary metabolites almost play no role in growth, photosynthesis, reproduction, solute transport, translocation, nutrient assimilation, and differentiation or other primary functions but secondary metabolism facilitates the primary metabolism in plants and plays a pinnacle role in keeping all the plants’ systems working properly. These chemicals are extremely diverse but not ubiquitous like primary metabolites rather they are restricted or limited distribution to few plant families, genus or species. They can sometimes be used as taxonomic characters in classifying plants. Secondary metabolites can be classified in different ways (e.g., based on structure, composition, solubility in various solvents, synthetic pathways, etc.). Based on their biosynthetic origins, plant secondary metabolites can be divided into three major groups such as terpenoids, phenolics (flavonoids and allied phenolic and polyphenolic compounds), and nitrogen-containing alkaloids and sulfur-containing compounds.

Secondary metabolites play an important role in plant defense against UV radiation, toxicity (as deterrent), herbivory, pests (deterrent), pathogens and pests as well as in interspecies defenses (allelopathy); in metal transportation; in symbiotic interaction between microbes and plants, nematodes, insects, and higher animals; formation of UV nectar guide in flowers (e.g., 4-deoxyaurones of Bidens ferulifolia), pollinator attractants and also as sexual hormones and differentiation effectors, etc. Humans use secondary metabolites as therapeutic, flavoring, and recreational agents.

The active components in a drug are the chemicals that create the intended therapeutic effects, and any side effects. The constituents of herbal drugs of medicinal value generally are diverse including carbohydrates, gums, acids, glycosides, anthraquinone derivatives, alkaloids, tannins and other phenols, enzymes, proteins, resins, fixed oils, fats, waxes, volatile oils, etc. These metabolites and their synthetic pathways are shown in Fig. 2.3.
Fig. 2.3

A schematic diagram showing the metabolic pathways of synthesis of primary and secondary metabolites in plants

The shikimic acid pathway is a multi step metabolic pathway used by bacteria, fungi, algae, some protozoan parasites and higher plants for the synthesis of folates and aromatic amino acids (phenylalanine, tyrosine and tryptophan). This pathway is not found in animals and the amino acids produced in this pathway must be supplied through animal’s diet (essential amino acids). The shikimic acid pathway participates in the biosynthesis of most plant phenolics. The malonic acid pathway is an important source of phenolic secondary products in fungi and bacteria, but is of less significance in higher plants. Diethyl ester of malonic acid is used in syntheses of vitamins B1 and B6, barbiturates, and numerous other valuable compounds. The mevalonic acid pathway, also known as the isoprenoid pathway or HMG-CoA reductase (3-hydroxy-3-methyl-glutaryl-coenzyme A reductase, HMGCR) pathway is present in eukaryotes, archaea, and some bacteria. The mevalonate pathway begins with acetyl-CoA and ends with the synthesis of two five-carbon building blocks called isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). These two building blocks are used in the synthesis of a diverse class of isoprenoids including cholesterol, heme, vitamin K, coenzyme Q10, and all steroid hormones. IPP and DMAPP also serve as the basis for the biosynthesis of molecules used in processes as diverse as protein prenylation, cell membrane maintenance, hormone synthesis, protein anchoring, and N-glycosylation. MEP (methylerythritol phosphate) pathway is an alternative or non-mevalonate metabolic pathway that forms IPP and DMAPP used for isoprenoid biosynthesis (including the dimeric isoprenoids, the terpenoids) by bacteria, plants, and apicomplexan protozoa (e.g., malaria parasites).

2.2.3 Plant’s Defensive or Survival Secondary Metabolites

Plants produce a diverse group of secondary metabolites with a prominent function in the protection against predators and microbial pathogens due to their toxic nature and repellence to herbivores and microbes; some of them are involved in defense against abiotic stresses, some others are important for the communication of the plants with other organisms and a few are insignificant for growth and developmental processes (Rosenthal 1991; Wink 1999; Schafer and Wink 2009). Secondary metabolites such as terpenes, phenolics and nitrogen (N) and sulfur (S) containing compounds defend plants against a variety of herbivores and pathogenic microorganisms as well as various kinds of abiotic stresses. A clear majority of the different terpenes produced by plants as secondary metabolites are involved in defense as toxins and feeding deterrents to a large number of plant feeding insects and mammal herbivores.

Monoterpenes esters pyrethroids occur in the leaves and flowers of Chrysanthemum species show strong neurotoxin insecticidal responses to insects; and monoterpenes α-pinene, β-pinene, limonene, myrecene, etc., present in resin ducts of pine and fir (Gymnosperms), are toxic to numerous insects. Essential oils (a mixture of volatile monoterpenes and sesquiterpenes) like limonene and menthol (monoterpenes present in glandular trichomes on epidermis) of lemon oil and peppermint oil, respectively with well-known insect resistant properties help in protecting the plant from insect infestation. Sesquiterpenes costunolides (a cyclic ester characterized by a five-membered lactone ring) of Asteraceae are antiherbivore and feeding repellant agents; abscisic acid (ABA-a sesquiterpene growth regulator) plays regulatory role in plant response to water stress by modifying the membrane properties, minimizes the interference of absorption of 400–700 nm phototsynthetically active radiation (PAR) by significantly increasing the level of UV-B absorbing flavonols (quercetin and kaempferol), and reduces damage due to UV-B by increasing the concentration of hydroxycinnamic acids (caffeic and ferulic acids), antioxidant enzymatic activities and sterols. Abietic acid (a diterpene) of pines and leguminous trees are chemical deterrents to continued predation; phorbol (a diterpene ester) of Euphorbiaceae works as skin irritants and internal toxins to mammals.

Terpenoid aldehyde (or polyphenolic aldehyde) gossypol produced by cotton (Gossypium hirsutum) has strong antifeedant, antifungal and antibacterial properties. Several triterpene steroid alcohols (sterols) such as better tasting glucosides (sterols) of milkweeds protect them against herbivory insects and cattle; phytoectysones produced by spinach (Spinacia oleracea) disrupt larval development and increase insect mortality; limonoids (a group of bitter triterpenes substances) present in lemon and orange peels (fresh scent) are triterpenoids act as antiherbivore compounds; azadirachtin of neem ( Azadirachta indica ) is a very powerful insect repellent and feeding deterrent limonoid. Insect repellent citronella, an essential oil of lemongrass (Cymbopogon citratus), contains high limonoid levels. Triterpenoid cardiac glycosides like digitoxin and digoxin of Foxglove ( Digitalis purpurea ) are highly toxic to vertebrate herbivores including humans if ingested in high quantities.

A clear majority of secondary metabolites play vital functions in plant growth and defenses (Fig. 2.4). For example, gibberellins (diterpenes) and brassinosteroids (BRs), a sixth class of plant hormones, are important plant hormones, and sterols (triterpene) are essential components of cell membranes and carotenoids (tetraterpenes) act as important pigment class in photosynthesis and protect plants from harmful effects of photooxidation. The ecdysones (steroids) of fern Polypodium vulgare play defensive role against insects (Heftmann 1975; Slama 1980). Solanine (triterpene steroids/glycoalkaloid poison) acts as a plant defense molecule. Rubber (polyterpene) provides protection to plants as a defense against herbivores and in mechanism for wound healing (Klein 1987; Eisner and Meinwald 1995).
Fig. 2.4

Different terpenes secondary metabolites involved in defense as toxins and feeding deterrents to plant feeding insects and mammal herbivores

Phenolics, a chemically heterogeneous group of compounds, have several important defensive roles in the plants against pests and diseases (defense-related phenolics include flavonoids, anthocyanins, phytoalexins, tannins, lignin, and furanocoumarins). For example, coumarins (oxygen heterocycle) protect plants against insect herbivores and fungi, and furanocoumarins (abundant in the members of Apiaceae such as celery, parsnip and parsley) become phytotoxic after activation by UV-A. Lignin, a highly complex and branched polymer of phenylpropanoic alcohols (e.g., econiferyl-, sinapyl-, and p-caumaryl alcohols), protects plants from herbivorous animals by its physical toughness while its chemical durability makes it relatively indigestible to herbivores and insects pathogens. Flavonoid pigments (invisible to human eye but visible to bees) (flavones—luteolin, apigenin, tangeritin; and flavonols—quercetin, kaempferol, myricetin, fisetin, galangin, isorhamnetin, pachypodol, rhamnazin, pyranoflavonols, furanoflavonols) function as shields against harmful UV-B radiation. Anthocyanins (flavonoids pigments) protect plant foliage from the damaging effects of ultraviolet radiation (e.g., cyanin glycoside). Phytoalexins (isoflavonoids) with antibiotic and antifungal properties are produced in plants in response to pathogen attack (e.g., medicarpin in alfalfa, rishitin in tomatoes and potatoes, camalexin in Arabidopsis thaliana). Miean and Mohamed (2001) analyzed the flavonoids (myricetin, quercetin, kaempferol, luteolin, and apigenin) contents in 62 edible tropical plants and among them, the highest total flavonoids content was in onion leaves (1497.5 mg/kg quercetin, 391.0 mg/kg luteolin, and 832.0 mg/kg kaempferol). Rotenone of Derris sp., an isoflavone, can act as an insect feeding deterrent. Tannins are plant phenolic polymers with defensive properties and act as feeding repellents to a great diversity of animals. There are three major classes of tannins (hydrolysable gallic acid polymer, nonhydrolyzable, or condensed flavones polymer and phlorotannins phloroglucinol polymer) having three different basic unit or monomer of the tannin. A tannin molecule requires at least 12 hydroxyl groups and at least five phenyl groups to function as protein binders. Basic units of tannin are gallic acid, flavone, and phloroglucinol (Fig. 2.5).
Fig. 2.5

The basic units of tannin: gallic acid , flavone and phloroglucinol

Phenolic compounds like ferulic and caffeic acids when released by some plants show allelopathic activity, i.e., inhibit the germination and growth of their neighboring plants and thus may increase its access to nutrients, light, and water (agents of plant–plant competition). Structure of different phenolic compounds is shown below (Figs. 2.6, 2.7 and 2.8).
Fig. 2.6

Different phenolic compounds released by some plants show allelopathic activity

Fig. 2.7

Building blocks of lignin : Coniferyl-, sinapyl- and p-coumaryl alcohols

Fig. 2.8

Different plant phenolics : Lignin, Coumarin, Furanocoumarin isomers, Psoralen (furanocoumarin), Tannic acid, Casuarictin (ellagitannin), Proanthocyanidins, etc.

Nitrogen-containing secondary metabolites including alkaloids (nicotine, caffeine, threonine, atropine, capsaicin, etc.), cyanogenic glucosides, and nonprotein amino acids are of considerable interest because of their role in the antiherbivore defense and toxicity to humans (Fig. 2.9). Defensive proteins (including defensins, amylase inhibitors, lectins, ricin, proteinase inhibitors, etc.) are cycteine rich small molecules (contained in seeds or other organs of plants) that inhibit pest enzymes activities and/or digestive enzyme activities of herbivores. Alkaloids are believed to function as defenses against herbivores because of their general toxicity and deterrence capability.
Fig. 2.9

Nitrogen-containing secondary metabolites (alkaloids): nicotine, caffeine, threonine, atropine, capsaicin, etc.

Nitrogenous protective compounds other than alkaloids are found in plants, e.g., cyanogenic glycosides and glucosinolates. Cyanogenic glycosides release the poisonous gas hydrogen cyanide (HCN). The presence of cyanogenic glycosides deters feeding by insects and other herbivores such as snails and slugs.

Sulfur-containing secondary metabolites include GSH, GSL (glutathione, glucosinolates), phytoalexins, thionins, defensins, allinin, etc., and they have direct or indirect link with the defense mechanism of plants against microbial pathogens (Fig. 2.10). The glucosinolates , or mustard oil glycosides, break down by the hydrolytic enzyme thioglucosidase or myrosinase to release defensive substances including S or sulfur. These are found principally in the Brassicaceae and related plant families. Like cyanogenic glycosides, glucosinolates are stored in the intact plant separately from the enzymes that hydrolyze them, and they are brought into contact with these enzymes only when the plant is crushed.
Fig. 2.10

Sulfur containing secondary metabolites : Glutathione and Glucosinolates

Plants defensive strategies against herbivory can be divided into two categories: constitutive and induced defenses. The constitutive defensive mechanisms are always present in the plant, often be species-specific and may exist as stored compounds, conjugated compounds (to reduce toxicity), or precursors of active compounds that can easily be activated if the plant is damaged. Most of the defensive secondary compounds are constitutive defenses. Induced defenses are initiated only after actual damage occurs, e.g., lectins and protease inhibitors as well as the production of toxic secondary metabolites. In principle, induced defenses require a smaller investment of plant resources than constitutive defenses, but they must be activated quickly to be effective. Plant hormone Jasmonic acid (JA) acts as a major signaling agent in most plant defenses against insect herbivores that triggers the production of many proteins involved in plant defenses. Several other signaling compounds (e.g., ethylene, salicylic acid, methyl salicylate, etc.) are also induced by insect herbivory. In many cases, the concerted action of these signaling compounds is necessary for the full activation of induced defenses.

High concentrations of secondary metabolites might result in a more resistant plant; the production of secondary metabolites is thought to be costly and reduces plant growth and reproduction (Karban and Baldwin 1997; Siemens et al. 2002). Defense metabolites can be divided into constitutive substances (also called prohibitins or phytoanticipins) and induced metabolites formed in response to an infection involving de novo enzyme synthesis (phytoalexins) (Grayer and Harborne 1994; Van Etten et al. 1994). Phytoanticipins are high energy and carbon consuming but recognized as the first line of chemical defense that potential pathogens must overcome (Mauricio 1998) while phytoalexin production may take 2 or 3 days (Grayer and Harborne 1994). The cost of defense has also been invoked to explain why plants have evolved induced defense, where concentrations generally increase only in stress situations (Harvell and Tollrian 1999). Defensive chemicals (including structures) are costly as they require resources that could otherwise be used by plants to maximize growth and reproduction, yet many make this investment in defenses against predators.

Unlike simple chemicals such as terpenoids, phenolics, alkaloids, etc., proteins require a great deal of plant resources and energy to produce; consequently, many defensive proteins are only made in significant quantities after a pathogen or pest has attacked the plant. Once activated, however, defensive proteins and enzymes effectively inhibit fungi, bacteria, nematodes, and insect herbivores.

2.2.4 Pollinator Attracting Secondary Metabolites

In addition to other functions, secondary metabolites produced in plants are used for communication as signal compounds to attract different pollinating agents including insects (honey bees, bumble bees, moths), birds (hummingbirds, honey eaters’ sunbirds, flower peckers, honeycreepers, bananaquits), lizard (Hoplodactylus geckos), mammals (bats, fruit bats, lemurs in Madagascar, and some Australian marsupials such as sugar gliders, honey possums, and some marsupial mice), etc., to enhance fertilization (Fig. 2.11). Nectar sugars, floral pigments, and headspace volatiles are important in this regard to filter flower visitors. Secondary terpene metabolites volatile oils (floral scents and fragrance) and pigments (carotenes) as well as phenolics and flavonoids are involved primarily to provide either visual or olfactory attraction in terms of flower aroma and color. The secondary metabolites involved in scent production are usually low‐molecular weight volatile products with phenylpropanoids, benzenoids, and terpenes; and the major scent compound emitted by Antirrhinum flowers is methylbenzoate. Many flavonoids are responsible for color, aroma of flowers, fruit to attract pollinators and consequently fruit dispersion. Rutin is a bioflavonoid plant pigment (a glycoside combining the flavonol quercetin and the disaccharide rutinose-α-l-rhamnopyranosyl-(1→6)-β-d-glucopyranose). It functions as a visual pollinator attractant in many plant species (e.g., Forsythia intermedia). Plants emit volatile organic compounds (VOCs) for the attraction of pollinating agents, e.g., linalool, a terpene alcohol (two enantiomers of a terpene alcohol) with pleasant scent found in flowers of many plant species, functions as a pollinator attractant; 4-deoxyaurone (sulfuretin), a flavonoid function as UV nectar guide. Flower nectar and secondary metabolites in it are also important in attracting pollinators. Introduction of secondary metabolites in nectar may introduce specialization in plant–pollinator interactions, protects nectar from larceny, and preserves nutrients in nectar from degradation and reduction in disease levels in pollinators. When insects, bats, birds and other animals visit flowers to feed on the nectar and pollen, they usually pollinate the flowers in the process, so that both partners benefit from this mutualistic association.
Fig. 2.11

Pollinator attracting secondary metabolites : Methylbenzoate, Rutin, Linalool (enantiomer), 4-deoxyaurone (sulfuretin), etc.

Miscellaneous secondary metabolites

Canavanine of Canavalia ensiformis (deterrence of mammalian herbivores), alliin of onion and garlic (antioxidant and imparts aroma), and azetidine-2-carboxylic acid of Convallaria majalis (deters the growth of predators) are nonprotein amino acids, while sinigrin of Brassicaceae members is a glucosinolate, acts as defensive factor against insects, as well as positive feeding stimulus to cabbage butterflies and aphids. All these are grouped as miscellaneous secondary metabolites.

2.2.5 Factors Affecting the Metabolic Pathways of Medicinal Plants

The secondary plant products or bioactive constituents in plants may be considered as (a) superfluous metabolites, i.e., substances that have no value as such and perhaps their presence is due to the lack of excretory mechanism in them and ultimately results in the “residual lockup” superfluous metabolites, and (b) characteristic survival substances, i.e., substances which exert a positive survival value on the plant wherein they are present. They offer a natural defense mechanism whereby these host plants are survived from destruction owing to their astringent, odorous, and unpalatable features, e.g., poisonous alkaloidal containing plants, astringent containing shrubs, and pungent volatile oil-containing trees, etc.

Genetic effects exert both qualitative and quantitative alterations of the active constituents in medicinal plants, e.g., (i) eugenol is naturally present in two different species in varying quantities as follows: Eugenia caryophyllus (70–95%) and Syzgium aromaticm (~85%); (ii) Reserpine-rescinnamine group of alkaloids in Rauvolfia serpentina (~0.15%) and Rauvolfia vomitoria (~0.20%); (iii) Rutin in Fagopyrum esculatum (3–8%) and Sophora japonica (20%); (iv) menthol in Mentha piperita (50–60%) and Mentha arvensis (75–90%).

The environment factors also contribute to the quantitative aspect of secondary metabolites or active constituents of medicinal plants, e.g., (i) modified strains of Claviceps purpurea can produce ~0.35% of ergotamine in comparison to the normal one producing ~0.15% of total ergot alkaloids; (ii) eucalyptol (cineole, cajeputol) is present in the fresh leaves of Eucalyptus globulus to the extent of 70–85%. It has been observed that the chemical races of some species of Eucalyptus invariably display significant variations in the content of essential oils. The composition of soil (mineral contents); climate (dry, humid, cold); associated flora (R. serpentina and R. vomitoria) and lastly the methods of cultivation (using modified strains, manual and mechanical cultivation) also affect the quantitative aspect of secondary metabolites production. For instance, a soil rich-in-nitrogen content gives rise to a relatively higher yield of alkaloids in the medicinal plants; whereas a soil not so abundant in nitrogen content and grown in comparatively dry zones may yield an enhanced quantum of volatile oil; ontogeny (or aging of plant) of a medicinal plant has a direct impact on the concentration of the active constituent (but not always true that older the plant greater would be the active principal), e.g., (i) cannabidiol, present in Cannabis sativa (C. sativa var. indica) and possessing euphoric activity, content attains a maximum level in the growing season and subsequently declines; however, the concentration of dronabinol (or tetrahydrocannabinol) starts to enhance reciprocally till the plants get fully matured; (ii) Morphine, the well-known narcotic-analgesic present in the air-dried milky exudate of Papaver somniferum or P. album is found to be the highest just 2–3 weeks after flowering. An undue delay in harvesting from this “critical-period” would ultimately result in the decomposition of morphine and similar is the case with the allied alkaloid like codeine and thebaine.

Metabolites of medicinal plants produced in different metabolic pathways have been proved to be the active or potentially active drug components in both traditional and modern medicines. Pharmaceutical as well as many food industries exploit these pools of biogenic resources of medicinal plants. Under outdoor conditions, medicinal plants are exposed to a multitude of environmental stress factors, both abiotic and biotic (herbivores and myriads of pathogenic microbes), during the growing season. The way plants sense stress depends on the duration and magnitude of the stress episode and its sustained effects depend on its severity, timing, duration, and the physiological status of the plant (Niinemets 2010). Stress factor by reducing the net photosynthetic rate reduces carbon and energy allocation for the synthesis of secondary metabolites and bringing alterations in the activity of key enzymes of secondary metabolic pathways also can modify the secondary metabolite pool under different stress regimes (Singsaas and Sharkey 2000). The quality and quantity of production of these chemical metabolites in plants are influenced by a multitude of internal and external environmental factors including circadian rhythm, developmental stage and age, tissue damage, season, altitude, temperature, water availability, soil nutrients, UV radiation, atmospheric composition, etc. Different stress regimes differentially affect quality and quantity of the secondary metabolite pool in plants. Co-occurrence of multiple stress factors and their effects may be additive or diverse and cannot always be extrapolated from responses to individual stress factor.

The secondary metabolites in plants are synthesized by different biochemical pathways and are strongly influenced by both environmental and biotic factors (Pavarini et al. 2012). The secondary metabolism of plants, and the expressed metabolite levels, may change considerably due to the influence of several biotic and abiotic stress signals.

2.2.5.1 Light (Visible and UV Radiation)

The visible spectrum of solar radiation energy (400–700 nm) is an important environmental factor required for photosynthesis, biomass accumulation, growth, and development of plants. Longer light exposure produced higher levels of ginsenosides in Panax quinquefolius (Fournier et al. 2003). Continuous solar radiation activated the flavonoid biosynthesis pathway in Vaccinium myrtillus (Jaakola et al. 2004). Ocimum basilicum on exposure to red light (600–700 nm) accumulates rosmarinic acid accumulation (Shiga et al. 2009). Light effects also have crucial importance on preharvest of Camellia sp. (Tea) shoots where synthesis and accumulation of phenolic derivatives (i.e., flavanols and catechin) become upregulated in leaf on overexposure to light and light exclusion of shoots just before harvest enhances the proportion of purine alkaloid (caffeine) level in leaves yielding high quality tea (Anan and Nakagawa 1974; Ashihara et al. 2008).

Plants have developed their ability to sense different light spectra and ultraviolet (10–400 nm , the range covers EUV to UVA) light present in the solar radiation and plants have evolved biochemical protective mechanisms against extreme light intensities and potentially damaging elevated doses of ultraviolet (UV) radiation (shorter than that of visible light but longer than X-rays). UV-B radiation (280–315 nm) impacts on the levels of a broad range of secondary metabolites, including phenolic compounds, terpenoids, and alkaloids (Rozema et al. 1997; Kazan and Manners 2011). Coumarins such as psoralen, bergapten, and xanthotoxin biosynthesized on exposure to UV radiation in leaves of celery (Apium graveolens) vegetables of Apiaceae can damage eukaryotic DNA (Taiz and Zeiger 2006). Catharanthus roseus (cell culture) on exposure to UV radiation enhanced the production of catharanthine (Ramani and Chelliah 2007; Jenkins 2009). Phenylpropanoid derivatives selectively absorb in the UV-B spectral region without decreasing penetration of photosynthetic radiation into the leaf. Flavonoids, hydroxycinnamic acids and their esters have also been implicated in this role in a broad range of plant species (Burchard et al. 2000; Kliebenstein 2004). Glycoalkaloids such as α-solanine and α-chaconine reportedly accumulate in potato tubers exposed to mechanical stress or light and the compounds lead to gastrointestinal or neurological disorders in humans.

2.2.5.2 Soil Nutrients

The level of secondary metabolites in plant tissues is reported to vary with resource availability (Coley et al. 1985). For example, proanthocyanidins increase quantitatively following nutritional stress involving phosphate deficiency (Kouki and Manetas 2002); biosynthesis of phenolic compounds increases under iron stress (low iron level) (Dixon and Paiva 1995). High conditions of Cu and Mn nutrition decreased both tannins and flavonoids contents in Eugenia uniflora (Santos et al. 2011) while enrichment of carbon dioxide in atmosphere elevated tannin levels in Quercus spp. (Stiling and Cornelissen 2007).

2.2.5.3 Moisture

Drought events of relative ranges of magnitudes and durations are commonly experienced in many environments and can drastically impact plant survival and/or stress tolerance. Moisture stress causes reduction in the rate of photosynthesis as well as plant growth and development and one might expect a negative relationship between water shortage and the synthesis of secondary metabolites. Reduced rate of water availability and high temperature influence high phenolic production in plants (Glynn et al. 2004; Alonso-Amelot et al. 2007). Phenolic and saponin levels and the corresponding bioactivity were found to vary seasonally in medicinal bulbs (Ncube et al. 2011). In the same study, high phenolic compounds were recorded in all the species during the winter season, where moisture stress is a typical characteristic. Drought stress lowers monoterpene emissions in Quercus ilex (Lavoir et al. 2009).

2.2.5.4 Temperature

Temperature stress in plants is generally known to induce or enhance the active oxygen species-scavenging enzymes like superoxide dismutase, catalase, peroxidase, and several antioxidants. Temperature stress may lead to many physiological, biochemical, and molecular changes in plant metabolism (protein denaturation or perturbation of membrane integrity) and many of these changes can alter the secondary metabolite concentrations in the plant tissues (Zobayed et al. 2005). High temperature (35 °C) treatment increased the leaf total peroxidase activity together with an increase in hypericin, pseudohypericin, and hyperforin concentrations in the shoot tissues of St. John’s Wort (Zobayed et al. 2005). Also, an exponential increase in a variety of VOCs with a linear increase in temperature has been described in a range of plant species (Parker 1977; Sharkey and Loreto 1993; Sharkey and Yeh 2001). Cold stress has been shown to stimulate an increase in phenolic production and their subsequent incorporation into the cell wall (Christie et al. 1994). Levels of anthocyanins increase following cold stress and are thought to protect plants against this effect (Pennycooke et al. 2005; Huang et al. 2012). Ncube et al. (2011) attributed the high levels of total phenolic compounds obtained during the winter season in their study as being consistent with this fact and support similar findings from previous other studies (Prasad 1996; Pennycooke et al. 2005). Lower temperature was reported to increase the level of artemisin in Artemisia spp. (Wallaart et al. 2000; Brown 2010).

2.2.5.5 Altitude

Altitude starting from sea level to timberline and beyond influences plant life as it (with the increase) changes climate, quality and amount of solar radiation, availability of soil water and nutrients, etc., to plants and also metabolism and metabolites content. Altitude affected the flavonoids contents in Leontodon autumnalis (Zidorn and Stuppner 2001; Grass et al. 2006). Higher altitudes increased the flavonoids and phenolic acids contents in flowers of Matricaria chamomilla (Ganzera et al. 2008) whereas higher altitudes in association with lower temperatures affected phenolics derivatives in Arnica montana (Albert et al. 2009).

2.2.5.6 Ozone

Ozone has also been demonstrated to affect secondary metabolism in plants (Eckey-Kaltenbach et al. 1994; Jordan et al. 1991). Elevated O3 levels increased the concentrations of terpenes, but decreased the concentrations of phenolics in Ginkgobiloba leaves grown under greenhouse conditions (He et al. 2009).

2.2.5.7 Salt Stress

Salt stress is a contributing factor to secondary metabolism in plants. Plants adjust metabolism to acclimate to different salt levels in soil and other growth media. High levels of alkaloids were reported for Achnatherum inebrians plants cultivated under salt stress (Zhang et al. 2011).

2.2.5.8 Biotic Stress Factors

Herbivore and pathogenic attacks have been repeatedly shown to cause an increased release of inducible secondary compounds in plants (Hagerman and Butler 1991; Bernays and Chapman 2000). Biotic effects include more sophisticated interactions with plant biochemistry and plant physiology (Briskin 2000). In a larger sense, it can be assumed that biotic effects are related either to plant interactions with microorganisms or plant physiological aspects, as phenology and ontogeny (Pavarini et al. 2012). Saponins occur constitutively in many plant species as part of their defense system and saponin content in plants seems to be dynamic, responding to many external factors including various biotic stimuli connected to herbivory attack and pathogenic infection, as well as involved in plant mutualistic symbioses with rhizobial bacteria and mycorrhizal fungi (Szakie et al. 2011).

2.3 Chemistry of Plant Constituents, Their Classification and Application

Classification of phytochemicals based on their elemental constituents (e.g., C & H; C, H & O; C, H, O, N, S & P containing compounds; O/N containing heterocyclic compounds; other miscellaneous compounds)

Living organisms are made of combinations of inanimate inorganic elements and out of the 92 naturally occurring elements, cells of living organisms are basically made of only a small selection of these elements, e.g., C, H, O, N, P, S, Ca, Fe, Cu, Mg, Zn, etc.; the first six of the series (C, H, O, N, P and S) make up bulk of the tissue component, but four of which (C, H, O and N) make up about 95% of the body weight of an organism. Cell sap contains electrolyte like Cl, K and Na as major elements. Atoms of different elements are linked together in groups in many ways to form molecules. Therefore, the chemical bonds (ionic, covalent, polar covalent, etc.) that hold atoms together in molecules in a living cell play the crucial role. Except water (which accounts for about 70% of a cell’s weight), almost all the molecules in a cell are based on carbon. It has the unique capacity to form large molecules and because of its nano size, and four vacancy electrons in its outermost shell, it can form highly stable covalent bonds with neighbor carbon as well as other atoms covalent C–C bonds to form chains and rings and hence generate large and complex organic molecules with no obvious upper limit to their size. A few basic categories of carbon-based molecules, e.g., hydrocarbons, carbohydrates, lipids, proteins, nucleic acids, secondary metabolites, etc., formed from different elements, give rise to all the extraordinary richness of form and behavior shown by living things. Based on the origin and the type of functions they perform in cells, some of these may be grouped as primary and others as secondary metabolites.

2.3.1 Primary Metabolic Products Consisting of C & H; C, H & O; N, S & P Elements (Carbohydrates, Lipids, Amino Acids, Proteins, Nucleic Acids, Organic Acids)

2.3.1.1 Hydrocarbons (C & H) and Derivatives

Hydrocarbons are organic compounds consisting of only carbon and hydrogen, e.g., the simplest form of lipids containing only C and H. Some latex-producing plants (hydrocarbon plants) of families, such as Euphorbiaceae, Apocynaceae, Asclepiadaceae, Sapotaceae, Moraceae, Dipterocarpaceae, etc., convert a substantial amount of photosynthetes (products of Calvin cycle) into latex which contains liquid hydrocarbons of high molecular weight (10,000 da). Hydrocarbons formed of isoprene units belong to the large group of terpenes.

Classification

Hydrocarbons include four classes of compounds such as alkanes, alkenes, alkynes, and aromatic hydrocarbons (Fig. 2.12). Hydrocarbons include linear or branched carbon chain; saturated (e.g., alkanes, CnH2n+2, ethane-C2H6) or unsaturated (e.g., alkenes, C n H2n, ethylene-C2H4; alkynes, C n H2n-2, ethyne-C2H2); and cyclic (benzene-C6H6, toluene-C7H8), alicyclic (e.g., cyclobutane-C4H8) hydrocarbons and carotenoids. Several hydrocarbons may be substituted with oxygen-containing groups (e.g., xanthophylls). Hydrocarbons may be classified as follows:
Fig. 2.12

Different classes of hydrocarbons : alkanes, alkenes, alkynes and aromatic hydrocarbons; saturated, unsaturated, cyclic and alicyclic

Alkanes are saturated hydrocarbons with a general formula C n H2n+2, e.g., methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), etc. Alkenes are unsaturated hydrocarbons and contain a carbon–carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms, e.g., the molecular formulae of ethene (IUPAC name) or ethylene and propene are C2H4 and C3H6, respectively. Alkyne is an unsaturated hydrocarbon containing at least one carbon–carbon triple bond with the general chemical formula C n H2n−2, e.g., acetylene, propyne, butyne, etc. Cycloalkanes contain carbon–hydrogen bonds and carbon–carbon single bonds and the carbon atoms are joined in a ring, e.g., cyclopropane, –butane, –pentane, –hexane, etc., are some common examples. Cycloalkenes or cycloolefin are alkenehydrocarbons with ring structure having at least one C=C double bond in the ring, but has no aromatic character. Cyclopropene, cyclopentene, cyclohexene, etc. are some of the examples cycloalkenes. Aromatic hydrocarbons (or arene or aryl hydrocarbon) are compounds that contain benzene (a cyclic hydrocarbon with the formula C6H6) as a part of their structure. Many of these compounds have a sweet or pleasant odor. Aromatic hydrocarbons can be monocyclic (MAH) or polycyclic (PAH). PAHs consisted of fused aromatic rings and do not contain heteroatoms or carry substituents. They are most widespread organic pollutants produced largely a result of natural emissions as well as anthropogenic activities (e.g., fossil fuel-burning, oil refining, coke and asphalt production, aluminum production, etc.). The most significant endpoint of PAHs (e.g., benz[a]anthracene, benzo[a]pyrene) toxicity is cancer (e.g., increased incidences of lung, skin, and bladder cancers are associated with chronic occupational exposure to PAHs). Some non-benzene-based compounds (heteroarenes) are also called aromatic compounds in which one carbon atom may be replaced by one of the heteroatoms (oxygen, nitrogen, or sulfur). For example, furan and pyridine are five- and six-membered ring heterocyclic compounds containing one O and one N atom, respectively in the heterocycle. Some examples of different classes of hydrocarbons are given in Table 2.1.
Table 2.1

Examples of different classes of hydrocarbons

Several hydrocarbons may be substituted with non-hydrogen atoms and thus produce hydrocarbon derivatives. Hydrocarbon derivatives contain different elements (e.g., oxygen, nitrogen, halogen atoms, etc.) or functional groups (e.g., hydroxyl, carbonyl, carboxyl groups, etc.) instead of hydrogen and in this way almost an innumerable number of carbon compounds are formed (e.g., alcohols, alkyl halides, amines, amides, carboxylic acids, esters, aldehydes, ketones, ethers, etc.).

Alkanes–alkenes molecules found in many living organisms are directly derived from fatty acids (e.g., undecane in ants and eicosane in Bryonia dioica). They are distinct from the terpenoid hydrocarbons. Terpenoids contained in the latex of Hevea spp. of Euphorbiaceae are plant-derived hydrocarbon products (e.g., rubber). Some algae also produce hydrocarbons, e.g., the green form of single cell alga Botryococcus braunii produces hydrocarbon (chain length between 34 and 38 carbons containing many double bonds). Hydrocarbons in plants may be formed as products of fatty acid cleavage during peroxidation processes. Alkanes as well as alkenes appear during hydroperoxide decomposition. Plant-derived aliphatic hydrocarbons include vegetable, olive, and other cooking oils.

There may be monocyclic and polycyclic hydrocarbons of biological origin (Fig. 2.13). Monocyclic hydrocarbons include several branched alkylbenzenes of Archaebacteria (Thermoplasma and Sulfolobus) with two methyl groups branched on a saturated chain of 9–12 carbon atoms, and algal pheromone ectocarpene is an unsaturated heptacyclic hydrocarbon found in the brown algae Ectocarpus, Adenocystis, and Sphacelaria (Müller et al. 1971), resveratrol (3,4′,5-trihydroxystilbene) is present in grapes and blueberries (Vaccinium). It has numerous pharmacological properties including anticancer, antiviral, neuroprotective, antiaging, and anti-inflammatory, etc. Diarylheptanoids, a group of compounds having phenyl rings at 1,7 positions of n-heptane. Curcumin of turmeric is a diarylheptanoid. Because of its strong antioxidant properties, numerous therapeutic activities have been assigned to turmeric for a wide variety of diseases and conditions (Aggarwal et al. 2007).
Fig. 2.13

Monocyclic and polycyclic hydrocarbons of biological origin with anticancer, antiviral, neuroprotective, antiaging, anti-inflammatory, strong antioxidant, etc., activities

Polycyclic hydrocarbons consist of fused rings containing only carbon (naphthalene, perylene, denthyrsinin) or heterocycles including foreign atoms such as O (coumarin, osthole, psoralen), N (2-Heptyl-3-hydroxy-4-quinolone), etc. Naphthalene is a constituent of Magnolia flowers (Azuma et al. 1996); it may protect tissue against insect herbivores, and attracts insects to pollinate by the UV absorption of accumulated naphthalene in the floral parts and floral scent. Several forms of phenanthrenes are present in higher plants of several families like Orchidaceae, Dioscoreaceae, Combretaceae, Euphorbiaceae, Juncaceae, and Hepaticae. Denthyrsinin is reported in the orchid species Cymbidium pendulum , Dendrobium spp., Eulophia nuda , Nidema boothii , Scaphyglottis livida , Thunia alba , etc. It, as others, displayed potent cytotoxic activities (Kovács et al. 2008).

Heterocyclic hydrocarbons include coumarins, and a coumarin is the simplest compound of this group (Fig. 2.14). Several other are coumarin derivatives by various additions. It is found in many plants including tonka bean (Dipteryx odorata) of Fabaceae, vanilla grass (Anthoxanthum odoratum) and buffalo grass (Hierochloe odorata) of Poaceae, woodruff (Galium odoratum) of Rubiaceae. All these plants are strongly scented due to the presence of coumarin which has been used in perfumes since 1882 (imitation of vanilla products). Coumarin is used as rodenticide, and extracts from these plants are potential harmful as coumarin is the precursor for several anticoagulants, notably warfarin. Osthole [7-methoxy-8-(3-methylpent-2-enyl) coumarin] is a coumarin derivative found in Cnidium monnieri, a plant used in traditional Chinese medicine to treat skin affections. Osthole was shown to exhibit several biological functions, including antiosteoporotic, antiallergic, anti-inflammatory, and antitumor functions. Recently, it was found that osthole might be a potent antidiabetic agent (Lee et al. 2011).
Fig. 2.14

Heterocyclic hydrocarbons coumarin and several coumarin derivatives possessing different biological functions: antiosteoporotic, antiallergic, anti-inflammatory, and antitumor activities

Umbelliferone (or 7-hydroxycoumarin) occurs in many familiar plants from the Umbelliferae family such as carrot or coriander but also from other families such as Asteraceae (Pilosella officinarum). Umbelliferone absorbs ultraviolet light strongly but despite possible harmful mutagenic properties, it is used in sunscreens. Psoralen (or psoralene) is a furanocoumarin. It is a derivative from umbelliferone by addition of a furan ring. Psoralen has been described in the seeds of the Fabaceae ( Psoralea corylifolia). It is also present in many plants of Rutaceae (Ruta, Citrus), Moraceae, Leguminoseae ( Psoralea , Coronilla), Apiaceae, etc. Psoralen-rich plants are used in Chinese and Indian medicines and psoralene, due to its UV absorption properties, is used in treatment of psoriasis, eczema, and vitiligo and in some cutaneous lymphoma.

The core of quinolines is the 1-azanaphthalene nucleus. The simplest one is quinolin. That compound is rarely found in living material but is present, as its derivatives, in the plant Rutales and even in some insects. Quinolin is also present in some insects, as phasmids, where it plays a role against predators. The bacteria Pseudomonas aeruginosa was shown to produce a new cell-to-cell signal molecule, e.g., 4-quinolone base structure with an alkyl chain (2-heptyl-3-hydroxy-4-quinolone) and has been designated as the Pseudomonas quinolone signal (Pesci et al. 1999). Compounds related to quinolines, the benzoxazinones, are present as inactive glucosides (phytoanticipins), mainly in Gramineae (rye, wheat, corn). They are sometimes described as cyclic hydroxamic acids. In rye, the principal compound is the glucoside of DIBOA (2, 4-dihydroxy-1,4-benzoxazin-3-one), in wheat and corn, it is the glucoside of the methoxylated form, DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one).

Carotene hydrocarbon compounds consist of C40 chains (tetraterpenes consisting of eight 5C isoprenoid units) with conjugated double bonds (carotenes), while their oxygenated derivatives are known as xanthophylls. They are the predominant class of tetraterpenes (or terpenoids), show strong light absorption and often are brightly colored (red, orange), and are found as pigments in bacteria, algae and higher plants. Carotenoids perform three major functions in plants (e.g., accessory pigments for light harvesting by expanding the absorption spectra of photosynthesis, prevention of photooxidative damage by dissipating excess light, and pigmentation attracting insects). Carotenoides like α-carotene, β-carotene and lycopene are major carotenes while lutein, zeaxanthin, and cryptoxanthin are some major xanthophylls. The human intake of carotenoids may be appreciated using databases such as that established for Swiss vegetables (Reif et al. 2013). In mammals, carotenoids exhibit immunomodulatory actions, likely related to their anticarcinogenic effects. β-carotene was thus shown to enhance cell-mediated immune responses (Hughes 1999). The decrease in prostate cancer risk has been linked to the consumption of tomatoes (lycopene rich vegetable), but there is yet limited direct evidence in favor of such link (Kavanaugh et al. 2007).

Hydrocarbons are found at the outer surface in higher plant leaves, e.g., C27, C29, and C31 n-alkanes are the most abundant (from 11 to 19%) in needle wax of the Serbian spruce Picea omorika of Pinaceae. Volatile oils of plant and animal origin are the oxygenated derivatives and hydrocarbons. Ethylene occurs in plants functions as a natural growth regulator that promotes the ripening of fruit. Several alkenes with 8 or 11 carbon atoms and 3 or 4 double bonds play a role in algae gamete attraction (pheromones): cystophorene in Cystophora sp., finavarrene in Ascophyllum sp. and Sphaerotrichia sp., fucoserratene in the brown seaweed Fucusserratus and in the freshwater diatom Asterionellaformosa (Bacillariophyceae). Cyclic hydrocarbons may be monocyclic or polycyclic. Monocyclic hydrocarbons have mostly two methyl groups branched on a saturated chain of 9–12 carbon atoms. Algal pheromone, ectocarpene, is an unsaturated heptacyclic hydrocarbon found in the brown algae Ectocarpus, Adenocystis, and Sphacelaria. Resveratrol (3, 4, 5-trihydroxystilbene), a polycyclic species, is present in grapes and wine and blueberries and shows numerous pharmacological properties such as anticancer, antiviral, neuroprotective, antiaging, anti-inflammatory, etc. Other examples include phenanthrenes (a polycyclic aromatic hydrocarbon composed of three fused benzene rings) that are present mainly in Orchidaceae family as well as in Dioscoreaceae, Combretaceae, Euphorbiaceae, Juncaceae, and Hepaticae.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous, present in the atmosphere, surface water, sediments and soil, food and lipid tissues; and human exposure to PAHs is mainly from food and inhaled air including the occupational exposure (EC 2002). Food can be contaminated from industrial as well as domestic food processing including drying, smoking, roasting, grilling, frying, barbecuing) (Harvey 1997; Howsam and Jones 1998). They have been detected in a wide variety of herbs like lime, pansy, mint, lemon balm, panax, Fructus liquidambaris, liquorice root, mulberry twig, cassia seed, eucommia bark, rose flower, indigowoad leaf, fleeceflower root, and in many crude drugs (Kataoka et al. 2010; Krajian and Odeh 2013; Zongyan et al. 2014). Anthraquinone (a PAH) having anthracene nucleus is found in several plant species (Aloes), fungi, and lichens. They are toxic and many of the compounds in this class are both genotoxic and carcinogenic (induce mutations, promote tumor formation). They are actively involved in enzyme induction, immunosuppression, and teratogenicity.

The hydrocarbon carotenoids are known as carotenes (tetra terpenes), while oxygenated derivatives of these hydrocarbons are known as xanthophylls (e.g., lutein, zeaxanthin, neoxanthin, violaxanthin, flavoxanthin, α- and β-cryptoxanthin, etc.) (Fig. 2.15). About 700 carotenoids have been identified, 50 are regularly consumed in the human diet and 24 have been detected in human plasma so far. Carotenoids are important components of photosynthetic pigments in plants and in mammals, carotenoids exhibit immunomodulatory actions, likely related to their anticarcinogenic effects. β-carotene was thus shown to enhance cell-mediated immune responses The decrease in prostate cancer risk has been linked to the consumption of tomatoes, vegetable rich in lycopene, as prostatic tissues. Lutein and zeaxanthin are isomers (differing only in the placement of one double bond) and represent xanthophylls. They are found in high quantities in green leafy vegetables and fruits such as spinach, kale, yellow carrots papaya, peaches, prunes, and squash. Animals get lutein from plants (in egg yolks and animal fats). Broiler feed now is fortified by lutein to improve the color of broiler chicken skin and egg yolk.
Fig. 2.15

The hydrocarbon carotenes and xanthophylls

2.3.1.2 Carbohydrates (C, H & O)

Carbohydrates are polyhydroxy aldehydes or ketones or substances that yield such compounds on hydrolysis. Carbohydrates are plant products and, in most of the cases, hydrogen (H) and oxygen (O) remain as associates with carbon in the ratio (1:2:1) similar to that of water (C–H2O), e.g., glucose, fructose (C6H12O6), sucrose (C12H22O11) and starch (C6H10O5) n as well as gum, mucilage and pectin (derived carbohydrates). Carbohydrates are widely distributed in plants, remain both as transitory and stored constituents, and provide energy, carbon skeleton for metabolic synthesis and building blocks of the cell wall materials and other metabolites.

Sugars, starches, gums, and other carbohydrates are of very little pharmacologic action and of little importance as remedies, but of importance in dietetics for energy for the body and brain cells, they are used in pharmacy mainly as pharmaceutic necessities, such as suspending and emulsifying agents (Tragacanth, Acacia, Agar, Alginates, etc.), adhesives and binders (tragacanth, dextrins, acacia, etc.), demulcents (acacia, sterculia, etc.), thickening agents, diluants and tablet disintegrants (alginates, starch, etc.).

Classification

Carbohydrates are classified into following groups:
  1. (i)

    Monosaccharides molecule consisting of only one unit (sugars containing 3–9 carbon atoms—triose to nanos, 5 and 6 being most common),e.g., glucose, fructose;

     
  2. (ii)

    Oligosaccharides molecule consisting 2–10 monosaccharide units (disaccharide to decasaccharide), e.g., sucrose, gentianose;

     
  3. (iii)

    Polysaccharides molecule consisting 11 to −n number of monosaccharide units, e.g., starch, cellulose.

     

The simple carbohydrates or sugars are water soluble and sweet in taste, e.g., glucose, fructose, and similar other carbohydrates. Drugs containing sugars include liquorice, which contains free glucose and fructose and gentian, which contains sugars like gentianose (trisaccharide) and gentiobiose (disaccharide).

Carbohydrates may also be grouped as sugars and non-sugars or polysaccharides on the basis of their taste and solubility. Sugars are sweet crystalline substances and soluble in water and further divided into monosaccharides (e.g., glucose, galactose, fructose) and oligosaccharides (e.g., sucrose, raffinose, stachyose etc.) while non-sugars are long chain polymers of monosaccharides units, polysaccharides [C6H10O5] n , linked to each other by glycosidic linkages. They are tasteless amorphous substances and insoluble in water and further divided into homopolysaccharides (e.g., starch, cellulose, inulin, etc.) and heteropolysaccharides (e.g., pectin, lignin, glycoprotein, etc.).
  • (i) Monosaccharides (CH 2 O) 3–10 : Monosaccharides consist of single unit and divided into (a) aldoses (each contains an aldehyde group, –CHO, e.g., aldotrioses—glycerose; aldotetroses—erythrose, threose; aldopentoses—ribose, arabinose, xylose; aldohexoses—glucose, galactose, mannose; aldoheptoses-l-glycero-d-manno-heptose, etc.); and (b) ketoses (each contains a keto group, >C=O, e.g., ketotrioses—dihydroxyacetone; ketotetroses—erythrulose; ketopentoses—ribulose, xylulose; ketohexoses–fructose; ketoheptoses—sedoheptulose, mannoheptulose; octoses—d-manno-octulose2-keto-3-deoxy-manno-octonate; nonoses-d-glycero-d-galacto-nonulose, sialose, l-ribo-d-manno-nonose (ketose), etc.), decose-3,6-Dideoxy-l-threo-l-talo-decose (aldose). Empirical and structural formulae of monosaccharides are shown in Table 2.2.
    Table 2.2

    Empirical and structural formulae of different classes of monosaccharides

A triose is a monosaccharide containing three carbon atoms, the simplest monosaccharides. There are only two trioses, an aldotriose (glyceraldehyde) and a ketotriose (dihydroxyacetone). In green plants, trioses are formed by the fixation of carbon dioxide in the process of photosynthesis. And they are also important in respiration as lactic acid and pyruvic acids are derived from aldotriose and ketotriose, respectively. Glyceraldehyde is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid and is an intermediate compound in carbohydrate metabolism. Dihydroxyacetone, the simplest ketose, is an isomer of glyceraldehyde. Trioses and trióse phosphates are important metabolic intermediates. A monosaccharide with four carbon atoms is a tetrose. They either have an aldehyde functional group in position C1, aldotetroses, e.g., d-erythrose, d-threose, or a ketone functional group in position C2, ketotetroses, e.g., d-erythrulose. A pentose is a five-carbon monosaccharide. Pentoses are organized into two groups, e.g., aldopentoses have an aldehyde functional group at position C1 and ketopentoses have a ketone functional group in position C2 or C3. d-arabinose, d-lyxose, d-xylose, d-ribose, l-arabinose, l-lyxose, l-xylose, and l-ribose are eight examples of aldopentose stereoisomers (they aldopentoses have three chiral centers and so 23 = 8 isomers) and d-ribulose, d-xylulose, and l-ribulose, l-xylulose are four examples of ketopentose stereoisomers (these aldopentoses have two chiral centers and so 22 = 4 isomers). Ribose and deoxyribose are constituents of RNA and DNA, respectively. A polymer of pentose sugars is called a pentosan.

Hexose is a monosaccharide with six carbon atoms. Hexoses are classified by functional group, with aldohexoses having an aldehyde at position C1 (e.g., glucose), and ketohexoses having a ketone at position C2 (e.g., fructose). The aldohexoses have four chiral centers for a total of 16 possible aldohexose stereoisomers (24). The D/L configuration is based on the orientation of the hydroxyl group at position 5, and does not refer to the direction of optical activity. The 8 d-aldohexoses are allose, altrose, glucose, mannose, gulose, idose, galactose, talose, etc. Of these d-isomers, all except d-altrose are naturally occurring. The ketohexoses have three chiral centers and therefore eight possible stereoisomers (23). Of these, only the four d-isomers are known to occur naturally (d-psicose, d-fructose, d-sorbose, d-tagatose). Only the naturally occurring hexoses are capable of being fermented by yeasts. Hexose sugars can form dihexose sugars with a condensation reaction to form a 1, 6-glycosidic bond. Many of these simple sugars are found in many fruits and vegetables and are the common building blocks for the more complex sugars. Glucose (also known as d-glucose, dextrose, or grape sugar), an important carbohydrate in biology, is the most widely distributed sugar in the plant and animal kingdoms and it is the sugar present in blood as blood sugar. Cells use it as a source of energy and a metabolic intermediate. Only the “right-handed form” of glucose, d-glucose (dextrose) is very common in nature but not the l-glucose. Fructose (levulose or fruit sugar), a ketohexose, is abundant in honey and some fruits. Fructose and glucose are the main carbohydrate constituents of honey. Fructose is more easily appropriated by diabetics than are cane sugar, glucose, and many starchy foods. It has been used by Strauss as a test of the functional power of the liver, the assertion being made that if the levulose is recoverable from the urine unchanged, the liver is seriously impaired. Manna, derived from a tree of the ash family Oleaceae (manna ash—Fraxinus ornus), contains the sugar, mannite (C6H14O6), is laxative. A sugary extract from the sap is extracted by making a cut in the bark. The sugar mannose and the sugar alcohol mannitol both are derived from the extract.

Heptulose is a monosaccharide with seven carbon atoms, e.g., mannoheptulose. It is found as d-mannoheptulose in appreciable amount in avocado fruit ( Persea gratissima Gaertn. of Lauraceae) and trace amount in guava, passion fruit, mango, papaya, fig, alfalfa, and primerose. Mannoheptulose is a hexokinase inhibitor. By blocking the enzyme hexokinase, it prevents glucose phosphorylation. As a result, the breakdown of glucose is inhibited. Mannoheptulose has been reported to inhibit insulin secretion from pancreas and to induce hyperglycemia. This inhibition occurs because when mannoheptulose is present, the glycolysis is inhibited (because there is no production of glucose-6-P) and therefore no increase in ATP concentration which is required to close the K+-ATP channel in the beta cells of the pancreas causing a diminution of calcium entry and insulin secretion. d-alto and d-talo are two other heptuloses.

Volemitol (d-glycero-d-manno-heptitol, α-sedoheptitol) is an unusual seven-carbon sugar alcohol that fulfills several important physiological functions in certain species of the genus Primula, major nonstructural carbohydrate in leaves of all stages of development, followed by sedoheptulose (d-altro-2-heptulose, 36 mg/g fresh weight). Volemitol is important in certain Primula species as a photosynthetic product, phloem translocate, and storage carbohydrate. The physiological roles of alditols are manifold and largely resemble those of disaccharides and oligosaccharides. They include photosynthetic assimilation, translocation, and storage of carbon, and reducing power, as well as protection against different types of stresses.

Octose is a monosaccharide with eight carbons and includes d-glycero-d-manno-Octulose, d-glycero-l-galacto-Octulose.

The aldehyde (–CHO) and ketone (>C=O) functional groups in the pentose as well as in the hexose carbohydrates react with the neighboring hydroxyl functional groups to form intramolecular hemiacetals and hemiketals, respectively resulting ring structures. The pentose (ribose) and hexose (glucose) ring forms are created when the oxygens on C4 or C5, links with the carbon comprising the carbonyl group (aldose C1, ketose C2) and transfers its hydrogen to the carbonyl oxygen to create a hydroxyl group. The resulting ring structure is related to furan, five-sided ring (furanose) in case of C1→C4 and pyran, and six-sided ring (pyranose) in case of C1→C5 links. The ring spontaneously opens and closes, allowing mutarotation to occur about the bond between the carbonyl group and the neighboring carbon atom yielding two distinct configurations, e.g., α and β. The rearrangement produces α-ribose or glucose when the hydroxyl group is on the opposite side of the –CH2OH group, or β-ribose or glucose when the hydroxyl group is on the same side as the –CH2OH group. Cyclic forms with a 7-atom ring (the same of oxepane), rarely encountered, are called heptoses. Some of the structural formulae of Penta furanoses (Fig. 2.16) and Hexa pyra- and furanoses (Fig. 2.17) are shown below.
Fig. 2.16

Structures of different penta furanoses —Furan, Pyran, d-ribose, α- and β-d-ribofuranose, β-d-ribofuranose, β-d-2-deoxyribo furanose

Fig. 2.17

Structure of different hexa pyra- and furanoses

The ring forms of β-d-ribose and β-d-2-deoxyribose (missing oxygen at position 2) are two ribo-pentose sugars and they are the structural components of RNA and DNA, respectively (Fig. 2.17).

Monosaccharides with same empirical formulae may differ in their structural formulae (isomers) leading to changes in their physical and chemical properties. For example, many saccharides differ in their structural formulae only in the orientation of the hydroxyl groups (–OH) and this structural difference makes a big difference in the biochemical, organoleptic, (taste) and physical properties (melting point, optical activity, etc.). These carbohydrates form stereoisomers and the German chemist Emil Fischer received Nobel Prize in 1902 in chemistry as he identified the stereoisomers in aldohexoses in 1894. Structures that have opposite configurations of a hydroxyl group at only one position, such as glucose and mannose, are called epimers. Isomers which differ only in their configuration about their carbonyl carbon atom are called anomers. The D refers to dextrorotatory (rotates polarized light to the right) character natural glucose, but it now denotes a specific configuration.

Sugars may be modified by natural processes into compounds like sugar alcohols, amino sugars, and uronic acids. Sugar alcohols are a type of carbohydrates called polyols; part of their chemical structure resembles sugar, and part of it resembles alcohol. Sugar alcohols are one type of reduced calorie sweetener. Both monosaccharides and disaccharides (maltitol and lactitol) can form sugar alcohols. Some of the common sugar alcohols are methanol (1-carbon), ethylene glycol (2-carbon), glycerol (3-carbon), erythritol (4-carbon), threitol (4-carbon), arabitol (5-carbon), xylitol (5-carbon), ribitol (5-carbon), mannitol (6-carbon), sorbitol (6-carbon), galactitol (6-carbon), fucitol (6-carbon), iditol (6-carbon), inositol (6-carbon, a cyclic sugar alcohol); volemitol (7-carbon), isomalt (12-carbon), maltitol (12-carbon), lactitol (12 carbon), maltotriitol (18-carbon), maltotetraitol (24-carbon), polyglycitol, etc. The simplest sugar alcohols, ethylene glycol and methanol, are sweet but highly toxic chemicals used in antifreeze. The more complex sugar alcohols are generally nontoxic. Sugar alcohols occur naturally in plants (e.g., sorbitol from corn syrup and mannitol from seaweed) or they are manufactured from sugars and starches. Sugar alcohols (e.g., glycetol, polyols, polyhydric alcohols, or polyalcohols) are the hydrogenated forms of the aldose or ketose sugars, e.g., sorbitol (glucitol), where the aldehyde (–CHO) group is replaced with a –CH2OH group (Fig. 2.18).
Fig. 2.18

Structure of different sugar alcohols —sorbitol, talitol and mannitol

The sugar alcohols commonly found in foods are sorbitol, mannitol, xylitol, isomalt, and hydrogenated starch hydrolysates (HSH). Their relative sweetness may be graded as sugar = xylitol > erythritol > maltitol > mannitol > sorbitol > isomatol > HSH > lactitol, etc. and that of their energy content (calories per gram) as sugar (4%) > HSH > sorbitol > xylitol > maltitol > isomaltol > lactitol > mannitol > erythritol, etc. These sugar substitutes provide fewer calories (1.5–3 calories per gram) than table sugar (4 calories per gram), mainly because they are not well absorbed and may even have a small laxative effect. In commercial foodstuffs, sugar alcohols are commonly used in place of table sugar (sucrose) as thickeners and sweeteners. Many so-called dietetic foods that are labeled sugar free or no sugar added in fact contain sugar alcohols.

Erythritol, a four-carbon polyol, 60–70% as sweet as table sugar, is only partially absorbed by the body and so it has only 0.2 calories per gram, i.e., 95% less than table sugar. Erythritol is used as a food additive throughout the world. It is used in food for diabetics because it does not affect blood sugar and does not cause dental caries. Although erythritol is well tolerated by humans but appeared toxic to Drosophila melanogaster like insecticide. Xylitol, a five-carbon polyol, is a very common ingredient in sugar-free candies and gums because it is approximately as sweet as sucrose, but contains 40% less food energy. This sugar alcohol is safe for humans; however, xylitol in small doses can cause seizures, liver failure, and death in dogs. Sorbitol, a six-carbon polyol, found primarily in stone fruits and also manufactured from corn syrup, is used in diet sodas, sugar-free ice creams and desserts, as well as in mints, cough syrups, and gum. Maltitol, a 12-carbon polyol derived from chicory and roasted malt, is very similar to actual sugar in terms of mouthfeel, sweetness, and cooking (except for browning) but calories and so it is used in copious amounts in sugar-free desserts and other products. Unlike sugars, sugar alcohols do not contribute to dental caries, blood sugar, and insulin levels, but may cause bloating and diarrhea when consumed in excessive amounts.

An amino sugar contains an amino (–NH2) group in place of a hydroxyl group of the sugar molecule. More than 60 amino sugars are known, many of them have been isolated and identified in recent times as components of antibiotics. Examples of amino sugars include d-glucosamine, d-mannosamine, d-galactosamine, N-acetyl-d-glucosamine (main component of chitin), α-d-glucosamine α, -d-N-acetylglucosamine, sialic acid, l-daunosamine (a deoxy hexosamine and a component of birch juice), etc. (Fig. 2.19).
Fig. 2.19

Structure of different sugar amines d-glucosamine, d-mannosamine, d-galactosamine, N-acetyl-d-glucosamine, d-glucosamine, d-N-acetylglucosamine, sialic acid, l-daunosamine

Glucosamine is amino sugars or aminosaccharide and is a well-known amino sugar that produces glycoconjugates like glycosylated lipids and proteins. Glucosamine has a structural role in composing the hard exoskeleton of chitins of arachnids, crustaceans, and insects. Wheat, rice, and barley grains as well as bovine and shark meats are the important sources of glucosamine. It can help in the treatment of osteoporosis, or osteoarthritis. Galactosamine is one of eight essential amino acids that functions in cell-to-cell interaction, research has shown that it may help those with joint inflammations, lacking in galactosamine may even be one of the factors related to heart disease, it may also function as a toxin leading to liver failure. Sources of galactosamine include bovine cattle, oxen, and shark meat as well as red algae. Sialic acid is a very important sugar amine for mental and physical health. Growth, development, and hair as well as skin pigmentation are affected due to sialic acid deficiency in children while improved sialic acid concentrations in infants proved to improve their synaptogenesis and neurological development. However, sialic acid allows different viruses to enter cell, e.g., the Influenza virus. N-acetyl-d-glucosamine is the main component of the polysaccharide in chitin, the substance that makes up the tough outer skeleton of arthropods and insects. Aminoglycosides are a class of antimicrobial compounds that inhibit bacterial protein synthesis.

Sugar acids are monosaccharides with a carboxyl group (Fig. 2.20). Main classes of sugar acids include: Aldonic acids, in which the aldehyde functional group of an aldose is oxidized (e.g., glyceric acid—3C, xylonic acid—5C, gluconic acid—6C and ascorbic acid—6C, unsaturated lactone); ulosonic acids, in which the first hydroxyl group of a 2-ketose is oxidized creating an α-ketoacid (e.g., neuraminic acid—5-amino-3,5-dideoxy-d-glycero-d-galacto-non-2-ulosonic acid, ketodeoxyoctulosonic acid, KDO or 3-deoxy-d-manno-oct-2-ulosonic acid); uronic acids, in which the terminal hydroxyl group of an aldose or ketose is oxidized (e.g., glucuronic acid—6C, galacturonic acid—6C, iduronic acid—6C) and aldaric acids, in which both ends of an aldose are oxidized (e.g., tartaric acid—4C, meso-galactaric acid, mucic acid—6C, d-glucaric acid-saccharic acid—6C).
Fig. 2.20

Structure of different sugar acids —d-gluconic acid, d-glucuronic acid, l-galactonic acid, d-arabonic acid, d-glucaric acid, l-glucaric acid, d-galactaric acid and l-galactaric acid

  • (ii) Oligosaccharides [C 6 H 12 O 6 ] 2–10 : Oligosaccharide molecules consist of 2–10 monosaccharide units and depending upon the number of monosaccharide units formed on hydrolysis they are grouped into: (a) disaccharides—consisted of two molecules of the same or different monosaccharides (e.g., sucrose formed by two molecules of glucose; lactose formed by glucose and galactose; maltose and isomaltose formed by two molecules of glucose; cellobiose formed by two molecules of β-d glucose); trehalose; (b) trisaccharides—consisted of three molecules of the same or different monosaccharides (e.g., raffinose also called melitose formed by galactose, glucose and fructose); (c) tetrasaccharide-consisted of four molecules of the same or different monosaccharides (e.g., stachyose formed by 2 galactose, 1 glucose and 1 fructose molecules) and so on up to decasaccharides (heparin) through penta-, hexa-, hepta-, octa-, and nanosaccharides. Undigestible oligosaccharides are trisaccharide raffinose, the tetrasaccharide stachyose, and the pentasaccharide verbacose.

  • Disaccharides [C 6 H 12 O 6 ] 2 : These molecules consist of two simple sugars, e.g., sucrose, maltose, trehalose, etc. Some of them are shown with structural formulae, structural components, bonds, and sources in Table 2.3.
    Table 2.3

    Structure of different disaccharides—formulae, structural components, bonds, and sources

    Disaccharides

    Name and empirical formulae

    Structural formulae, components and glycosidic bond

    Sources

    Sucrose consisting of glucose and fructose molecules, C12H22O11

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    Common table, cane, beet sugar

    Maltose consisting of two α-d-glucose molecules C12H22O11

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    Product of starch hydrolysis

    Trehalose consisting of two α-d-glucose molecules C12H22O11

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    Found in fungi

    Lactose consisting of galactose and glucose C12H22O11

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    Main sugar in milk

    Cellobiose consisting of two β-D-glucose molecules C12H22O11

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    Product of cellulose hydrolysis

    Melibiose consisting of α-d-galactose and glucose

    C12H22O11

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    Found in legumes

Different disaccharides shown in Table 2.3 are described in the following paragraphs.

Sucrose or saccharose is ordinary table sugar refined from sugarcane or sugar beets and consists of glucose and fructose molecules linked by α-1→2 glycosidic bond. It is found in abundance in the sap of the sugar maple, in sugarcane, in sorghum, and in the root of the sugar beet. It is the main ingredient in turbinado sugar, evaporated or dried cane juice, brown sugar, and confectioner’s sugar. It dissolves in half its weight of water and is insoluble in alcohol. It ferments with yeast but does not reduce Fehling’s solution.

Lactose or milk sugar has a molecular structure consisting of galactose and glucose linked by β-1→4 glycosidic bond and requires for solution five times its weight of water. It is not very sweet, and is chiefly used as a nutritive in infant feeding and typhoid fever. In pharmacy, it is employed as a diluent. Lactose intolerance is an intestinal distress caused by a deficiency of intestinal enzyme lactase needed to absorb and digest lactose in milk. Undigested lactose ferments in the colon and causes abdominal pain, bloating, gas, and diarrhea. Yogurt does not cause these problems because lactose is consumed by the bacteria that transform milk into yogurt.

Maltose, maltobiose or malt sugar, is a disaccharide formed from two units of α-d-glucose molecules joined with an α-(1→4) glycoside bond. Maltose disaccharide is produced when amylase breaks down starch in germinating seeds and also when glucose is caramelized. Due to O-glycosidic link (free hemiacetal group), maltose can reduce Fehling’s reagent. Figure 2.21 shows the hydrolyzing reaction mechanism of amylase leading to producing maltose.
Fig. 2.21

Amylase reaction consisting of hydrolyzing amylose, producing maltose

Trehalose (mycose or tremalose) is a natural alpha-linked disaccharide formed by two α-d-glucose molecules connected through α1→1 glucosidic bond. It is a nonreducing disaccharide, less soluble than sugar, and resistant to acid hydrolysis (except at high temperatures >80 °C). Trehalose, widely distributed in nature, can be found in animals (shrimp, grasshoppers, locusts, butterflies, bees, as blood sugar), plants (sunflower seeds, moonwort, Selaginella, sea algae), fungi and microorganisms (mushrooms, yeast and it is metabolized by a number of bacteria, including Streptococcus mutans. The major dietary source is mushrooms. The trehalose is then broken down into glucose by the catabolic enzyme trehalase for use and so it is more efficient than sugar as energy source. It is implicated in anhydrobiosis and cryptobiosis, i.e., it helps plants and animals to withstand desiccation and freezing, respectively. It forms a gel phase as cells dehydrate that prevents disruption of internal cell organelles. Selaginella (the resurrection plant) growing in desert and mountainous areas may revive again after drying out for years following a rain because of the function of trehalose. Trehalose is used as excipient during freeze drying of a variety of products in the pharmaceutical industry and as an ingredient for dried, baked, and processed food, as well as a nontoxic cryoprotectant of vaccines and organs for surgical transplants. It has high water retention capabilities, and is used in food, beverages, and cosmetics. As a multifunctional sugar with nearly half the sweetness of sucrose, high thermostability, and a wide pH-stability range, trehalose strongly improves the taste, texture, and appeal of foods. Trehalose has the added advantage of being an antioxidant.

Cellobiose, a repeating unit of cellulose, is a disaccharide consisting of two β-d-glucose molecules that have a β1→4 glycosidic bond as in cellulose. The beta glycosidic bond makes d-cellobiose strong, stable and highly crystallizing character. Cellobiose has no taste, whereas maltose and trehalose are about one-third as sweet as sucrose.

Melibiose, hydrolysis of raffinose, is a reducing disaccharide formed by galactose α1→6 glucose linkage. It can be formed by invertase-mediated hydrolysis of raffinose, which produces melibiose and fructose. This sugar is produced and metabolized only by enteric and lactic acid bacteria and other microbes. Melibiose, a nondigestible saccharide, enhances the intestinal absorption of quercetin glycosides, an important antioxidant, anti-inflammatory, and anticarcinogenic agent.

Trisaccharides [C6H12O6]3

Raffinose , also called melitose, is a trisaccharide that is widely found in legumes and cruciferous vegetables, including beans, peas, cabbage, brussels sprouts (a cabbage cultivar), broccoli, asparagus, other vegetables, and whole grains. It consists of galactose connected to sucrose by α1→6 glycosidic linkage (Fig. 2.22). Humans and other monogastric animals (pigs and poultry) cannot digest saccharides with this linkage as they do not possess the α-galactosidase (α-GAL) enzyme and these oligosaccharides pass undigested through the stomach and upper intestine. In the lower intestine, they are fermented by gas-producing bacteria that do possess the α-GAL enzyme and make carbon dioxide, methane, and/or hydrogen—leading to the flatulence commonly associated with eating beans and other vegetables. Tablets Beano containing the enzyme α-GAL are frequently used as digestive aids to prevent gas and bloating. The enzyme is derived from selected strains of the food grade fungus Aspergillus niger.
Fig. 2.22

Structure of trisaccharide—raffinose

Tetrasaccharides [C6H12O6]4

Stachyose, a tetrasaccharide, on hydrolysis produces four molecules of monosaccharides consisting of two α-d-galactose units, one α-d-glucose unit, and one β-d-fructose unit sequentially linked by 3 glycoside bonds as gal(α1→6)gal(α1→6)glc(α1↔2β) fru (Fig. 2.23). Stachyose is a tetrasaccharide Stachyose together with related oligosaccharides such as raffinose occurs naturally in numerous vegetables, e.g., green beans, soybeans and other beans, artichoke, and also other plants. Stachyose is less sweet than sucrose, at about 28% on a weight basis. It is mainly used as a bulk sweetener. Stachyose is not completely digestible by humans and delivers 1.5–2.4 kcal/g or 6–10 kJ/g.
Fig. 2.23

Structure of tetrasaccharide—stachyose

Stachyose undergoing partial hydrolysis may yield different di- and trisaccharides like galactobiose (galactose + galactose), sucrose (glucose + fructose), melibiose (galactose + glucose) manninotriose (2galactoses + glucose) and raffinose (galactoses + glucose + fructose).

Examples of other tetrasaccharides include Lychnose (1-α-Galactosyl-raffinose = O-α-d-Galp-(1→6)-O-α-d-Glup-(1→2)-O-β-d-Fruf-(1→1)-O-α-d-Galp), Maltotetraose (O-α-d-Glcp-(1→4)-O-α-d-Glcp-(1→4)-O-α-d-Glcp-(1→4)-d-Glcp), Nigerotetraose (O-α-d-Glcp-(1→3)-O-α-d-Glcp-(1→3)-O-α-d-Glcp-(1→3)-d-Glcp), Nystose (β-d-Fructosyl-1-kestose, O-α-d-Glcp-(1→2)-β-d-Fruf-(1→2)-β-d-Fruf-(1→2)-β-d-Fruf), Sesamose (O-α-d-Galp-(1→6)-O-α-d-Galp-(1→6)-O-β-d-Fruf-(2→1)-O-α-d-Glcp), etc.

Pentasaccharides [C6H12O6]5

Verbascose (O-α-d-galactopyranosyl-(1→6)-[O-α-d-galactopyranosyl-(1→6)-]2-O-α-d-glucopyranosyl-(1→2)-β-d-fructofuranoside), a raffinose family oligosaccharides (RFO), is a non‐digestible pentasaccharide, galactose‐galactose‐galactose‐glucose‐fructose, isolated from roots of mullein Verbascum Thapsus of Scrophulariaceae found in legumes; fermented by intestinal bacteria and causes flatulence (Fig. 2.24).
Fig. 2.24

Structure of pentasaccharide verbascose—verbascose

  • (iii) Polysaccharides [C 6 H 10 O 5 ] 11 -n: Polysaccharides are polymers of monosaccharide units, linked to each other by glycosidic linkages. Many polysaccharides, unlike sugars, are tasteless and insoluble in water. Dietary fiber includes polysaccharides and oligosaccharides that are resistant to digestion and absorption in the human small intestine but which are completely or partially fermented by microorganisms in the large intestine. Polysaccharides are of two types, e.g., reserve (starch, glycogen, etc.) and structural (cellulose). Reserve polysaccharides provide energy and building material while structural polysaccharides supply dietary fiber. The polysaccharides described below play important roles in nutrition, food preparation, and biology. They are further divided into:

  1. (a)

    Homopolysaccharides: polysaccharides consist of only one type of monosaccharide (e.g., pentosans, polymer of pentose and includes arabans, xylans, etc., and glucosans, polymer of glucose: starch, straight chain amylose and branched chain-amylopectin starch are formed by α-d glucose and connected respectively by α-1-4, α-1-4 and α-1-6 linkages, dextrin-intermediate product of starch digestion formed by α-d glucose, cellulose—formed by β-d glucose, glycogen—formed by α-d glucose, highly branched, linked by α-1,4 and α-1,6 glycosidic bonds, chitin-constructed from units of N-acetylglucosamine linked together in β-1,4 fashion like cellulose; fructosan: levan, inulin, polymer of fructose known as fructosans, polymer of fructose—storage carbohydrate for energy source in many plant species and also important as dietary fibers commercially extracted from chicory root; galactosan, polymer of galactose: agar—structural carbohydrate of the cell wall of agarophytes algae, consisting of chains of repeating alternate units of β-1,3-linked-d-galactose and α-1,4-linked 3,6-anhydro-l-galactose (a linear polymer of galactose);

     
  2. (b)

    Heteropolysaccharides (heteroglycans): polysaccharides consist of two or more types of monosaccharides or their derivatives and are closely associated with lipid or protein. The major heteropolysaccharides include pectins, a structural heteropolysaccharide in the primary cell walls of terrestrial plants rich in galacturonic acid; hemicelluloses are polysaccharides in plant cell walls that have beta-(1–>4)-linked backbones with an equatorial configuration and include xyloglucans, xylans, mannans, and glucomannans, and beta-(1–>3,1–>4)-glucans, lignin, a phenylpropanoid-derived heteropolymer important for the strength and rigidity of the plant secondary cell wall a complex polymer of aromatic alcohols, the connective tissue polysaccharides, blood group substances, glycoproteins (gamma globulin), and glycolipids (found in the central nervous system of animals and in a wide variety of plant gums), mucopolysaccharides (glycosamino glycans GAG)—made of sugar amino sugars and uronic acids, hyaluronic acid formed by thousands of N-acetyl glucosamine and glucuronic acid), chondroitin, and sulfated heteropolysaccharides included chondroitin sulfate, dermatan sulfate, keratan sulfate, heparin, etc.

     

Starch

Starch is the major form of reserve polysaccharide carbohydrate in plants and composed of α-d-glucose units. Starch is divided two groups: amylose, a strait chain or linear polysaccharide linked by α-1→4 glycosidic bond, and amylopectin, a highly branched chain polysaccharide linked by α-1→4 and α-1→6 glycosidic bonds. Natural starches contain 10–20% amylose and 80–90% amylopectin. Amylose forms a colloidal dispersion in hot water whereas amylopectin is completely insoluble. Starch is abundantly present in the roots, rhizomes, and seeds of many plants. Corn starch or amylum is employed as a dusting powder for the skin, or for pills to prevent their sticking together, or in the form of starch water as a soothing injection in irritative conditions of the lower bowel. Cornstarch and arrowroot starch (obtained from the rhizomes or rootstock of Maranta arundinacea of Marantaceae) are used as foods.

Amylose molecules consist typically of 200–20,000 α-d-glucose units which form a helix as a result of the bond angles between the glucose units (Fig. 2.25).
Fig. 2.25

Structure of polysaccharide—amylose

Amylopectin is also made up of α-d-glucose but differs from amylose in being highly branched. Short side chains of about 30 glucose units are attached with α1→6 linkages approximately every twenty to thirty glucose units along the chain (Fig. 2.26). The side branching chains are clustered together within the amylopectin molecule. Amylopectin molecules may contain up to two million glucose units.
Fig. 2.26

Structure of polysaccharide—amylopectin

Starches are transformed into many commercial products by hydrolysis using acids or enzymes as catalysts. In hydrolysis, water is added to glycosidic bonds to break the long polysaccharide chains into smaller chains of simple carbohydrates. The resulting products are assigned a dextrose equivalent (DE) value which is related to the degree of hydrolysis. A DE value of 100 corresponds to completely hydrolyzed starch, which is pure glucose (dextrose). Derived from dextrose (glucose), dextrins are a group of low-molecular-weight carbohydrates produced by the hydrolysis of starch. Dextrins are polymers of α-d-glucose units linked by α-1→4 glycosidic bonds. Dextrins are usually made from corn, potato, arrowroot, rice, wheat, or barley. There are several types of dextrin, e.g., white, yellow, or brown powders, partially or fully soluble in water yielding optically active solutions of low viscosity. Dextrin is used in many glue products due to its adhesive qualities and safety. The indigestible form of dextrin is often used as a fiber supplement. Dextrin fiber offers a lot of health benefits including weight loss, toxin cleansing, etc. Dextrin promotes healthy intestinal flora (probiotics), supports heart health, colon health, healthy cholesterol levels, and relieves constipation.

Maltodextrin is partially hydrolyzed starch that is not sweet and has a DE value less than 20. Syrups, such as corn syrup made from corn starch, have DE values from 20 to 91. Commercial dextrose has DE values from 92 to 99. Corn syrup solids, which may be labeled as soluble corn fiber or resistant maltodextrin, are mildly sweet semicrystalline or powdery amorphous products with DEs from 20 to 36 made by drying corn syrup in a vacuum or in spray driers. Resistant maltodextrin or soluble corn fiber are not broken down in the digestive system, but they are partially fermented by colonic bacteria thus providing only 2 calories per gram instead of the 4 calories per gram in corn syrup. High fructose corn syrup (HFCS), commonly used to sweeten soft drinks, is made by treating corn syrup with enzymes to convert a portion of the glucose into fructose. Commercial HFCS contains from 42 to 55% fructose, with the remaining percentage being mainly glucose. There is an effort underway to rename HFCS as corn sugar because of the negative public perception that HFCS contributes to obesity. Modified starch is starch that has been changed by mechanical processes or chemical treatments to stabilize starch gels made with hot water. Without modification, gelled starch–water mixtures lose viscosity or become rubbery after a few hours. Hydrogenated glucose syrup (HGS) is produced by hydrolyzing starch, and then hydrogenating the resulting syrup to produce sugar alcohols like maltitol and sorbitol, along with hydrogenated oligo- and polysaccharides. Polydextrose (poly-d-glucose) is a synthetic, highly branched polymer with many types of glycosidic linkages created by heating dextrose with an acid catalyst and purifying the resulting water-soluble polymer. Polydextrose is used as a bulking agent because it is tasteless and is similar to fiber in terms of its resistance to digestion. The name resistant starch is applied to dietary starch that is not degraded in the stomach and small intestine, but is fermented by microflora in the large intestine. The relative sweetness of various starch hydrolysates and other carbohydrates in relation to sucrose (=100%) may be graded as follows: Fructose (173), invert sugar—a mixture of glucose and fructose found in fruits (120), HFCS (42% fructose) (120), sucrose (100), xylitol (100), tagatose (92), glucose (74), high-DE corn syrup (70), sorbitol (55), mannitol (50), trehalose (45), regular corn syrup (40), galactose (32), maltose (32), and lactose (15).

Glycogen

Glycogen, a multibranched polysaccharide of α-d-glucose units (up to 120,000 glucose residues) linked by α1→4 and also α1→6 glycosidic bonds, is a storage polysaccharide of animals, fungi, and cyanobacteria (Fig. 2.27). Glycogen is identical to amylopectin, but the branches in glycogen tend to be shorter (~13 glucose units), extensively branched and compact than starch. The glucose chains are organized globularly like branches of a tree originating from a pair of molecules of glycogenin, a protein that acts as a primer at the core of the structure. In humans, glycogen is stored primarily in the hepatocytes of the liver (~8% of its fresh weight) and the skeletal muscle cells (1–2% of its fresh weight) and functions as the secondary long-term energy storage, the primary energy stores being fats held in adipose tissue. The amount of glycogen stored in the body, especially within the muscles, liver, and red blood cells mostly depend on physical training and basal metabolic rate. Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain glial cells in the brain and white blood cells. The uterus also stores glycogen during pregnancy to nourish the embryo. Muscle glycogen is converted into glucose by muscle cells, and only liver glycogen converts to glucose which can be made accessible to other organs including the central nervous system. When glucose cannot be stored as glycogen or used immediately for energy, it is converted to fat.
Fig. 2.27

Structure of polysaccharide—glycogen structure showing α-1→4 and also α-1→6 glycosidic linkages

Dextran

Dextran is a complex branched polysaccharide, similar to amylopectin, consisting of a linear backbone (main chain) of α-1→6 glycosidic bond linked d-glucopyranosyl repeating units and the dextran may have branches of smaller chains of d-glucose linked to the backbone by α-1→2, α-1→3 or α-1→4 glycosidic bonds. It differs from dextrin, starch (amylose and amylopectin), etc., in main chain formation and chain branching. Dextrin is a hydrolysate polymer product of starch (amylose), composed of α-d-glucose units, linked by α-1→4 glycosidic bonds and chain length is less than starch (Fig. 2.28). Dextran was first discovered by Louis Pasteur as a microbial product in wine. Historically, dextrans had been long recognized as contaminants in sugar processing and other food production. Dextran is synthesized from sucrose by certain lactic acid bacteria, the best-known being Leuconostoc mesenteroides and S. mutans.
Fig. 2.28

Structure of polysaccharide—a Dextran (~20 glucose units). b Dextrin (n < 300–600 glucose units). c Starch (300–600 glucose units)

Dextran is an oral bacterial product that adheres to the teeth, creating a film called dental plaque (plaque rich in dextrans). Dextran is also formed by the lactic acid bacterium Lactobacillus brevis to create the crystals of tibicos, a water kefir fermented beverage which supposedly has some health benefits. It is also used commercially in confections, in lacquers, as food additives, and as plasma volume expanders. It is used medicinally as an antithrombotic (antiplatelet) to reduce blood viscosity, and as a volume expander in hypovolaemia. It is used in some eye drops as a lubricant and in certain intravenous fluids to solubilize other factors, e.g., iron (= iron dextran such as Cosmofer), or as a derivative in Monofer, intravenous solutions with dextran function both as volume expanders. Dextran is used in the osmotic stress technique for applying osmotic pressure to biological molecules. It is also used in some chromatography matrices (e.g., Sephadex). Dextran is used to make microcarriers for industrial cell culture. Although there are relatively few side effects, the side effects associated with dextran use can be very serious and these include anaphylaxis, volume overload, pulmonary edema, cerebral edema, or platelet dysfunction. An uncommon but significant complication of dextran osmotic effect is acute renal failure.

Inulin

Inulins are a group of naturally occurring storage polysaccharides present in many vegetable and fruit plants including wheat, onion, banana, garlic, asparagus, leeks, chicory, and Jerusalem artichokes. Most often, inulin is extracted from chicory. Inulins are polymers of fructose units, also called fructans, and typically have a terminal glucose (Fig. 2.29). The fructose units in inulins are joined by a β-2→1 glycosidic bond. In general, plant inulins contain between 20 and several thousand fructose units. Smaller compounds, consisting of 10 or fewer fructose units, are called fructooligosaccharides, and among them, the simplest being 1-kestose, which has 2 fructose units and 1 glucose unit (a trisaccharide found in vegetables consisting of β-d-fructofuranose having β-d-fructofuranosyl and α-d-glucopyranosyl residues attached at the 1- and 2-positions respectively). Oligofructose has the same structure as inulin, but the chains consist of 10 or fewer fructose units. Inulins with a terminal glucose are known as α-d-glucopyranosyl-[β-d-fructofuranosyl](n-1)-d-fructofuranosides, GpyFn. Inulins without glucose are β-d-fructopyranosyl-[d-fructofuranosyl](n-1)-d-fructofuranosides, FpyFn. Hydrolysis of inulins may yield fructooligosaccharides, which are oligomers with a degree of polymerization (DP) of ≤10.
Fig. 2.29

Structure of a Inulin (n = 11–∞). b Oligofructose (n = 4–10). c 1-Kestose (2 fructose + 1 glucose)

Because of the β-2→1 linkages, inulins and oligofructose are nondigestible by human intestinal enzymes, and form a class of dietary fibers fructans, contributing to its functional properties, e.g., reduced calorie value, dietary fiber and prebiotic effects. They are nondigestible by human intestinal enzymes, but they are totally fermented by colonic microflora. The short chain fatty acids (SCFA) and lactate produced by fermentation contribute 1.5 kcal per gram of inulin or oligofructose. Without color and odor, inulin has little impact on sensory characteristics of food products. Oligofructose has approximately 30–50% of the sweetness of table sugar. Inulin is less soluble than oligofructose. When thoroughly mixed with liquid, inulin forms a gel and a white creamy texture similar to fat that provides a fat-like mouth feel. It can also improve the stability of foams and emulsions. Inulin and oligofructose are used to replace sugar, fat, and flour and reduce the calories of foods like ice cream, dairy products, confections, and baked goods. In addition to being a versatile ingredient, inulin has many health benefits. Inulin increases calcium absorption and possibly magnesium absorption, while promoting the growth of beneficial intestinal bacteria. Chicory inulin is reported to increase absorption of calcium in girls with lower calcium absorption and in young men. Inulin and its analog sinistrin are used to help measure kidney function by determining the glomerular filtration rate (GFR). Inulin is postulated to benefit the immune system through the direct interaction between the inulin and its metabolites with the gut-associated lymphoid tissues and especially Peyer’s patches, though this link has not been established in humans. Inulin is reported to decrease amount of cholesterol and triglycerides, and hence benefits lipidemia and cardiovascular system. It is also used for rehydration and remineralization following important loss of water, like diarrhea and diaphoresis. Due to the body’s limited ability to process fructans, inulin has minimal increasing impact on blood sugar. It is considered suitable for diabetics and potentially helpful in managing blood sugar-related illnesses. The consumption of large quantities, however, can lead to gas and bloating due to overgrowth of intestinal methanogenic bacteria.

Most plants that synthesize and store inulin do not store other forms of carbohydrate such as starch. Some plants store carbohydrates in the form of inulin as an alternative, or in addition, to starch. Inulin is used by some plants as a means of storing energy and is typically found in roots or rhizomes. For these plants, inulin is used for reserving energy as well as regulating cold resistance. It is osmotically active for it is soluble in water. The plants can change the osmotic potential of cells by changing the DP of inulin molecules with hydrolysis process. Being able to change osmotic potential without changing the total amount of carbohydrate, plants can withstand cold and drought during winter periods. Nonhydrolyzed inulin can also be directly converted to ethanol in a simultaneous saccharification and fermentation process, which may have great potential for converting crops high in inulin into ethanol for fuel.

Cellulose

Cellulose is a polysaccharide consisting of a linear chain of several hundred to many thousands β-d-glucose units linked by β-1→4 glycoside bonds (Fig. 2.30). The absence of side chains allows cellulose molecules to lie close together and form rigid structures. Cellulose is an important structural component of the primary cell wall of green plants, many forms of algae and the oomycetes fungi. Some species of bacteria secrete it to form biofilms. The cellulose content of cotton fiber is 90%, that of wood is 40–50% and that of dried hemp is approximately 45%. Cellulose is the most abundant organic polymer on Earth.
Fig. 2.30

Structure of cellulose showing β-1→4 glycoside bond

Humans cannot digest cellulose due to the lack of the enzyme cellulase and it mainly acts as a dietary fiber. Cellulose provides a lot of volume or bulk in food but because it is indigestible to humans, it has no caloric value and for this reason, cellulose has become a popular bulking agent in diet foods. Consumers who eat foods with high cellulose content feel full physically and psychologically without having consumed many calories. With rising awareness about fiber intake, cellulose has become one of the most popular food additives. Adding cellulose to food allows an increase in bulk and fiber content without a major impact on flavor. Because cellulose binds and mixes easily with water, it is often added to increase the fiber content of drinks and other liquid items when the gritty texture of regular fiber supplements would be undesirable. The gelling action of cellulose when combined with water provides both thickening and stabilizing qualities in the food to which it is added. Cellulose gel acts similarly to an emulsion, suspending ingredients within a solution and preventing water from separating out. Cellulose is often added to sauces for both the thickening and emulsifying action. Cellulose gum or cellulose gel, which are hydrated forms of cellulose, are often used in sauces or other wet items like ice cream and frozen yogurt.

Animals like ruminants and termites can digest cellulose with the help of symbiotic microorganisms that live in their guts, e.g., anaerobic bacteria, protozoa, fungi, etc. in ruminants and Trichonympha (a genus of parabasalian protists) in termite species. Cellulose is mainly used to produce paperboard and paper. Smaller quantities are converted into a wide variety of derivative products such as cellophane and rayon. Conversion of cellulose from energy crops into biofuels such as cellulosic ethanol is under active investigation. Cellulose for industrial use is mainly obtained from wood pulp and cotton. Cellulose may be modified in the laboratory by treating it with nitric acid (HNO3) to replace all the hydroxyl groups with nitrate groups (–ONO2) to produce cellulose nitrate (nitrocellulose or guncotton) which is an explosive component of smokeless powder. Partially nitrated cellulose, known as pyroxylin, is used in the manufacture of collodion, plastics, lacquers, and nail polish.

Carboxymethyl cellulose (CMC)

Carboxymethyl cellulose (CMC) or cellulose gum is a chemical derivative of cellulose where some of the hydroxyl groups (–OH) are substituted with carboxymethyl groups (–CH2–COOH) (Fig. 2.31). The properties of cellulose gum depend on the degree of substitution and the length of the cellulose chains. Cellulose gum is nontoxic and becomes very viscous when combined with water. Cellulose gum is a versatile, cost-effective and easy-to-use thickening agent that has numerous industrial applications. It is found in a range of products, including tobacco, paper, and yogurt. Cellulose gum stabilizes proteins, adds texture and mouth feel, forms oil-resistant film, and retains moisture in industrial and processed food products. It is used as a thickener for foods and as an emulsion stabilizer in products like ice cream. Cellulose gum is also a constituent of many nonfood products, such as personal lubricants, toothpaste, laxatives, diet pills, water-based paints, detergents, textile sizing, and various paper coatings.
Fig. 2.31

Structure of carboxymethyl cellulose (CMC)

Hemicellulose

Hemicelluloses (also known as polyoses) are polysaccharides (heteropolymer matrix polysaccharide) in plant cell walls that have β-1→4-linked backbones with an equatorial configuration (Fig. 2.32). Hemicelluloses include heteromannans, xyloglucans, heteroxylans, xylans, mannans, glucomannans, and β-1→3, 1→4-glucans. These types of hemicelluloses are present in the cell walls of all terrestrial plants, except for β-1→3 and 1→4-glucans, which are restricted to Poales and a few other groups. The chemical structure of hemicelluloses consists of long chains of a variety of pentoses, hexoses, and their corresponding uronic acids.
Fig. 2.32

Most common molecular motif of hemicellulose

Hemicelluloses constitute roughly one-third of the wall biomass. Hemicelluloses consist of shorter chains of 500–3000 sugar units while cellulose consists of longer chains of 7000–15,000 glucose units; hemicellulose is a branched polymer but cellulose is unbranched; cellulose is crystalline, strong, and resistant to hydrolysis; hemicellulose has a random, amorphous structure with little strength, easily hydrolyzed by dilute acid or base as well as myriad hemicellulase enzymes. In contrast to cellulose, which contains only anhydrous glucose, hemicellulose contains many different sugar monomers, e.g., xylose, mannose, galactose, rhamnose, arabinose and glucose as well. Hemicelluloses contain most of the d-pentose sugars, and occasionally small amounts of l-sugars as well. The most common hemicelluloses contain xylans, uronic acid, and arabinose (Fig. 2.33). Xylose is the predominant sugar monomer in most cases, but in softwoods, mannose can be the most abundant monomer unit. Hemicellulose also contains the acidified form of sugars like glucuronic acid and galacturonic. The polysaccharides yielding pentoses on hydrolysis are called pentosans and xylan is an example of a pentosan consisting of d-xylose units with β-1→4 linkages.
Fig. 2.33

Structure of xylan

Hemicelluloses comprise almost one-third of the carbohydrates in woody plant tissue. The fine structure of these polysaccharides varies depending on the plant species and tissue type (Fig. 2.34). Hemicellulose found in hardwood trees is predominantly xylan with some glucomannan, while in softwoods, it is mainly rich in galactoglucomannan and contains only a small amount of xylan. Hemicelluloses may be found in fruit, plant stems, and grain hulls. The most important biological role of hemicelluloses is their contribution to strengthening the cell wall by interaction with cellulose and, in some walls, with lignin. The hemicelluloses are used in numerous industrial applications such as food additives as well as in medicinal applications. Although hemicelluloses are not digestible, they can be fermented by yeasts and bacteria.
Fig. 2.34

Structure of cellulose , hemicellulose and pectin

Arabinoxylan

Arabinoxylans are polysaccharides found in the bran of grasses and grains such as wheat, rye, and barley. Arabinoxylans consist of a xylan backbone with l-arabinofuranose (l-arabinose in its 5-atom ring form) attached randomly by α-1→2 and/or α-1→3 linkages to the xylose units throughout the chain (Fig. 2.35). Since xylose and arabinose are both pentoses, arabinoxylans are usually classified as pentosans. Arabinoxylans are important in the baking industry. The arabinose units bind water and produce viscous compounds that affect the consistency of dough, the retention of gas bubbles from fermentation in gluten-starch films, and the final texture of baked products.
Fig. 2.35

Structure of Arabinoxylan

Chitin

Chitin is an unbranched polymer of N-Acetyl-d-glucosamine (Fig. 2.36). It is found in fungi and is the principal component of arthropod and lower animal exoskeletons, e.g., insect, crab, and shrimp shells. It may be regarded as a derivative of cellulose, in which the hydroxyl groups of the second carbon of each glucose unit have been replaced with acetamido [–NH(C=O) CH3] groups.
Fig. 2.36

Structure of Chitin

β-Glucan

Beta-glucans consist of linear unbranched polysaccharides of β-d-glucose like cellulose, but with one β-1→3 linkage for every three or four β-1→4 linkages (Fig. 2.37). β-glucans form long cylindrical molecules containing up to about 250,000 glucose units. β-glucans occur in the bran of grains such as barley and oats, and they are recognized as being beneficial for reducing heart disease by lowering cholesterol and reducing the glycemic response. They are used commercially to modify food texture and as fat substitutes.
Fig. 2.37

Structure of β-glucan

Glycosaminoglycans

Glycosaminoglycans (GAGs) or mucopolysaccharides are long unbranched chain of negatively charged polysaccharides containing repeating disaccharide units that contain (except for keratan) either of two amino sugar compounds (N-acetylglucosamine or N-acetylgalactosamine) and a uronic sugar (glucuronic acid or iduronic acid) (Fig. 2.38). The physiologically most important GAGs are hyaluronic acid, dermatan sulfate, chondroitin sulfate, heparin, heparan sulfate, and keratan sulfate. Chondroitin sulfate is composed of β-d-glucuronate linked to the third carbon of N-acetylgalactosamine-4-sulfate as illustrated here. Heparin is a complex mixture of linear polysaccharides that have anticoagulant properties and vary in the degree of sulfation of the saccharide units.
Fig. 2.38

Glycosaminoglycans (GAGs) or mucopolysaccharides

GAGs are highly polar, highly viscous molecules, and attract water. They are therefore useful to the body as a lubricant or as a shock absorber. GAGs are found in the lubricating fluid of the joints and as components of cartilage, connective tissues, synovial fluid, vitreous humor, bone, heart valves, skin, and cell membranes. These substances have anticoagulant, anti-lipemic and antithrombogenic properties in addition to facilitating wound healing. They help with the transport of oxygen around the body and play a vital role in all cell growth. Mucopolysaccharides often appear as a thickening agent in shampoos and conditioners because they improve circulation of nutrients to the hair and accelerate the removal of naturally produced waste products. Mucopolysaccharides are marketed as nutritional supplement for conditions that affect human joints and connective tissue such as arthritis and osteoporosis. Mucopolysaccharides are also a natural humectant so hold moisture in the hair thus improving condition. GAGs are available different animal and plant sources including beef cartilage (beef trachea), bone marrow, neck meat, and broths that contain boiled bone and connective part, green-lipped mussels, all crustaceans (lobster, shrimp, crab, shell, etc.), shell, and seaweed. Aloe vera is also a plant source for GAGs,

Agar and carrageenan

Agar (agar agar) is a linear polymer chain (galactosan) of agarobiose (Figs. 2.39 and 2.40), a disaccharide consisting of β-(1→3)-d-galactose and α-(1→4)-L-galactose. Most of the α-(1→4) residues are modified by the presence of a 3→6 anhydro bridge.
Fig. 2.39

Agarobiose, a disaccharide unit in agar consisting of β-(1→3)-d and α-(1→4)-l galactose residues

Fig. 2.40

A fragment of agar molecule showing 4 repeating monosaccharide units [α-(1→4)-lgalactose-β-(1→3)-d-galactose-α-(1→4)-l galactose-β-(1→3)-d-galactose-]

Agar is extracted from the cell wall of marine algae agarophytes (Gelidium and Gracilaria) that belong to the Rhodophyta. Agar is approximately 80% fiber, so it serves as an exceptional intestinal regulator as a laxative, an appetite suppressant, vegetarian gelatin substitute, as a thickener for soups, jellies and ice cream, as a binder for puddings, custards and other desserts, in fruit preserves, as a clarifying agent in brewing, and as a filler in paper sizing fabrics. Highly refined agar is used in microbiology as a medium for culturing bacteria, fungi, etc., used to measure microorganism motility and mobility; in biotechnology for cellular tissue culture, micropropagule development, and for DNA fingerprinting. Agar is used as an ingredient in desserts in Japan and other Asian countries. The gels produced with agar have a crispier texture than the desserts made with animal gelatin.

Carrageenans or carrageenins are a family of high-molecular-weight linear sulfated polysaccharides that are extracted from red edible seaweeds (e.g., Chondrus crispus, Eucheuma denticulatum, Kappaphycus alvarezii). Carrageenan compounds differ from agar in that they have sulfate groups (–OSO3) in place of some hydroxyl groups. There are three main classes of carrageenan, kappa, iota, and lambda, each of which has different degrees of sulfation (1, 2 and 3 sulfate group per disaccharide, respectively) and gel strengths. All carrageenans are polysaccharides made up of repeating galactose units and 3→6 anhydrogalactose (3→6-AG), both sulfated and nonsulfated. The units are joined by alternating α-1→3 and β-1→4 glycosidic linkages (Fig. 2.41).
Fig. 2.41

Three main classes of carrageenan —kappa, iota and lambda

Carrageenans are widely used in the food industry, for their gelling, thickening, suspending, binding and stabilizing properties. It gives foods a smooth texture and accentuates flavor. It is used in dairy-based foods, like ice cream, yogurt, and cottage cheese, because it reacts well with milk proteins. Carrageenan is also found in jelly, pie filling, chocolate, salad dressing, and even as a fat substitute in processed meat. Because it comes from algae, it can be used as a substitute for gelatin for vegetarian and vegan products. However, degraded carrageenan could cause gastrointestinal problems and only the undegraded variety has been deemed safe for human by the Food and Drug Administration (FDA), USA. The use of carrageenan in infant formula, organic or otherwise, is prohibited in the EU, but is permitted in other foodstuffs. Nonfood items like toothpaste, personal lubricants, and air freshener gels may also include carrageenan to thicken and stabilize the product, and make it smoother. Some types of firefighting foam also use carrageenan, which thickens the foam and helps it become sticky and more effective. In chemistry, gels made with it can be used to carry microbes or immobilize cells.

Alginic acid

Alginic acid (algin or alginate) is an anionic polysaccharide distributed widely in the cell walls of brown algae such as giant kelp (Laminaria spp., Macrocystis pyrifera of Phaeophyceae). It is sold in filamentous, granular, or powdered forms. Alginic acid is a linear copolymer with homopolymeric blocks of (1→4)-linked β-d-mannuronate (M) and its C-5 epimer α-l-guluronate(G) residues (i.e., linear polymer of mannuronic and glucuronic acids), respectively, covalently linked together in different sequences or blocks (Fig. 2.42). The monomers can appear in homopolymeric blocks of consecutive G-residues (G-blocks), consecutive M-residues (M-blocks) or alternating M, and G-residues (MG-blocks). Alginates are insoluble in water, but absorb water readily, capable of absorbing 200–300 times its own weight in water. It is widely used in processed foods and in medicinal, industrial and household products, including swabs, filters, and fire retardants. It is useful as gelling and thickening agents. Alginates are used as foam, clotting agents, and gauze in absorbable surgical dressings and packing. Alginate dressings are derived from seaweed made of soft nonwoven fibers, and are available as pads, ropes or ribbons. Alginate dressings are extremely lightweight, absorb many times their own weight, form a gel-like covering over the wound, and maintain a moist environment. They are best used for wounds with significant exudate, especially useful for packing exudative wounds; do not physically inhibit wound contraction as does gauze and are highly absorbent. Alginates are used in the manufacture of textiles, paper, and cosmetics. The sodium salt of alginic acid, sodium alginate, is used in the food industry to increase viscosity and as an emulsifier. Alginates are found in food products such as ice cream and in slimming aids where they serve as appetite suppressants. In dentistry, alginates are used to make dental impressions.
Fig. 2.42

Chemical structures and components of the alginic acid

Galactomannan

Galactomannans are reserve polysaccharides consisting mainly of the monosaccharides mannose and galactose units (Fig. 2.43). The mannose elements form a linear chain a mannose backbone consisting of β-1→4 linked-d-mannopyranosyl residues with α-1→6 linked d-galactopyranosyl residues as side chain.
Fig. 2.43

A generalized structure of locust bean gum galactomannan-β-1→4-linked-d-mannopyranose backbone with a branch of α-1→6-linked d-galactopyranose

Galactomannans are present in several vegetable gums that are used to increase the viscosity of food products. Several galactomannans are known from natural sources and they may be arranged into four groups according to their molecular mannose to galactose ratio, e.g., fenugreek gum (~1:1), guar gum (~2:1), tara gum (~3:1), locust bean gum (LBG) or carob gum (~4:1) etc. These galactomannans are obtained from four main plant sources of Fabaceae, e.g., fenugreek from the seeds of Trigonella foenum-graecum, guar gum from the seeds of Cyamopsis tetragonoloba , Tara gum from seeds of Cesalpinia spinosa and LBG from the seeds of Ceratonia siliqua (Carob tree). Extraction of galactomannans involves de-hulling of seeds, crushing to remove the embryo, followed by milling of the endosperm to produce crude flour. The flour can be purified by dissolving in hot water followed by filtration and precipitation with isopropanol to remove impurities.

Galactomannans, common food fibers, are often used in food products to increase the viscosity of the water phase and also used in foods as stabilizers. Guar and LBG are commonly used in ice cream to improve texture and reduce ice cream meltdown. LBG is also used extensively in cream cheese, fruit preparations, and salad dressings. The use of tara gum as a food ingredient growing but is still to a much lesser extent than guar or LBG. Medical potential of mannans is as a drug nanocarrier system.

The properties of galactomannans from different source species vary depending on the ratios of mannose to galactose, number and distance of the side chains on mannose backbone chain.

Fenugreek (seeds of T. foenum-graecum of Fabaceae) is a good source of soluble fiber, it prevents the post prandial glucose surge, lowers the Glycemic Index (GI) level of the food product, eases heartburn, minimizes the risk of heart disease, suppresses appetite, helps in weight and cholesterol management, prevent constipation, smoothes menstrual pain, and increases libido, etc. In addition, it is an ideal gum to be used as thickener or to increase viscosity in gravies, sauces, spread, and beverages.
  • Guar gum , mannose:galactose 2:1 (Fig. 2.45)
    Fig. 2.45

    Guaran, the main component in guar gum ; Guar gum, mannose: galactose 2:1

Guar gum, or guaran is a galactomannan. Guaran is the main component in guar gum. It is primarily the ground endosperm of guar or cluster beans ( C. tetragonoloba of Fabaceae). The guar seeds are dehusked, milled, and screened to obtain the guar gum. Guar gum is water soluble and exhibits a viscosifying effect in water, almost eight times the water-thickening potency of corn starch. Guar gum can be used as an emulsifier or as a stabilizer in various multiphase formulations. Guar gum retards ice crystal growth nonspecifically by slowing mass transfer across the solid/liquid interface. It shows good stability during freeze–thaw cycles. Guar has been used as an appetite depressant. Its thickening ability is utilized in various lotions and creams. Coarse Guar gum is often used as a binding and disintegrating ingredient in compressed tablets.
Tara gum is produced by separating and grinding the seed endosperm of C. spinosa of Fabaceae. Tara gum consists of a linear main chain of β-1→4-d-mannopyranose units attached by α-1→6 linkages with d-galactopyranose units. The ratio of mannose to galactose in Tara gum is 3:1. Tara gum is safe for human consumption as a food additive in the form of thickening agent and stabilizer. Generally, Tara gum presents a high viscosity, an intermediate acid stability, and resists the depolymerization effect of organic acids down to a pH of 3.5, stable to high temperature heat treatment (up to 145 °C).
Medicinal uses include gargling infusions of the pods for inflamed tonsils or washing wounds in and also used to treat fevers, colds, and stomach aches. The tree can also be a source of lumber and firewood, and as a live fence. Water from boiled dried pods is used as insecticides. The seeds can be used to produce black dye while dark blue dye can be obtained from the roots.
  • Locust bean gum or carob gum , mannose:galactose 4:1 (Fig. 2.47)
    Fig. 2.47

    Locust bean gum , mannose to galactose ratio = 4:1

LBG is extracted from the seeds of the carob tree ( C. siliqua L.), mostly found in the Mediterranean region.

Guar is a legume that has been traditionally cultivated as livestock feed. Guar gum is also known by the name  C. tetragonoloba  which is the Latin taxonomy for the guar bean or cluster bean. Guar gum is the ground endosperm of the seeds. Approximately 85% of guar gum is guaran, a water-soluble polysaccharide consisting of linear chains of mannose with β-1→4 linkages to which galactose units are attached with α-1→6 linkages. The ratio of mannose to galactose is 2:1. Guar gum has five to eight times the thickening power of starch and has many uses in the pharmaceutical industry, as a food stabilizer, and as a source of dietary fiber.

Pectin

Pectin is a polymer of 300–1000 α-d-galacturonic acid units with a variable number of methyl ester groups and joined by α-1→4 glycoside linkages (Fig. 2.48). The degree of esterification (DE) affects the gelling properties of pectin. The structure shown below has three methyl ester forms (–COOCH3) for every two carboxyl groups (–COOH), hence it is has a 60% DE, normally called a DE-60 pectin.
Fig. 2.48

A pectinmolecule with 3/5th degree (60%) of esterification

Pectin acts as a cementing material in the cell wall and middle lamella of all plant tissues. Pectin is present in all plants but the content and composition vary depending on the species, variety, maturity of the plant, plant part, tissue, and growing condition. Pectin is higher in legumes and citrus fruits than cereals. Other sources of pectin include banana, beets, cabbage, and carrots. Fruits that are high in pectin include apples, crab apples, blackberries, guava, gooseberries, cranberries, grapes, medlars, plums, and quince. Pectin content is low in fruits like apricots, blueberries, cherries, peaches, pears, raspberries, rhubarb, and strawberries. Any citrus fruit peel is also very high in pectin. The white portion of the rind of lemons and oranges contains approximately 30% pectin. Generally, unripe fruit will have more pectin than ripe fruit. Pectin is an important ingredient in industrial yogurt, cakes, ketchup, fruit jellies, jams, and fruit preserves.

Pectin is a soluble dietary fiber and it has several health benefits. It lowers serum cholesterol (LDL), blood glucose levels, heart disease and gallstones, improves insulin resistance, constipation, and relieves diarrhea. Pectin acts as detoxicant, regulator, and protectant of the gastrointestinal tract, immune system stimulant, antiulcer, and antinephrotic agent. Several studies have reported significant decrease in serum LDL and increase or no change in HDL cholesterols in people by pectin rich dietary supplement like fruits and vegetables (e.g., apples, carrots etc.). Since pectin is fermented only in the colon and releases fatty acids that lowers the risks of colon cancer and also for that pectin is one of the strong candidates for coating colon-specific oral drug delivery systems.

Xanthan gum

Xanthan gum, secreted by the bacterium Xanthomonas campestris, is a polysaccharide with a β-d-glucose backbone like cellulose, but every second glucose unit is attached to a trisaccharide consisting of mannose, glucuronic acid, and mannose and each of the repeat units is composed of glucose, mannose, and glucuronic acid in the molar ratio 2:2:1 (Fig. 2.49). The mannose closest to the backbone has an acetic acid ester on carbon 6, and the mannose at the end of the trisaccharide is linked through carbons 6 and 4 to the second carbon of pyruvic acid.
Fig. 2.49

The repeating unit of xanthan gum (2-β-d-glucose units backbone + 1 mannose + 1 glucuronic acid + 1 mannose

Xanthan gum produced by the bacterium X. campestris as slimy substance on cruciferous vegetables such as broccoli, cabbage and cauliflower causing black rot. Unlike other gums, it is very stable under a wide range of temperatures and pH. The negatively charged carboxyl groups on the side chains because the molecules to form very viscous fluids when mixed with water. Xanthan gum is commonly used as a food additive, thickener (rheology modifier) for sauces, in salad dressing, to prevent ice crystal formation in ice cream, and as a low-calorie substitute for fat, stabilizer in cosmetic products and as a binder in toothpaste. Xanthan gum is frequently mixed with guar gum because the viscosity of the combination is greater than when either one is used alone. Xanthan gum is a highly efficient laxative but may act as common allergen to sensitive people.

Glucomannan

Glucomannan is mainly a straight chain polysaccharide with a small amount of branching consists of d-glucose (G) and d-mannose (M) in a proportion of 5:8 joined by β-1→4 glycoside linkages (Fig. 2.50). The degree of branching is about 8% through β-1→6-glucosyl linkages. The basic polymeric repeating unit has the GGMMGMMMMMGGM pattern. Short side chains of 11–16 monosaccharides occur at intervals of 50–60 units of the main chain attached by β-1→3 linkages as well as acetate groups on carbon 6 occur at every 9–19 units of the main chain. Hydrolysis of the acetate groups favors the formation of intermolecular hydrogen bonds that are responsible for the gelling action.
Fig. 2.50

A portion (GGMM) of the glucomannan repeating unit, an acetate group on C6 of 2nd glucose

Glucomannan is a dietary fiber obtained from tubers or corm (40%) of Amorphophallus konjac of Araceae cultivated in Asia. Another culinary source is salep, ground from the roots of certain orchids and used in Turkish cuisine. Glucomannan (a hemicellulose) is present in large amounts in the wood of conifers and in smaller amounts in the wood of dicotyledons. Glucomannan is also a constituent of bacterial, plant, and yeast membrane with differences in the branches or glycosidic linkages in the linear structure. Glucomannan, a food additive, is used as an emulsifier and thickener. It has some clinically supported health benefits and different products containing glucomannan are sold as nutritional supplements for constipation, obesity, high cholesterol, acne vulgaris, and type 2 diabetes. The U.S. FDA did not but the Canadian authority approved some of the products containing glucomannan for the purposes of appetite reduction, weight management, and treatment of constipation and management of high cholesterol levels. Flour from the konjac tubers is used to make Japanese shirataki noodles, also called konnyaku noodles, which are very low in calories. Glucomannan is used as a hunger suppressant because it produces a feeling of fullness by creating very viscous solutions that retard absorption of the nutrients in food. One gram of this soluble polysaccharide can absorb up to 200 ml of water, so it is also used for absorbent articles such as disposable diapers and sanitary napkins.

2.3.1.3 Lipids (C, H, O, N, S & P)

Lipids are a heterogeneous group of compounds including esters of fatty acids and glycerol or alcohols and their derivatives, and the fat-like substances that are soluble in nonpolar organic solvents. These heterogeneous groups of substances are of plant and animal origin and share a common character of solubility in organic solvents like alcohol, ether, chloroform, benzene etc., but not in water. Lipids are one of the four major classes of biologically essential organic molecules (e.g., carbohydrates, proteins and nucleic acids) found in living organisms; their amounts and quality in diet are able to influence cell, tissue and body physiology. Lipids are macromolecules, but not polymers, with a molecular weight 100–5000 D, vary considerably in polarity, including hydrophobic (triglycerides, sterol esters) and hydrophilic molecules (phospholipids). The little or absent water solubility of many of them means that they are subject to special treatments at all stages of their utilization during digestion, absorption, transport, storage, and use. Lipids are the most energy-rich and important sources of metabolic energy (9.50 kcal/g) compared to protein (5.60 kcal/g) and carbohydrate (4.10 kcal/g).

Classification

Generally, lipids may be classified as (a) True lipids—esters of fatty acids and glycerol, (b) Lipoids or lipid-like substances and (c) Pseudolipids-substances soluble in lipid solvents. The esters of fatty acids and glycerol (triglycerides) and fatty acids and long chain monobasic alcohols (waxes) are simple or true lipids; true lipids molecules in association with phosphoric acid, carbohydrate, protein, etc. constitute complex lipids (e.g., phospholipid, glycolipid, lipoprotein, etc.). Lipid degradation products (e.g., fatty acids, glycerol, alcohol, fatty aldehydes and ketone bodies, etc.), organic solvent soluble substance (pigments), lipid soluble vitamins (A, D, E and K), and steroids (e.g., hormones, cholesterol, etc.) constitute derived lipids. Aliphatic hydrocarbon and terpenes and similar other fat-soluble substances are grouped as miscellaneous or pseudolipids. Lipids are also classified as glycerol-based and non-glycerol-based lipids. Glycerol-based lipids include simple (e.g., fats, oils, etc.) and complex lipids (e.g., phospholipids, sulfolipids, glycolipids, lipoprotein, etc.) while non-glycerol-based lipids include waxes, cerebrosides, steroids, terpenes, sphingomyeline, etc. (Fig. 2.51).
Fig. 2.51

Broad classes of lipids

On the basis of structural features, lipids may be grouped as single component lipids or lipid monomers including higher hydrocarbon, fatty acids, aliphatic alcohols, amino alcohols, aldehydes, ketones, isoprene compounds, polyols etc. and multicomponent lipids including simple or true lipids (e.g., oils, fats, waxes, etc.) and heterolipids (e.g., phospholipids, glycolipids,lipoprotein, sphingophospholipids, etc.) (Table 2.4).
Table 2.4

Classification of lipids with properties and examples

Classes of lipids

Class

Properties and examples

I. One component lipids or lipid monomers

1.1 Aliphatic hydrocarbons, the simplest form of lipids, e.g., alkanes, alkenes, cyclic hydrocarbons, etc.

1.2 Aliphatic alcohols, aldehydes and ketones, e.g., long chain monohydric/dihydric alcohols of beeswax, insect sex hormone pheromones

1.3 Isoprenoids and their derivatives (terpenes—pigments, sterols—cholesterols, steroid hormones—vitamins—glycosides—alkaloids)

1.4 Fat-soluble vitamins (A, D, E and K)

1.5 Amino alcohols (sphingosines)

1.6 Polyols

1.7 Fatty acids (saturated and unsaturated fatty acids), aldehydes and ketone bodies

II. Multicomponent lipids

1. Simple lipids (esters composed of lipid monomers—fatty acids with glycerol), fats and oils are true lipids, fats contain long chain saturated fatty acids, solid at room temperature (lard, tallow) while oils contain relatively short chain unsaturated fatty acids, liquid at room temperature (olive oil, soybean oil)

1.1 Waxes (esters of fatty acids with long chain mono or dihydric alcohols)

1.2 Simple diol lipids or acyl diols (esters of dibasic alcohols)

1.3 Glycerides or acyl glycerides (esters of fatty acids with tribasic alcohols glycerol)

1.4 Sterids, esters of sterols with fatty acids, cholesterol esters; stigmasterol, ergosterol, and β-sitosterol are plant sterols

2. Complex lipids or heterolipids

2.1 Phospholipids (triesters of glycerol that contain charged phosphate diester groups)

2.1.1 Phosphoglycerides (phosphoesters of glycerides)

2.1.2 Diol phosphatides (phosphoesters of diol lipids)

2.1.3 Sphingophosphatides or sphingophospholipids (phospholipids of N-acyl-sphingosine)

2.2 Glycolipids (esters of fatty acids, glycerol and carbohydrates, a macromolecular complex)

Lipoprotein (esters of fatty acids, glycerol and proteins; a macromolecular complex)

Hydrocarbons form the simplest form of lipids and include alkanes, alkenes, cyclic hydrocarbons, and others. Several hydrocarbons may be substituted with oxygen-containing groups. These molecules are found mainly in petroleum but living organisms, eukaryotic or prokaryotic, and contain frequently hydrocarbons which are directly derived from fatty acids. They are distinct from the terpenoid hydrocarbons. They have usually a straight chain of up to about 36 carbon atoms but may also be branched, with one or more methyl groups attached at almost any point of the chain. Hydrocarbons are found at the outer surface in higher plant leaves. Resveratrol (3,4,5-trihydroxystilbene), a bicyclic hydrocarbon, is the most studied because of its presence in grapes and wine and some berries (blueberries, Vaccinium) and its numerous pharmacological properties (anticancer, antiviral, neuroprotective, antiaging, and anti-inflammatory).

Higher aliphatic alcohols, aldehydes, and ketones compounds occur in free state and also as structural constituents of multicomponent lipids (Fig. 2.52). They are oxygen derivatives of hydrocarbons. Higher aliphatic alcohols are compositional constituents of beeswax. Higher ketones occur in free form in bacteria while branched unsaturated ketones are structural components of insect sex hormone pheromones.
Fig. 2.52

Structures of higher aliphatic alcohols, aldehydes and ketones compounds, and insect pheromones

Isoprenoids and their derivatives constitute a vast group of biologically important lipids consisting of terpenes and steroids (Fig. 2.53). Cholesterol is synthesized from the triterpene squalene and lanosterol precursors. Carotenoids, structurally similar to tetraterpenes, are important and wide spread plant pigments, include phytoene, lycopene, and α-, β-, γ-carotenes, of which β-carotene is the provitamin A. Isoprenoid alcohols include farnesol (sesquiterpene), geraniol, nerol, linalool (monoterpenes), etc. are used in the fabrication of perfumes. Menthol (monoterpene) is widely used in pharmaceutical and confectionary industries. Phytol (in chlorophyll) and retinol (in phylloquinone—vitamin K1) are two diterpene alcohols. Camphor (monoterpene ketone) is used in drug; abscisic acid (monocyclic sesquiterpene derivative) is a phytohormone. Steroids (triterpene cyclic derivative) whose skeletal framework is that of gonane and steroids with an alcoholic group is called sterols, e.g., cholesterol, which is an important constituent of plasma membrane. Bile alcohols, bile acids (cholic acid, chenodeoxycholic acid), hormones (pregnenolone, progesterone), vitamins (vitamin D-calciferol), steroid glycosides (cardiac glycoside), steroid alkaloids (nitrogen-containing alkaloids of nightshades, lilies, periwinkles, Phyllobates frog), etc., are important derivatives of cholesterol.
Fig. 2.53

Biologically important lipids consisting of terpenes and steroids

Vitamins that are soluble in fat are considered as lipids. This group of lipids include vitamin A (retinol), D (calciferols), E (α,β,γ-tocoferols), K (naphthoquinones) and vitaminoids ubiquinone (CoQ, CoQ.H2) and F (oleic, linoleic, linolenic acids) (Fig. 2.54). Toxic effects of vitamin A include drowsiness, headache, vomiting, peeling of the skin, sun sensitivity, that of vitamin D include hypercalcemia, E-supra oxidant effect, etc.
Fig. 2.54

Lipids—fat-soluble vitamins

All fat-soluble vitamins are isoprenoid compounds and they are found in vegetable oils, colored vegetables, fruits, grains, nuts, seeds, meat, liver and fish products, seafood, dairy products, etc. Fat-soluble vitamins can be stored in liver and in adipose tissue. They are not readily excreted in urine and excess consumption may cause excess accumulation leading to hypervitaminosis or vitamin intoxication. Vitamin A is important for vision, especially night vision, bone growth, and mucous membranes. As an antioxidant, it may reduce the risk of some forms of cancer. Vitamin D aids in the absorption of calcium and phosphorous. Teeth, bones, and cartilage require it. Vitamin E is also an antioxidant and helps to generate red blood cells and prevents blood from clotting. Vitamin K also works with the blood, aiding in the normal clotting process and bone maintenance.

Sphingosine and dihydrosphingosine, major membrane components, are examples of two unsaturated and saturated higher (18C) amino alcohols, respectively. They form part of multicomponent sphingolipids, a class of cell membrane lipids (Fig. 2.55). Sphingosine on phosphorylation leads to the formation of sphingosine-1-phosphate, a potent signaling molecule involved in diverse cellular processes.
Fig. 2.55

Structure of a class of cell membrane lipids

A polyol is an alcohol containing multiple hydroxyl groups. Polyols are neither sugars nor alcohols, they are low-calorie carbohydrates (Fig. 2.56). Polyols like erythritol, HSH, isomalt, lactitol, maltitol, mannitol, sorbitol, and xylitol replace sugar in sugar-free foods and medicines. The sugar alcohols differ in chain length, most have five- or six-carbon chains as they are derived from pentoses and hexoses (six-carbon sugars), respectively. Some others have higher number of carbon atoms, e.g., volemitol (7C), isomalt (12C), maltitol (12C), lactitol (12C), maltotriitol (18C), maltotetraitol (24C), and polyglycitol (nC). They have one OH group attached to each carbon. Higher numbers of molecules create more viscous solutes, depending on the type of polyol. Higher polyols constitute a relatively small group of lipid monomers. They occur in microorganisms, plants and in animal tissues in the form of diol lipids.
Fig. 2.56

Structure of different polyols

Fatty acids are derivatives of aliphatic hydrocarbons with a carboxyl group. Over 200 fatty acids are known and they are the chief hydrophobic components of simple and complex lipids. They may be saturated or unsaturated and they also differ among themselves in chain length, number and position of double bonds, and also in the nature of substituents, e.g., oxy-, keto-, epoxy groups, cyclic structures, etc. Fatty acids with odd number carbon are less frequent than that of even number, and palmitic (16C) and stearic (18C) acids are more frequent than others. Short chain fatty acids like butyric and capronic acids do not belong to lipids because they are water soluble. Some examples of fatty acids are shown in Table 2.5.
Table 2.5

Some examples of saturated and unsaturated fatty acids

Type

Name of the acid

Source

Carbon atoms

Double bonds

Molecular formula

Melting point (°C)

Saturated

Lauric

Coconut oil

12

0

CH3(CH2)10COOH

44

Myristic

Butter fats

14

0

CH3(CH2)12COOH

58

Palmitic

Most fats and oils

16

0

CH3(CH2)14COOH

63

Stearic

Most fats and oils

18

0

CH3(CH2)14COOH

70

Unsaturated

Oleic

Olive oil

18

1

CH3(CH2)7CH=CH(CH2)7COOH(cis)

4

Linoleic

Vegetable fats

18

2

CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH(cis)

−5

Linolenic

Soybean and canola oils

18

3

CH3CH2CH=CHCH2CH=CH CH2CH=CH(CH2)7COOH(all cis)

−11

Arachidonic

Lard

20

4

CH3(CH2)4(CH=CHCH2)4 CH2CH2COOH(all cis)

−50

Multicomponent lipids contain more than one component or lipid monomer in the molecule. Molecules of simple and complex or hetero lipids are multicomponent lipids. They are eaters of fatty acids alcohols of different chain length, number of –OH group, etc. Fatty acid chains differ by length, e.g., SCFA with <6 carbons (i.e., butyric acid), medium chain fatty acids (MCFA) with 6–12 carbons, long chain fatty acids with 13–21 carbons (tallow or lard whose chains are 17 carbons long) and very long chain fatty acids with >22 carbons (hexacosanoic acid, a 26-carbon long chain saturated fatty acid).

Fatty acids

A fatty acid is a carboxylic acid with a saturated (without carbon–carbon double bonds) or unsaturated (having carbon–carbon double bonds) aliphatic tail, mostly of even number of carbon atoms ranging from 4 to 28 (Fig. 2.57). Fatty acids are usually derived from triglycerides or phospholipids. When they are not attached to other molecules, they are known as “free” fatty acids.
Fig. 2.57

The way of numbering of carbon atoms starting from carboxyl carbon onward and the number of double bond from other end

Fatty acids are important sources of cellular energy because they yield large quantities of ATP during their catabolism. Many cell types can use either glucose or fatty acids as for energy source (heart and skeletal muscle prefer fatty acids; brain cells also use fatty acids in addition to glucose and ketone bodies). Fatty acids with aliphatic tails of <6 carbon atoms are SCFA (e.g., butyric acid), with aliphatic tails of 6–12 carbon atoms are MCFA, with aliphatic tails of 13 to 21 carbon atoms are long chain fatty acids (LCFA), and with aliphatic tails > 22 carbon atoms are very long chain fatty acids (VLCFA).

Examples of some saturated and unsaturated fatty acids are shown in Table 2.6.
Table 2.6

Examples of saturated and unsaturated fatty acids

Saturated fatty acids

Chemical structure

C:D

Source

Butyric acid

CH3(CH2)2COOH

4:0

Butterfat

Caproic acid

CH3(CH2)4COOH

6:0

Butterfat

Caprylic acid, 8

CH3(CH2)6COOH

8:0

Coconut oil

Capric acid, 10

CH3(CH2)8COOH

10:0

Coconut oil

Lauric acid, 12

CH3(CH2)10COOH

12:0

Coconut oil

Myristic acid, 14

CH3(CH2)12COOH

14:0

Palm kernel oil

Palmitic acid, 16

CH3(CH2)14COOH

16:0

Palm oil

Stearic acid, 18

CH3(CH2)16COOH

18:0

Animal fats

Arachidic acid, 20

CH3(CH2)18COOH

20:0

Peanut oil, fish oil

Behenic acid,22

CH3(CH2)20COOH

22:0

Rapeseed oil

Lignoceric acid,24

CH3(CH2)22COOH

24:0

Small amounts in most fats

Cerotic acid

CH3(CH2)24COOH

26:0

beeswax

Unsaturated fatty acids

Chemical structure

C:D

Source

Myristoleic acid

CH3(CH2)3CH=CH(CH2)7COOH

14:1

Nutmeg butter, palm oil, coconut oil, butter fat

Palmitoleic acid

CH3(CH2)5CH=CH(CH2)7COOH

16:1

In animal oils, vegetable oils, and marine oils

Sapienic acid

CH3(CH2)8CH=CH(CH2)4COOH

16:1

sebaceous origin

Oleic acid

CH3(CH2)7CH=CH(CH2)7COOH

18:1

olive oil

Elaidic acid

CH3(CH2)7CH=CH(CH2)7COOH

18:1

in hydrogenated vegetable oils

Vaccenic acid

CH3(CH2)5CH=CH(CH2)9COOH

18:1

butterfat

Linoleic acid

CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH

18:2

grape seed oil

Linoelaidic acid

CH3(CH2)4CH=CHCH2CH=CH(CH2)7CO

18:2

In partially hydrogenated vegetable oils

α-Linolenic acid

CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH

18:3

linseed oil

Arachidonic

CH3(CH2)4CH=CHCH2CH=CHCH2CH=CHCH2

CH=CH(CH2)3COOH

20:4

liver fats

Eicosapentaenoic

CH3CH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CHCH2CH=CH(CH2)3COOH

20:5

fish oil

Erucic acid

CH3(CH2)7CH=CH(CH2)11COOH

22:1

rapeseed oil

Docosahexaenoic

CH3CH2CH=CHCH2CH=CHCH2CH=CHC H2CH=CHCH2CH=CHCH2CH=CH(CH2)2COOH

22:6

fish oil

Fatty acids that are required by the human body but cannot be made in sufficient quantity from other substrates, and therefore must be obtained from food, are called essential fatty acids. There are two series of essential fatty acids: one has double-bond three-carbon atoms removed from the methyl end; the other has double-bond six-carbon atoms removed from the methyl end. Humans lack the ability to introduce double bonds in fatty acids beyond carbons 9 and 10, as counted from the carboxylic acid side. Two essential fatty acids are linoleic acid (LA) and alpha-linolenic acid (ALA). They are widely distributed in plant oils. The human body has a limited ability to convert ALA into the longer chain n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which can also be obtained from fish. Fatty acid composition (approximate) of some common animal fats and vegetable oils is shown in Table 2.7.
Table 2.7

Fatty acid composition of some common fats and oils (approximate)

Source

Type of fatty acids in fats and oils, %

Saturated

Unsaturated

LauricC12

MyristicC14

PalmiticC16

StearicC18

OleicC18

LinoleicC18

Animal fat

Lard

1

25

15

50

6

Butter

2

10

25

10

25

5

Human fat

1

3

25

8

46

10

Whale blubber

8

12

3

35

10

Vegetable oil

Corn

1

8

4

46

42

Olive

1

5

5

83

6

Peanut

7

5

60

20

Soybean

7

4

34

53

Simple diol lipids are esters of fatty acids with dibasic alcohols (ethyleneglycol). Small amount of diol lipids is present in plant and animal tissues and they are functionally active in cell generation and maturation of plant seeds. Glycerides or acylglycerides, examples of simple neutral lipids, are esters of fatty acids with tribasic alcohol glycerol (Fig. 2.58). Glycerides may be mono-, di-, and tri-acyl glycerides depending on the number of acyl groups (RCO–) in the molecule. Triglycerides are edible fats derived from oil seeds. Fats made up of shorter chain of unsaturated fatty acids are usually liquid at room temperature, whereas the fats with longer chain saturated fatty acids will be solid. Oil refers to fats that are liquids at normal room temperature, while fats are solids at normal room temperature, e.g., animal fats tallow and lard are high in saturated fatty acid content and are solids while olive and linseed oils are unsaturated and liquid. Fat is important foodstuff and serving structural, energy resource, and metabolic functions. Many cell types can use either glucose or fatty acids for this energy, heart, skeletal muscle, and brain cells prefer fatty acids as a source of fuel. Some fatty acids are essential because they cannot be synthesized in the body from simpler constituents. Two essential fatty acids (unsaturated) in human nutrition include α-linolenic acid (an omega-3 fatty acid) and LA (an omega-6 fatty acid). Unsaturated fatty acids of cis form are more common in nature than trans form. Consumption of cis fats is hygienic while trans fats increase the risk of coronary heart disease. Unsaturated fats can be altered by hydrogenation to fully saturated solid fat in order to increase the desirable physical properties, e.g., a desirable melting temperature (30–40 °C), shelf life, etc. However, trans fats are generated during hydrogenation as contaminants. Waxes are esters of higher monobasic alcohol and higher fatty acids. Waxes are hydrophobic and form water-repellent protective layer on plant leaves and fruits, skin and hair of animals, feather of birds, and external skeleton of insects.
Fig. 2.58

a Triacylglycerol showing 3 ester bonds between –COOH of fatty acids (left) and –OH of glycerol (right). b Representative triglyceride found in a linseed oil, a triester (triglyceride) derived of linoleic acid (green, below, 18C), alpha-linolenic acid (red, left above, 18C), and oleic acid (blue, right above, 18C). Linseed oil is liquid at room temperature because all these fatty acids have some degrees of unsaturation

Pharmaceutically, fixed oils and fats are used as emollients and vehicles for other medicaments. Some of them, such as castor oil and cod liver oil, possess therapeutic properties. Natural sources of fixed oils and fats include castor beans, olive fruits, peanuts, coconut seed kernel, cod liver, cacao seeds, and sheep wool. Industrially they are used as lubricants and in the manufacture of soaps, paints, and varnishes.

A wax, an ester of a long chain alcohol (C16–C32) and a fatty acid (C16–C26), is a simple lipid (Fig. 2.59). Waxes are found in nature as coatings on leaves and tender stems of plant, on skin, hair of mammals, feather of birds, and exoskeleton of insects also in fish and marine organisms (invertebrate-whale). The nature of the lipid constituents (also the chain length and degree of unsaturation and branching of the aliphatic constituents) varies greatly with the source of the waxy material and includes hydrocarbons, sterol esters, aliphatic aldehydes, primary and secondary alcohols, diols, ketones, β-diketones, triacylglycerols, etc. (Fig. 2.60).
Fig. 2.59

Structure of wax esters consisting of long chain fatty acid (C15 upper) and alcohol (C15 lower)

Fig. 2.60

The nature of the lipid constituents of the waxy material

Waxes on surface tissues of plants limit the diffusion of water and solutes, permit the controlled release of volatiles to deter pests or attract pollinating insects, provide protection from disease and insects, and help the plants resist drought. Waxes appear to have a variety of functions in fish, from serving as an energy source to insulation, buoyancy, and even echo location. Spermaceti or sperm whale oil (wax esters, 76%; triacylglycerols, 23%) was once in great demand as a lubricant but now is proscribed. The main purpose of the waxes is presumed to be to give a waterproof layer to the feathers. Waxes (microbial waxes) are not common in prokaryotes, but the pathogenic mycobacteria (e.g., Mycobacterium tuberculosis  and M. leprae.) produce waxes—mycoserosates (virulence factors consisting of branched chain alcohols C34–C36 chain length esterified with long chain fatty acids of C18–C26 chain length having 2–4 methyl branches). Sebaceous glands appear to be the only source of wax esters in mammalian tissues and these waxes keep skin surface moist and possess powerful antibacterial properties coating and prevent water loss. Carnauba wax is found on the leaves of Brazilian palm trees (Copernicia cerifera) as thick coating, harvested from the dried leaves and is used in floor and automobile waxes. It contains mainly wax esters (85%), accompanied by small amounts of free acids and alcohols, hydrocarbons and resins. The wax esters constitute C16 to C20 fatty acids linked to C30 to C34 alcohols, giving C46 to C54 molecular species. Jojoba is seed wax ester of jojoba plant (Simmondsia chinensis)—a significant crop of the semiarid regions of Mexico and the U.S.A. It consists mainly of C18–C22 fatty acids linked to C20–C24 fatty alcohols. The sunflower seed waxes (10–12%) from oil refineries are long chain fatty esters (C38–C54) comprised of fatty alcohol (C18–C30) and fatty acid (C16–C30). Lanolin or wool wax is obtained from the wool of sheep during the cleaning or refining process and is rich in wax esters (of 1- and 2-alkanols, and 1, 2-diols), sterol esters, triterpene alcohols, and free acids and sterols (contains up to 50% wax esters and 33% sterol esters). A high proportion of the sterol component is lanosterol. The fatty acid components are mainly saturated and iso- and anteiso-methyl-branched chain. Bees secret beeswax to make cells for honey and eggs. The wax is recovered as a by-product when the honey is harvested and refined. It contains a high proportion of wax esters (35–80%). The wax esters consist of C40 to C46 molecular species, some diesters with up to 64 carbons may be present, together with triesters, hydroxy-polyesters and free acids. Spermaceti wax is found in the head cavities and blubber of the sperm whale. Many of the waxes are used in ointments, hand creams, and cosmetics. Paraffin wax, used in some candles, is not based upon the ester functional group, but is a mixture of high-molecular-weight alkanes. Ear wax is a mixture of phospholipids and esters of cholesterol. Bees wax and Carnauba wax are examples of waxes. Waxes are used as hardeners of ointment and cosmetic creams. Structure of some waxes from different sources is given in Fig. 2.61.
Fig. 2.61

Structure of different waxes . Beeswax (ceryl myristate), carnauba wax (myricyl cerotate) and spermaceti wax (cetyl palmitate)

Paraffin wax is a white or colorless soft solid derivable from petroleum, coal or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms (Fig. 2.62). In chemistry, paraffin is used synonymously with alkane, indicating hydrocarbons with the general formula C n H2n+2. Paraffins are unreactive in nature. Molecular composition of waxes from different sources is shown in Table 2.8.
Fig. 2.62

The hydrocarbon C31H64 (with the general formula C n H2n+2) is a typical component of paraffin wax

Table 2.8

Molecular composition of different waxes

Waxes

Type

Alcohol

Fatty acid

Beeswax (ceryl myristate)

CH3(CH2)28CH2–OH

CH3(CH2)14COOH

Carnauba (myricyl cerotate)

CH3(CH2)28CH2–OH

CH3(CH2)24COOH

Spermacetic (cetyl palmitate)

CH3(CH2)14CH2–OH

CH3(CH2)14COOH

Sterids are esters of sterols with fatty acids. Sterol esters constitute a heterogeneous group of chemical compounds (Fig. 2.63). The phytosterols (as opposed to zoosterols) include campesterol, β-sitosterol, stigmasterol, and Δ5-avenasterol.
Fig. 2.63

Structure of sterols and sterids —cholesterol, cholesterol ester, campesterol, β-sitosterol, stigmasterol, ∆5-avenasterol, phytostanol ester and campesterol ferulate

Certain distinctive phytosterol esters occur in the aleurone cells of cereal grains, including trans-hydroxycinnamate, ferulate (4-hydroxy-3-methoxy cinnamate) and p-coumarate esters. Similarly, rice bran oil is a rich source of esters of ferulic acid and a mixture of sterols and triterpenols, termed “γ-oryzanol”. Leaf and other tissues in plants contain a range of sterol glycosides and acyl sterol glycosides, in which the hydroxyl group at C3 on the sterol is linked to the sugar by a glycosidic bond. Typical examples (glucosides of β-sitosterol and acyl β-sitosterol) are illustrated below (Fig. 2.64).
Fig. 2.64

Glucosides of β-sitosterol and acyl β-sitosterol

Phytosterols are bioactive compounds naturally occurring in vegetable oils and in cereal grains like corn and rice. In addition to the free form, plant sterols occur as different types of conjugates in which the 3ß-OH group is (1) esterified to fatty acids, (2) hydroxycinnamic acids or (3) glycosylated with a hexose (Fig. 2.65). Phytosterols/stanols and their esters have received much attention because of their capacity to reduce cardiovascular risk of coronary heart disease. Several functional foods (e.g., margarine) enriched with plant steryl fatty acid esters are commercially available. Ferulic acid esters, e.g., g-oryzanol in rice, exhibit cholesterol-lowering and antioxidative properties. Therefore, they are used in Asia for therapeutic reasons predominantly.
Fig. 2.65

Plant sterols as different types of conjugates in which the 3ß-OH group is (1) esterified to fatty acids , (2) hydroxycinnamic acids or (3) glycosylated with a hexose

Plant sterols have a role in plants like that of cholesterol in mammals, e.g., forming cell membrane structures. Plant sterols fall into one of three categories: 4-desmethylsterols (no methyl groups); 4-monomethylsterols (one methyl group) and 4,4-dimethylsterols (two methyl groups). The most common plant sterols are β-sitosterol, campesterol and stigmasterol and structurally these are very like cholesterol, belonging to the class of 4-desmethylsterols (Figs. 2.63 and 2.66). They include mainly campesterol, sitosterol, stigmasterol, and their respective stanols, which chemically resemble cholesterol. They are present in a normal diet but less than 0.1% of serum sterols are plant sterols. A phytochemical-rich, plant-based diet is of importance in reducing risks of hormone-related neoplasms.
Fig. 2.66

Some common plant sterols and stanols—brassicasterol, ergosterol, brassicastanol, cycloartenol, stigmastanol (sitostanol) and campestanol

Stanol esters are a heterogeneous group of phytosterol esters with a saturated sterol ring structure known to reduce the level of low-density lipoprotein (LDL) cholesterol in blood when ingested. They can be found in trace amounts in every cell type but are highly enriched in foam cells (fat-laden immune cells of the type macrophage) and are common components of human skin oil. Cholesterol esters are of more frequent occurrence, e.g., butter and yolk contain cholesterol esters. In blood, cholesterol esters constitute the transport lipoprotein. Plant sterols are cholesterol-like molecules found in all plant foods, with the highest concentrations occurring in vegetable oils, are plant equivalents of cholesterol, and have a very similar molecular structure. According to their structure, plant sterols can be divided into sterols and stanols (a saturated subgroup of sterols). Plant sterols and stanols are substances that occur naturally in small amounts in many grains (rice, wheat, oat), vegetables (broccoli, cauliflower, tomato, brussels sprout), fruits (avocado, apples, blueberries, dill), legumes (pea, bean, lentil), nuts, and seeds (almond, peanuts, pecans, walnuts, sunflower seeds, pumpkin seeds, sesame seeds), oils (vegetable oils, rice bran oil, wheat germ oil) as well as some fortified foods (orange juice, margarine, cookies, energy bars, yogurt drinks). It is important to eat high sterol foods because these foods contain important vitamins and minerals, as well as fiber. Plant sterol and stanol esters have been shown to reduce the level of low-density lipoprotein (LDL) cholesterol in blood, which helps to reduce the risk of heart disease. Plant sterol esters are used as dietary supplements and are added to certain oil-containing products like margarine, milk, or yogurt to make functional foods for controlling cholesterol levels. In the intestines, plant sterols interfere with cholesterol absorption and absorption rate of plant sterol is very low and more than 90% of sitosterol is passed through the fecal material. Cholesterol is esterified to long chain fatty acids and produces cholesterol esters. Cholesterol esters are much less polar than free cholesterol and appear to be the preferred form for transport in plasma and as a biologically inert storage (detoxification) form. They do not contribute to membranes but are packed into intracellular lipid particles. Cholesterol esters are major constituents of the adrenal glands, and they accumulate in the fatty lesions of atherosclerotic plaques. Stigmasterol is one of a group of plant sterols that include β sitosterol, campesterol, ergosterol (provitamin D2), brassicasterol, delta-7-stigmasterol, and delta-7-avenasterol that are chemically similar to animal cholesterol. Phytosterols are insoluble in water but soluble in most organic solvents and contain one alcohol functional group. Stigmasterol is an unsaturated plant sterol occurring in the plant fats or oils of soybean, calabar bean, and rapeseed, vegetables, legumes, nuts, seeds, and unpasteurized milk (pasteurization will inactivate stigmasterol) and in a number of medicinal herbs, including Ophiopogon japonicus, Mirabilis jalapa, and American Ginseng. Stigmasterol may be useful in the prevention of certain cancers, including ovarian, prostate, breast, and colon cancers, a diet high in phytoesterols may inhibit the absorption of cholesterol, possesses potent antioxidant, hypoglycemic and thyroid inhibiting properties.

Sterids such as fatty acid esters of stigmasterol, ergosterol, and β-sitosterol are the important sterols. Complex or heterolipids contain nonlipid components like phosphate, carbohydrate, protein, etc., in the molecule and constitute phospholipid, glycolipid, and lipoprotein molecules, respectively. Phospholipids are phosphate substituted esters of diverse organic alcohols (glycerol, sphingosine, diols etc.). These polar lipids are predominantly present in the cell membrane. In phosphoglycerides, one of the –OH groups forms an ester bond with phosphate instead of a fatty acid. Phosphatidic acid is the naturally occurring simplest phosphaglyceride. Phosphatidylethanolamine, phosphatidylcholine, phosphatidy linositides, cardiolipin, plasmogens, phosphatidylserines, etc. are some other examples of phosphoglyceride (Fig. 2.67).
Fig. 2.67

Structure of phosphaglyceride—phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositides, cardiolipin, plasmogens, phosphatidyl serines , etc.

Simple glycerides contain same fatty acids and mixed glycerides contain different fatty acids. Glycerophospholipids or phospholipids are similar to true lipid except that one hydroxyl group of glycerol is replaced by the ester of phosphoric acid and an amino alcohol (i.e., triesters of glycerol that contain charged phosphate diester groups depending on the amino alcohol, these can be lecithins (containing choline) or cephalines (containing ethanolamine or serine), abundant in cell membranes.); together with other lipids, they help to control the flow of molecules into and out of cells; sphingolipids or sphingomyelins are phospholipids of an 18 carbon amino alcohol (sphingosine) instead of glycerol and also contain charged phosphate diester groups, they are essential to the structure of cell membranes and are abundant in brain and nerve cell membranes (Fig. 2.68); steroids contain steroid nucleus, consisting of three cyclohexane rings and one cyclopentane ring fused together, e.g., cholesterol; Vitamins A, D2, E, and K1 are fat soluble, therefore, considered lipids. They have important roles in vision, bone growth, and blood clotting.
Fig. 2.68

Structure of sphingomyelins

Sphingomyelins are contained in the nerve tissue in large amounts and also in other organs like lung, liver, kidney, spleen, blood, etc.

Glycolipid, a lipid that contains carbohydrate groups, usually galactose but also glucose, inositol, or others; while it can describe those lipids derived from glycerol or sphingosine, with or without phosphates, the term is usually used to denote the sphingosine derivatives lacking phosphate groups (glycosphingolipids). Glycolipids are glycoconjugates of lipids that are generally found on the extracellular face of eukaryotic cellular membranes, and function to maintain stability of the membrane and to facilitate cell–cell interactions. Glycolipids can also act as receptors for viruses and other pathogens to enter cells. Gangliosides and cerebrosides that form glycosphingolipids (carbohydrate + sphingolipid) are two classes of glycolipids. Their role is to provide energy and also serve as markers for cellular recognition. The carbohydrates are found on the outer surface of all eukaryotic cell membranes. They extend from the phospholipid bilayer into the aqueous environment outside the cell where it acts as a recognition site for specific chemicals as well as helping to maintain the stability of the membrane and attaching cells to one another to form tissues. Glycolipids are membrane components composed of lipids that are covalently bonded to monosaccharides or polysaccharides. Glycolipids include glyceroglycolipids (galactolipids and sulfolipids), glycosphingolipids (cerebrosides and galactocerebrosides) and others (Fig. 2.69).
Fig. 2.69

Glycolipids , unsaturated triesters of glycerol, glyceroglycolipids, glycosphingolipids and sulfosphingolipids (sulfatide)

Glycolipids, different amides derived from sphingosine, contain polar carbohydrate groups; on cell surfaces, the carbohydrate portion is recognized by and connects to intracellular messenger.

A lipoprotein is not a molecule but a biochemical assembly of particulate nature comprised of several thousand molecules of both proteins and lipids (Fig. 2.70). These particles solve the problem of lipid–water incompatibility via the amphipathic nature of phospholipids. The monolayer lipids or their derivatives (e.g., cholesterol) may be covalently or non-covalently bound to the intrinsic proteins, which allow fats to move through the water inside and outside cells. The proteins serve to emulsify the lipid molecules. Examples include the plasma lipoprotein particles classified under high-density (HDL) and low-density (LDL) lipoproteins, which enable fats to be carried in the blood stream, many enzymes (lipoprotein lipase), transporters (chylomicrons, very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), LDL, HDL), structural proteins(α- and β-lipoproteins), antigens, adhesins and toxins, the transmembrane proteins of the mitochondrion and the chloroplast, and bacterial lipoproteins (Fig. 2.71).
Fig. 2.70

General structure (schematic) of lipoprotein

Fig. 2.71

Structure of lipoprotein chylomicron, LDL and HDL. ApoA, ApoB, ApoC, ApoE (apolipoproteins); T (triacylglycerol); C(cholesterol); outer phospholipid boundary (green in chylomicron or other colors in LDL, HDL)

The chief purpose of lipoproteins is to transport fats—mainly cholesterol and triglycerides—from place to place through the bloodstream. Fats are insoluble (they do not dissolve in water) so they have to be “packaged” in such a way that they can flow through the bloodstream. Lipoproteins form a container, made of specialized proteins called apolipoproteins, which enclose the fats, allowing them to be transported to their appropriate destinations. Chylomicrons are lipoproteins that deliver triglycerides from the intestines to the liver, muscle, and adipose (fat) tissue. The main apolipoprotein of chylomicrons is APO B-48. LDLs (low-density lipoproteins) carry cholesterol from the liver to tissues in the body. The main apolipoprotein of LDL is APO B-100. HDL (high-density lipoproteins) carries excess cholesterol from the body’s tissues back to the liver. The main apolipoprotein of HDL is APO-A.

The molecular components including their molecular diameter of different transporter lipoproteins are different (Table 2.9).
Table 2.9

Size and molecular components of different transporter lipoproteins

Characteristic and molecular components lipoproteins

Class

Density (g/ml)

Diameter (nm)

Protein (%)

Cholesterol (%)

Phospholipid (%)

Triglyceride (%)

HDL

>1.063

5–15

33

30

29

4

LDL

1.019–1.063

18–28

25

50

21

8

IDL

1.006–1.019

25–50

18

29

22

31

VDL

0.95–1.006

30–80

10

22

18

50

Chylomicrons

<0.95

100–1000

<2

8

7

84

Lipoproteins are larger and less dense, if they consist of more fat than of protein. These lipid transporter lipoproteins characteristically carry different types of lipids to specific organs. Chylomicrons carry triglycerides (fat) from the intestines to the liver, skeletal muscle, and to adipose tissue. VLDL carry (newly synthesized) triacylglycerol from the liver to adipose tissue. Intermediate-density lipoproteins (IDL) are intermediate between VLDL and LDL. They are not usually detectable in the blood. Low-density lipoproteins (LDL) carry cholesterol from the liver to cells of the body. LDLs are sometimes referred to as the “bad cholesterol“ lipoprotein. High-density lipoproteins (HDL) collect cholesterol from the body’s tissues, and bring it back to the liver. HDLs are sometimes referred to as the “good cholesterol“ lipoprotein.

2.3.1.4 Plant Organic Acids

Plants contain an innumerable number of organic acids in the form of carboxylic acids, phenols etc.; they may be saturated or unsaturated and some of them are volatile and others are nonvolatile. These acids are produced as metabolic intermediates and as amphibolites they participate in the synthetic or degradative pathways leading to the formation of more complex molecules or disintegrated into simple molecules in association with the liberation of energy. For example, Krebs cycle performs amphibolic (dual) functions, catabolic since the cycle degrades the acetyl residues into more simple substances (CO2 and H2O) with the liberation of energy and anabolic function since the cycle substrates are used in the synthesis of more complex materials (aspartic acid and glucose from oxaloacetate, glutamic acid from 2-oxogluterate, heme from succinate). In plants, acids are produced in respiratory glycolytic pathway (pyruvic acid), Krebs cycle (at least 8 di-and tricarboxylic acids are produced, e.g., citric-, isocitric-, cisaconitic-, α-ketogluteric-, succinic-, fumaric-, malic-, oxaloacetic-acids) , pentose phosphate shunt; in photosynthetic Calvin-Basham, Hatch-Slack, CAM pathways; in lipid metabolic pathways, in secondary metabolic pathways like shikimic acid, mevalonic acid, etc. pathways.

The acids are found in the fruits, leaves, stem, and root stocks. The acid may occur in the free form, but is often combined as a salt or an ester. Oxalic acid occurs very frequently and widely distributed in plants, usually as the calcium salt, and apparently less frequently as the sodium and potassium salts. Succinic acid is found in many plants, and glutaric and adipic have been isolated from the sugar beet. Malic acid is found as the free acid and as the salts of malic acid in many plants, and particularly in apples and pears. Citric acid is found in tomatoes and it is found in smaller quantities in other foods, e.g., in cabbage, asparagus, and string beans. Tomatoes, which have the highest amount of acid of our common vegetables, contain about 0.42% citric acid. The most common acids in fruits, citric, and malic, may occur in different proportions or one alone may be present. The total acidity of most fruits varies with the variety and the degree of ripeness. The relative proportions of the various acids may also vary with the degree of ripeness, variety, and climatic and soil conditions. Rhubarb contains some oxalic acid; cranberries and plums some benzoic. Other acids sometimes found in small quantities are succinic, lactic, isocitric, and acetic. Gooseberry contains both citric and malic acids.

The citric acid of lemons, the tartaric acid of grapes, benzoic, cinnamic, salicylic, tannic acid, and some of their salts are of interest pharmacologically. Glycyrrhizin, the sweet principle of glycyrrhiza (licorice, Glycyrrhiza glabra of Fabaceae), is really glycyrrhizic acid, and is sweet to taste only in the form of alkaline salts. It is precipitated and rendered tasteless by acids.

Classification

The plant acids may belong to different groups, e.g., the volatile and nonvolatile ones. Volatile acids volatilize and pass from the liquid to vapor state on exposure. The following acids of the C n H2 n O2 series are volatile, and are arranged according to their degree of volatileness from high to low; each acid contains only one carboxyl group per molecule and so they are also called monocarboxylic acids (Table 2.10).
Table 2.10

Straight chain saturated monocarboxylic acids and fatty acids

Straight chain saturated monocarboxylic acids and fatty acids

C-atom

Common name

IUPAC/other name

Molecular formula

Source and use

1

Formic acid

Methanoic acid

HCOOH

Ant, bee venom, hair of Urtica dioica

2

Acetic acid

Ethanoic acid

CH3COOH

Vinegar, plant juice

2

Glycolic acid

2-Hydroxy ethanoic acid

HOCH2COOH

Sugarcane

3

Propionic acid

Propanoic acid

CH3CH2COOH

Propionyl-CoA, fermentation by Propionibacterium, preservatives

4

Butyric acid

Butnoic acid

CH3(CH2)2COOH

Milk, rancid butter, cheese, human vomit, exhibit butyrate paradox

5

Valeric acid

Pentanoic acid

CH3(CH2)3COOH

Valeriana officinalis

6

Caproic acid

Hexanoic acid

CH3(CH2)4COOH

Goat or other barnyard animals fat

7

Ethanthic acid

Heptanoic acid

CH3(CH2)5COOH

8

Caprylic acid

Octanoic acid

CH3(CH2)6COOH

Coconuts and breast milk

9

Pelargonic acid

Nonanoic acid

CH3(CH2)7COOH

Pelargonium

10

Capric acid

Decanoic acid

CH3(CH2)8COOH

Coconut and palm carnel oil

11

Undecylenic acid

Undecanoic acid

CH3(CH2)9COOH

12

Lauric acid

Dodecanoic acid

CH3(CH2)10COOH

Coconut oil and hand wash soaps

13

Tridecyclic acid

Tridecanoic acid

CH3(CH2)11COOH

14

Myristic acid

Tetradecanoic acid

CH3(CH2)12COOH

Nutmeg

15

Pentadecanoic acid

CH3(CH2)13COOH

16

Palmitic acid

Hexadecanoic acid

CH3(CH2)14COOH

Palm oil

17

Margaric acid

Heptadecanoic acid

CH3(CH2)15COOH

18

Stearic acid

Octadecanoic acid

CH3(CH2)16COOH

Chocolate, waxes, soaps and oils

20

Arachidic acid

Icosanoic acid

CH3(CH2)18COOH

Peanut oil

Straight chain unsaturated monocarboxylic fatty acids

Acrilic acid

2 propenoic acid

–CH2=CHCOOH

Used in polymer synthesis

6

Oleic acid

 

CH3CH2CH=CHCH2COOH

 

18

Linolic acid

C18H32O2

Open image in new window

 

18

Linolenic acid

C18H30O2

Open image in new window

 

Other acids

Straight chain mono- and dicarboxylic amino acids

2

Glycine

Aminoethanoic acid

NH2CH2COOH

 

3

Alanine

2-amino propanoic acid

CH3CH(NH2)COOH

 

5

Valine

2-amino-3-methylbutanoic acid

HOOCCH(NH2)CH(CH3)2

 

6

Leucine

2-amino-4-methylpentanoic acid

HOOCCH(NH2)CH2CH(CH3)2

 

3

Serine

2-amino-3-hydroxypropanoic acid

HOOCCH(NH2)CH2OH

 

4

Threonine

2-amino-3-hydroxybutanoic acid

HOOCCH(NH2)CH(OH)CH3

 

3

Cysteine,

2-amino-3-sulfhydrylpropanoic acid

HOOCCH(NH2)CH2SH

 

6

Cystine,

2-amino-3-(2-amino-2-carboxy-ethyl)disulfanyl-propanoic acid

{SCH2CH(NH2)COOH}2

 

5

Methionine

2-amino-4-(methylthio)butanoic acid

HOOCCH(NH2)CH2CH2SCH3

 

Cyclic aromatic, heterocyclic, hydoxy monocarboxylic amino acids

9

Phenylalanine

2-amino-3-phenylpropanoic acid

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In the breast milk of mammals; form it flavonoid, lignan and cinnamic acid are derived; possesses analgesic and antidepressant effect

9

Tyrosine

L-2-amino-3-(4-hydroxyphenyl)propanoic acid

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Found in many high-protein food product; signal transduction processes

11

Tryptophane

2-amino-3-(1H-indol-3-yl)propanoic acid

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Histidine

   
 

Proline

   

Mono- and dicarboxylic keto acids

3

4

4

5

Pyruvic acid, Diacetic or Acetoacetic acid,

Oxalo acetic acid,

2-Oxogluterate

 

CH3COCOOH,

CH3COCH2COOH

COOHCOCH2COOH, COOH(CH2)2COCOOH

Metabolites of glycolysis, acetoacetyl CoA, Krebs cycle

Aromatic carboxylic acids

7

Benzoic acid,

Benzenecarboxylic acid

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Found in most berries, apples; used as an expectorant, analgesic, antiseptic and food preservative

7

Salicylic acid

2-Hydroxy benzoic acid

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Found in white willow (Salix alba), active metabolite of aspirin (acetylsalicylic acid)

Straight chain saturated dicarboxylic acids

2

Oxalic acid

COOH COOH

Leaves of Oxalis sp.

3

Malonic acid

COOHCH2COOH

 

4

Succinic acid

COOH(CH2)2COOH

 

4

Malic acid

COOHCHOHCH2COOH

Fruits of apple, pea, tomato, etc.

5

Glutaric acid

COOH(CH2)3COOH

 

4

Tartaric acid

COOH(CHOH)2COOH

Fruits of tamarind, grape, tomato, etc.

6

Adipic acid

COOH (CH2)4 COOH

 

Straight chain unsaturated dicarboxylic acids

4

Fumaric acid

COOHCH=CHCOOH

 

Straight chain saturated tricarboxylic acids

6

Citric acid

2-hydroxy propane-1,2,3-tricarboxylic acid

COOHCH2COHCOOHCH2COOH

Fruits of Citrus spp.

6

Isocitric acid

1-hydroxpropane-1,2,3-tricarboxylic acid

COOHCH2CHCOOHCHOHCOOH

 

6

Aconitic acid

Prop-1-ene-1,2,3-tricarboxylic acid

COOHCH=CCOOHCH2COOH

 

6

Propane-1,2,3-tricarboxylic acid (tricarballylic acid)

Propane-1,2,3-tricarboxylic acid

COOHCH2CHCOOHCH2COOH

 

9

Trimesic acid

Benzene-1,3,5-tricarboxylic acid

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6

Ascorbic acid

(5R)-[(1S)-1,2-dihydroxyethyl]-3,4dihydroxyfuran-2(5H)-one

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Citrus fruits; antioxidant, skin, cartilage, teeth, bone, blood vessels

Alpha hydroxy acids (AHAs)

3

Glyceric acid

2,3-dihydroxy propanoic acid

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2

Glycolic acid

2-hydroxyethanoic acid

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2

Lactic acid

2-hydroxy propanoic acid

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4

Malic acid,

2-hydroxy butanedioic acid

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4

Tartaric acid

2,3-dihydroxy butanedioic acid

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9

Aspirin (acetylsalicylic acid) 

2-(acetyloxy)benzoic acid

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Formic acid (methanoic acid) is the simplest carboxylic acid. It is an important intermediate in chemical synthesis and occurs naturally in ant and bee venom and also in the stings of Urtica dioica. Formic and acetic acids have been obtained from plants during distillation. Esters, salts, and the anions derived from formic acid are referred to as formates. Formic acid is a colorless fuming liquid with a pungent penetrating odor; it irritates the mucous membranes and blisters the skin. It is miscible with water and most polar organic solvents, and soluble in hydrocarbons. In the vapor phase, it consists of hydrogen-bonded dimers but the gaseous formic acid does not obey the ideal gas law due to its hydrogen bond. Solid formic acid consists of an effectively endless network of hydrogen-bonded formic acid molecules. Formic acid is used as a preservative, as an acid reducing agent, as a coagulant in the production of rubber, in processing textiles and leather; applied on silage to promote the fermentation of lactic acid and to suppress the formation of butyric acid and as antibacterial agent in livestock feed because it arrests certain decay processes and causes the feed to retain its nutritive value longer. Formic acid has been shown to be an astringent, effective treatment against warts but may cause severe metabolic acidosis and ocular injury.

Acetic acid (ethanoic acid) is the second simplest carboxylic acid. Acetic acid has a distinctive sour taste and pungent smell. A dilute (4–8%) solution of acetic acid is called vinegar; a salt, ester, or acylal of acetic acid is called acetate. Pure acetic acid (glacial acetic acid) is a corrosive, colorless liquid, and completely miscible with water. Biologically, acetic acid is an important metabolic intermediate, and it occurs naturally in body fluids and in plant juices. Acetic acid is produced and excreted by acetic acid bacteria (Acetobacter sp., Clostridium acetobutylicum). It is an important chemical reagent and industrial chemical used in the production of cellulose acetate for photographic film and polyvinyl acetate for wood glue, as well as synthetic fibers and fabrics. In the food industry, acetic acid is used as food preservative, additive, acidity regulator, and condiment. Dilute acetic acid is often used in descaling agents. Acetic acid is produced industrially both synthetically (75% by the carbonylation of methanol), and by bacterial fermentation (10%). The biological route remains important for the production of vinegar because food purity laws claim vinegar of biological origin for food industry. Concentrated acetic acid is corrosive and attacks the skin. Diluted acetic acid is used in physical therapy using iontophoresis.

Propanoic acid (propionic acid) is a naturally occurring carboxylic acid. It is a clear liquid with a pungent odor. The anion, salts, and esters of propanoic acid are known as propanoates or propionates. In industry, propanoic acid is mainly produced by the hydrocarboxylation of ethylene using nickel carbonyl as the catalyst. Large amounts of propanoic acid were once produced as a by-product of acetic acid manufacture. Propanoic acid is produced biologically as its coenzyme A ester, propionyl-CoA, forms from the metabolic breakdown of fatty acids containing odd numbers of carbon atoms, and also from the breakdown of some amino acids. Bacteria of the genus Propionibacterium produce propanoic acid as the end product of their anaerobic metabolism. This class of bacteria is commonly found in the stomachs of ruminants and the sweat glands of humans, and their activity is partially responsible for the odor of both Swiss cheese and sweat.

It is also biosynthesized in the large intestine of humans by bacterial fermentation of dietary fiber. Propanoic acid inhibits the growth of mold and some bacteria at the levels between 0.1 and 1% by weight. As a result, most propanoic acid produced is consumed as a preservative for both animal feed and food for human consumption.

Butyric acid (butanoic acid) is a fatty acid occurring in the form of esters in animal fats and plant oils. As a glyceride, it makes up 3–4% of butter; the disagreeable odor of rancid butter is that of hydrolysis of the butyric acid glyceride. Salts and esters of butyric acid are known as butyrates orbutanoates. Butyric acid is found in milk, especially goat, sheep and buffalo milk, butter, Parmesan cheese, and as a product of anaerobic fermentation (including in the colon and as body odor). The acid is of considerable commercial importance as a raw material in the manufacture of esters of lower alcohols for use as flavoring agents; its anhydride is used to make cellulose butyrate, a useful plastic. Butyric acid is manufactured by catalyzed air oxidation of butanal (butyraldehyde). Butyric acid is present in, and is the main distinctive smell of, human vomit. Butyrate is produced as end product of a fermentation process solely performed by obligate anaerobic bacteria (e.g., Clostridium). The role of butyrate differs between normal and cancerous cells, called the “butyrate paradox” in which butyrate inhibits colonic tumor cells, and promotes healthy colonic epithelial cells; but the signaling mechanism is not well understood.

Valeric acid (pentanoic acid) is a straight chain alkyl carboxylic acid. Like other low-molecular-weight carboxylic acids, it has a very unpleasant odor. It is found naturally in the perennial flowering plant valerian (Valeriana officinalis), from which it gets its name. Its primary use is in the synthesis of its esters. Volatile esters of valeric acid tend to have pleasant odors and are used in perfumes and cosmetics. Ethyl valerate and pentyl valerate are used as food additives because of their fruity flavors.

Caproic acid (hexanoic acid) is the carboxylic acid derived from hexane with the general formula. It is a colorless oily liquid with an odor that is fatty, cheesy, waxy, and like that of goats or other barnyard animals. It is a fatty acid found naturally in various animal fats and oils, and is one of the chemicals that give the decomposing fleshy seed coat of the ginkgo its characteristic unpleasant odor. It is also one of the components of vanilla. The primary use of hexanoic acid is in the manufacture of its esters for artificial flavors, and in the manufacture of hexyl derivatives, such as hexylphenols. The salts and esters of this acid are known as hexanoates or caproates. Two other acids named after goats are caprylic (C8) and capric (C10) acids.

2.3.1.5 Amino Acids and Proteins (C, H, O, N, S & P)

Amino acids

Amino acids are organic acids consisting of an α-carbon (Cα) attached to a carboxyl functional (α-COOH) group, an amino imino or amide functional group (α-NH2, =NH, =N–) group, a hydrogen atom, and a unique side chain (Cβ), (R-group) (Fig. 2.72).
Fig. 2.72

The general structure of an amino acid

About 300–500 amino acids are known and can be classified according to the core structural functional groups’ locations [alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-)amino acids], on the basis of their polarity, acid/basic character, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.). Proteinogenic amino acids—PAAs, (amino acids that are precursors to proteins or building blocks of protein) are incorporated into proteins cotranslationally. There are 23 proteinogenic amino acids, but the genetic code (nuclear genes) of eukaryotes directly encodes only 20 standard amino acids (l-α-amino acids) for incorporation into proteins during translation. Selenocysteine and pyrrolysine are incorporated into proteins by distinct posttranslational biosynthetic mechanisms, and they are added in place of a stop codon when a specific sequence is present, UGA (uracyl, adenine, adenine) codon (one of 3 stop codons—UAA, UAG, and UGA) and selenocysteine insertion sequence (SECIS) element for selenocysteine, and UAG PYLIS (pyrrolysine insertion sequence) downstream sequence for pyrrolysine (Böck et al. 1991; Théobald-Dietrich et al. 2005). N-formylmethionine is often the initial amino acid of proteins in bacteria, mitochondria, and chloroplasts (often removed posttranslationally). Standard or proteinogenic 20 amino acids (in alphabetic order) are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. Amino acids may also be classified as essential (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine), nonessential (alanine, asparagine, aspartic acid, and glutamic acid), and conditional (arginine, cysteine, glutamine, tyrosine, glycine, ornithine, proline, and serine) amino acids. Essential amino acids cannot be synthesized within the body and as a result, they must be supplied through diet; body can synthesize nonessential amino acids while conditional amino acids are usually not essential, except in times of illness and stress.

Selenocysteine contains a selenol group on its β-carbon; Pyrrolysine is formed by joining to the ε-amino group of lysine a carboxylated pyrroline ring (Fig. 2.73).
Fig. 2.73

Structure of selenocysteine, pyrrolysine and lysine

There are various groups of amino acids such as (i) 20 standard pyrrolysine, (ii) 22 proteinogenic amino acids (20 standard amino acids plus 2 nonstandard selenocysteine and pyrrolysine amino acids), (iii) over 80 amino acids created abiotically in high concentrations, (iv) about 900 are produced by natural pathways, and (v) over 118 engineered amino acids have been placed into protein (Lu and Freeland 2006). Pyrrolysine, an ɑ-amino acid, is used in the biosynthesis of proteins in some methanogenic archaea and bacteria (Rother and Krzycki 2010), and contains an α-amino group and a carboxylic acid group in the protonated (–+NH3) and deprotonated (–COO) forms, respectively under biological conditions.

In nature, there exist mostly α-amino acids with l conformation, but there is some nonalpha amino acids (amino group displaced further from the carboxylic acid end of the amino acid molecule and bonded to either α, β, or γ carbon) with non-l(d) conformation (Fig. 2.74).
Fig. 2.74

Structure of nonalpha amino acid molecule bonded to either α, β, or γ carbon with non-L(D) conformation

These amino acids may perform various functions such as a precursor to coenzyme A (β-alanine), neurotransmitter in animals (γ-Aminobutyric acid (GABA), is found as an intermediate in tetrapyrrole biosynthesis in haem, chlorophyll, cobalamin, etc. (δ-Aminolevulinic acid) and as an intermediate in folate biosynthesis (p-Aminobenzoic acid (PABA). In some fungi, α-amino isobutyric acid is produced as a precursor to peptides, and some exhibit antibiotic properties (Gao et al. 2011). It is similar to alanine, but possesses an additional methyl group on the α-carbon instead of hydrogen. Dehydroalanine is similar to alanine without an α-hydrogen but a methylene side chain. It is one of several naturally occurring dehydroamino acids (Fig. 2.75). d-alanine and d-glutamate, d-lysine, d-serine, d-tyrosine, d-proline and d-phenylalanine, d-tryptophan, d-methionine, d-leucineetc are some common d-amino acids. d-amino acids singly or in combination exhibit antibacterial action, effective in preventing biofilm, etc (Kolodkin-Gal et al. 2010; Hochbaum et al. 2011; Moran-Palacio et al. 2014). In animals, some d-amino acids are neurotransmitters.
Fig. 2.75

Structure of α-aminoisobutyric acid dehydroalanine

Taurine is an amino sulfonic acid but not an amino acid. However, it is often considered as amino acid because of its requirement is closer to those of essential amino acids to suppress amino acid auxotrophy in cats. The osmolytes, sarcosine, and glycine betaine are derived from amino acids, but have a secondary and quaternary amine respectively. Examples of posttranslationally incorporation or modification of amino acids into protein are carboxylation of glutamate (for better binding of calcium cations), hydroxylation of proline (critical for maintaining connective tissues), modification of a lysine residue (formation of hypusine) (Vermeer 1990; Bhattacharjee and Bansal 2005; Park 2006).
  • Classification:

The 20 proteinogenic amino acids are divided into following 5 groups on the basis of their side chains.

I. Nonpolar alphatic side groups amino acids

Glycine (gly, G)—has a single hydrogen atom as a side chain; alanine (ala, A)—has a methyl group (CH3) as a side chain; valine (val, V), leucine (leu, L), and isoleucine (ile, I)—have a branched aliphatic side chain; methionine (met, M)—has a sulfur-containing linear aliphatic side chain. Six amino acids with nonpolar aliphatic side groups are shown in Fig. 2.76.
Fig. 2.76

Structure of six amino acids with nonpolar aliphatic side groups

II. Polar-uncharged side groups amino acids

Serine (ser, S) and threonine (thr, T)—have a hydroxyl group in their side chain; cysteine (cys, C)—has a thiol (sulfhydryl) group in its side chain; proline (pro, P)—has an aliphatic side chain which is covalently attached to the α-amino group; glutamine (gln, Q) and asparagine (asn, N)—have an amide group in their side chains. Six amino acids with polar-uncharged side groups are shown in Fig. 2.77.
Fig. 2.77

Structure of six polar-uncharged side groups amino acids

III. Polar positively charged side groups amino acids

Lysine (lys, K) and asparagine (asn, N)—have nitrogen-containing groups in their side chains (amino group in lysine and guanidine group in arginine). These groups have high pKa and therefore tend to become protonated and positively charged at physiological pH. For this reason, lysine and arginine are referred to as basic amino acids. Histidine (his, H)—has an imidazole group in its side chain. This group has a pKa of ~6, and therefore has a 50% chance of being protonated (positively charged) or deprotonated (neutral) at physiological pH. This allows histidine to function in hydrogen transfer enzymatic catalysis, where it may function as the hydrogen donor, acceptor, or both. Three amino acids with positively charged side groups are shown in Fig. 2.78.
Fig. 2.78

Structure of polar positively charged side groups amino acids

IV. Polar negatively charged side groups amino acids

Aspartate (asp, D) and glutamate (glu, E)—have a carboxyl group in their side chains. This group has a low pKa and tends to become deprotonated and negatively charged at physiological pH and therefore, aspartate and glutamate are referred to as acidic amino acids. Two amino acids with negatively charged side groups are shown in Fig. 2.79.
Fig. 2.79

Structure of polar negatively charged side groups amino acids

V. Aromatic side groups amino acids

Phenylalanine (phe, F)—has a phenyl group in its side chain; tyrosine (tyr, Y)—has a phenol group in its side chain; tryptophan (trp, W)—has an indole group in its side chain. Three amino acids with aromatic side groups are shown in Fig. 2.80.
Fig. 2.80

Structure of aromatic side groups amino acids

Amino acids may also be classified on the basis of their chemical nature as acidic (aspartic acid, glutamic acid); basic (arginine, lysine, histidine); amides (asparagine, glutamine); aliphatic (alanine, glycine, isoleucine, leucine, valine); aromatic (phenylalanine, tryptophan, tyrosine); cyclic (proline); hydroxyl containing (serine, threonine, tyrosine); sulfur-containing (cysteine, methionine) amino acids; as well as on the basis of their hydrophobic or nonpolar (with alkyl group side chain—alanine, glycine, isoleucine, leucine, valine, methionine, proline; with alkyl group side chain—phenylalanine, tryptophan) and hydrophilic or polar (neutral with polar side chains such as –OH, –SH groups– serine, threonine, tyrosine cysteine, asparagine, glutamine; acidic-aspartic acid, glutamic acid; basic-arginine, lysine, histidine) properties.

Humans can synthesize 11 and the rest 9 must be supplied from out side through diet and so they are called essential amino acids. The 10 nonessential amino acids (body can synthesize them) are alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine and tyrosine. The nine essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (Young 1994; Reeds 2000; Fürst and Stehle 2004). Amino acids arginine is essential for the young but adult can synthesize it. The human body cannot store excess amino acids for future use like starch and fat and therefore the essential amino acids must be supplied through food every day for necessary protein synthesis. Both l and d amino acids, the mirror image or enantiomers, exist in nature, but only l-amino acids are constituents of proteins. At isoelectric point (pI) ionizable amino acids form zwitterions.

Sources of amino acids

Amino acids are the building blocks of proteins and dietary proteins of plant and animal origin are the best sources of amino acids. Meat (beef, pork, lamb, goat, deer, etc.), poultry (chicken, turkey and others), fish and seafood (tilapia, halibut, tuna, salmon cod, sole, flounder, perch, etc.), eggs and dairy products (milk, yogurt, cheese, etc.) are some of the common sources of animal proteins and consequently, the most amino acids. Plant sources containing the highest quantity of these amino acids are almonds, avocados, figs, raisins, quinoa, Spirulina, watercress, green leafy vegetables, seeds of hemp, chia, soybeans, sesame, sunflower, pumpkin, wheat as well as sunflower butter. Outside protein synthesis, amino acids perform critical roles in processes such as neurotransmitter transport and biosynthesis. Biosynthetically PAAs are derived from keto acids like pyruvate (leucine, isoleucine, and valine), oxaloacetate (aspartic acid, asparagines, lycine, threonine, and methionine), α-ketogluterate (glutamate, ornithine, arginine proline, and hydroxyproline), and shikimate (phenylalanine, tyrosine, and tryptophan) or directly from the intermediate of the Calvin cycle (glycine, serine, cysteine). Biosynthesis of the heterocyclic amino acid histidine follows separate pathway (Goodwin and Mercer 1983) (Table 2.11).
Table 2.11

Main plant and animal food sources of 9 essential amino acids

Amino acids

Plant and animal food sources

1. Histidine

Soy protein, rice, wheat, rye, corn, seaweeds, beans, peanuts, cantaloupe, seeds of hemp and chia buckwheat, potatoes, cauliflower and sesame; eggs, parmesan

2. Isoleucine

Soy protein and tofu, rye, cashews, almonds, oats, lentils, beans, brown rice, cabbage, seeds of pumpkin, sunflower, sesame, hemp and chia, spinach, cranberries, quinoa, blueberries, apples, and kiwis; eggs, whitefish, pork, parmesan

3. Leucine

Seaweed, pumpkin, pea, rice, watercress, turnip greens, seeds of sunflower, sesame and kidney beans, figs, avocados, raisins, dates, apples, blueberries, olives and bananas; Eggs, whitefish, parmesan (cheese), smelts (fish)

4. Lysine

Beans, lentils, chickpeas, watercress, seeds of hemp and chia, spirulina, parsley, avocados, soybean, almonds, cashews; eggs, whitefish, parmesan (cheese), smelts (fish)

5. Methionine

Sunflower seeds and seed butter, seeds of hemp, sesame and chia, beans, soy protein, oats, wheat, rice, onions, figs, cacao, Brazil nuts, seaweed, and raisins; eggs, whitefish, smelts

6. Phenylalanine

Spirulina and seaweeds, pumpkin, beans, soy protein, peanuts, sesame, rice, avocado, almonds, peanuts, quinoa, figs, raisins, leafy greens, berries and olives; eggs, whitefish

7. Threonine

Spirulina, watercress, pumpkin, leafy greens, seeds of hemp, chia, soybeans, sesame, sunflower, soy protein and sunflower butter, almonds, avocados, figs, raisins, quinoa, and wheat; sprouted grains; eggs, whitefish, smelts

8. Tryptophan

Soy protein, sesame, beans, chickpeas, chia seeds, oats, seaweed, seeds of hemp and chia, pumpkin, sweet potatoes, beats, spinach, watercress, parsley, asparagus, mushrooms, lettuces, leafy greens, avocado, figs, winter squash, celery, peppers, carrots, onions, apples, oranges, bananas, quinoa, lentils, and peas; eggs

9. Valine

Soy protein, peanuts, seeds of sesame, hemp, and chia, beans, spinach, broccoli, whole grains, sprouted grains and seeds, figs, avocado, apples, blueberries, cranberries, oranges and apricots; eggs, parmesan, beef

On worldwide basis, α-amino acids, mainly protein amino acids (PAAs), are produced in large scale (~0.5 million tons per year) due to their wide use in medicine, agriculture (growth-stimulating food additives), and food industry (flavoring substances and preservatives), e.g., tryptophan (0.2–0.3 thousand tons), glycine (7–10 thousand tons), lysine (50 thousand tons), methionine (150–200 thousand tons), glutamic acid (>200 thousand tons) per year (Saghyan and Langer 2016). Methionine is used in medicine for the treatment and prevention of hepatotoxicity and diabetes, while a mixture of methionine and cysteine is used for the treatment of different kinds of poisoning. A mixture of glycine and glutamic acid is used to control gastric acidity. Pure glutamic acid is used for the treatment of CNS disorders (epilepsy, psychosis in children with polio, and mental retardation), and its sodium salt as flavoring and preservative in food.

Animal feed and human food industries are major consumers of amino acids. Many bulk components of animal feeds (e.g., soybeans) lack or contain low levels of some essential amino acids like lysine, methionine, threonine, tryptophan, etc. and these are added in bulk quantities to feeds. Many amino acids are also used to chelate metal cations in order to improve the absorption of minerals in animals from supplements required for health improvement or production (Ashmead 1993; Leuchtenberger et al. 2005). The chelating ability of amino acids has been used in the human nutrition industry and in fertilizers to alleviate symptoms of mineral deficiencies in human (anemia) and plants (chlorosis) by facilitating the delivery of minerals such as iron. Glutamic acid and aspartame (aspartyl-phenylalanine-1-methyl ester) are used in food industry as flavor enhancer and low-calorie artificial sweetener, respectively (Stegink 1987; Garattini 2000). Amino acids produced industrially are also used in the synthesis of drugs and cosmetics (Leuchtenberger et al. 2005). Amino acid derivatives used in pharmaceutical industry include 5-HTP (5-hydroxytryptophan) for experimental treatment of depression (Turner et al. 2006), l-DOPA (l-dihydroxyphenylalanine) for treatment of Parkinson’s disease (Kostrzewa et al. 2005) and eflornithine drug (inhibits ornithine decarboxylase) the treatment of sleeping sickness (Heby et al. 2007). Amino acids have been investigated as precursor chiral catalysts (Blaser 1992). Amino acids may be used as components of a range of water-soluble and biodegradable polymers (e.g., polypeptides, polyamides, polyesters, polysulfides, polyaspartate, polyurethanes, polycarbonates, etc.) for environmentally friendly packaging materials, drug delivery vehicle, disposable diapers, prosthetic implants, polycarbonates, etc (Sanda and Endo 1999; Gross and Kalra 2002; Bourke and Kohn 2003; Thombre and Sarwade 2005).

The 20 protein amino acids convey a vast array of chemical and functional diversity, e.g., they are either involved in synthesize proteins and other biomolecules or are oxidized to urea and carbon dioxide as a source of energy (Sakami and Harrington 1963). The exact and specific amino acid content, the sequence of amino acids in the protein molecule, etc., is determined by the sequence of the bases in the gene that encodes that protein through mRNA transcript. The chemical properties of the amino acids of proteins determine the biological and other cellular activities of the protein. The amino acid sequences in the protein contain the necessary information that determine the mechanism of protein fold into a three-dimensional structure as well as the stability of the resulting structure. The removal of amino group by a transaminase initiates the oxidation pathway; the amino group then enters the urea cycle and the keto acid (the other product of transamidation) enters the citric acid cycle (Brosnan 2000). Gluconeogenesis converts glucogenic amino acids into glucose (Young and Ajami 2001).

Amino acids carry out many important bodily functions in addition to their function in proteins synthesis as their building blocks. (i) Alanine is involved in sugar and acid metabolism, boosts up the immune system by producing antibodies, and provides energy for muscles tissues, brain, and the central nervous system. Alanine is used in pharmaceutical preparations for injection or infusion, in dietary supplement, and flavor compounds in Maillard reaction products and it is a stimulant of glucagon secretion. (ii) Arginine assists in wound healing and helps in burn treatment, enhances the production of T-cells necessary in normal immune system activity, helps in vasolidation, chest pain, atherosclerosis (clogged arteries), heart disease or failure, erectile dysfunction, intermittent claudication/peripheral vascular disease, and vascular headaches (headache-inducing blood vessel swelling), enhancing sperm production, and preventing tissue wasting in people with critical illnesses. Arginine hydrochloride has high chloride content and has been used to treat metabolic alkalosis. (iii) Asparagine, along with glutamate, is an important neurotransmitter. Asparagine is required by the nervous system to maintain equilibrium and is also required for amino acid transformation from one form to the other. (iv) Aspartic acids are involved in transamination (aspartate ↔ oxaloacetate), also involved in immune system activity by promoting immunoglobulin production and antibody production, protects the liver and helps in detoxification of ammonia. Aspartate (conjugate base of aspartic acid) functions as a neurotransmitter. Along with few other amino acids, its primary role is to activate NMDA receptors (N-methyl-d-aspartate receptors) in brain (but not significant as glutamate’s), used in coding of DNA. Aspartate plays important roles as acids in enzyme active centers, as well as in maintaining the solubility and ionic character of proteins. Aspartate, glycine, and glutamine are precursors of nucleotides (Stryer et al. 2007). (v) Cysteine can inactivate insulin in bloodstream (because of nucleophilic thiol groups), by reducing one of three disulfide bonds in insulin structure, and can be utilized in medicine and pharmaceutic in a patient experiencing hypoglymecia attack due to the high level of insulin. Cysteine promotes iron production in iron deficiency anemia, and assists in lung diseases by increasing production of red blood cells. It is a key active site residue in many important proteins (e.g., glyceraldehyde-3-phosphate dehydrogenase, glutathione reductases). (vi) Glutamine is the most abundant amino acid in the body, circulates in the blood and is able to cross the blood–brain barrier directly. Glutamine performs various functions such as protein synthesis, helps to maintain neutral pH in the liver by balancing the acid and base levels, is capable of fueling cell like glucose, donates nitrogen to cells via anabolic reactions and provides carbons in the citric acid cycle, provides energy to the small intestine as a primary energy source; provides energy to kidney, activated immune cells, and cancer cells (but not as a primary energy source). Within a cell, glutamine is essential for cell growth and protein translation. (vii) Glutamic acid is highly involved in metabolism in citric acid cycle, transamination (alpha-ketoglutarate with alanine or aspartate producing glutamate and pyruvate or oxalate respectively) and plays role in DNA synthesis. Glutamic acid assists in wound and ulcer healing, in the excitatory neurotransmitter and the metabolism of sugars and fats; aids potassium move through the blood–brain barrier and functions a source of fuel for the brain, can be used in correcting personality disorders and treating childhood behavioral disorders, in treating epilepsy, mental retardation, muscular dystrophy, ulcers, and hypoglycemic coma. Other minor uses include flavor enhancer, GABA precursor (gamma-aminobutyric acid precursor), nutrients, and fertilizers for plants. (viii) Glycine serves an important role in maintaining central nervous and digestive systems, prevents the breakdown of muscle by increase creatine, and keeps the skin firm and flexible. Glycine regulates blood sugar levels and helps to provide glucose for the body. Glycine serves as an inhibitory neurotransmitter in the central nervous system, especially in the spinal cord. Glycine is a precursor of porphyrins such as heme (Shemin and Rittenberg 1946). (ix) Histidine is found in high concentrations in hemoglobin, it aids in the treatment of anemia and maintaining optimal blood pH; acts as a precursor of histamine and thus, involved in local immune responses. Histidine plays important roles in stimulating the inflammatory response of skin and mucous membranes; stimulates the secretion of the digestive enzymes gastrin and acts as the source and control for histamine levels. Histidine is required for growth and for the repair of tissues, as well as the maintenance of the myelin sheaths that act as protector for nerve cells. Histidine is also required to manufacture both red and white blood cells, it helps protect the body from damage caused by radiation and in removing heavy metals from the body, helpful in producing gastric juices, and people with a shortage of gastric juices or suffering from indigestion, arthritis and nerve deafness may benefit from this nutrient. (x) Isoleucine is needed for the formation of hemoglobin and to regulate blood sugar and energy levels; plays important roles in muscle strength and endurance, promotes muscle recovery after an intense workout and is a source of energy for muscle tissues. It is also involved in the formation of blood clots. Isoleucine deficiency may result in headaches, dizziness, fatigue, depression, confusion as well as irritability and may mimic the symptoms of hypoglycemia. (xi) Leucine is necessary in promoting growth in infant and regulating nitrogen concentration in adults. In many cases, functions of leucine are similar to that of isoleucine because of their similarity in branched hydrocarbon side chain. In addition, leucine facilitates skin healing and bone healing by modulating the release of natural pain reducers, enkephalins (a precursor of cholesterol) and increases the synthesis of muscle tissues by slowing down their degradation process. It is generally used as a flavor enhancer. (xii) Lysine inhibits viral growth and can be used in the treatment of Herpes Simplex and virus-associated Chronic Fatigue Syndrome, facilitates the formation of collagen (main component of fascia, bone, ligament, tendons, cartilage and skin), helps in absorption of calcium and thus facilitates the bone growth of infants, plays an essential role in the production of carnitine, converts fatty acids into energy, and helps to lower cholesterol. (xiii) Methionine is an antioxidant that neutralizes free radicals and removes waste in the liver, helps the breakdown of fat, reduces blood cholesterol levels, DNA and RNA synthesis. It is a precursor of several critical amino acids, hormones, and neurotransmitters in human body. Its AUG codon also serves as a “start” signal for ribosomal translation of mRNA. (xiv) Phenylalanine is a precursor of the amino acid tyrosine that gives rise to neurotransmitters (dopamine, norepinephrine and epinephrine), a powerful antidepressant and can enhance memory, thought, and mood, decreases blood pressure, promotes growth in infants and regulates nitrogen concentration in adults. Phenylalanine is a precursor of phenethylamine in humans and in plants, it is a precursor of various phenylpropanoids, which are important in plant metabolism. (xv) Proline plays role in protein’s higher structure and function, it is important in healing, cartilage building, and in flexible joints and muscle support, helps reduce the sagging, wrinkling, and aging of skin. (xvi) Serine is a precursor of glycine and cysteine, found in phospholipids, active sites of trypsin and chymotrypsin; can synthesize pyrimidines and proteins, cysteine and tryptophan; involved in fat and fatty acid formation, muscle synthesis. (xvii) Threonine is a precursor of isoleucine, aids the formation of elastin and collagen, aids in the formation of antibodies in the immune system, threonine, promotes growth and function thymus glands and absorption of nutrients. (xviii) Tryptophan is present in peptides, enzymes, and structural proteins. Tryptophan is a precursor of the neurotransmitter serotonin (Savelieva et al. 2008). (xix) Tyrosine helps in minimizing effects of the stress syndrome (depression treatment) as an adaptanogen, in drug detoxification, assists in treating vitiligo, pigmentation of skin, etc. Tyrosine is also an important precursor of epinephrine, norepinephrine, serotonin, dopamine, melanin, and enkephalins; affects the function of hormones by regulating thyroid, pituitary and adrenal glands. (xx) Valine is essential in muscle growth and development, muscle metabolism, and maintenance of nitrogen balance in the human body; can be used in the treatment of brain damage due to alcohol; can be used as an energy source in place of glucose.

Nonprotein amino acids (NPAAs)

The amino acids that are encoded directly by the codons of the universal genetic code are called standard or canonical amino acids while the nonstandard amino acids are nonproteinogenic except two such as selenocysteine (present in many non-eukaryotes and most eukaryotes) and pyrrolysine (present in some archaea and one bacterium). N-formylmethionine, a modified form of methionine, is often incorporated in place of methionine as the initial amino acid of proteins in bacteria, mitochondria, and chloroplasts. Nonprotein or nonproteinogenic or noncoded amino acids are not naturally encoded for protein synthesis or found in the genetic code of any organism. Despite the use of only 20 amino acids by the translational machinery to assemble proteins (the proteinogenic amino acids), over 140–200 amino acids are known to occur naturally in proteins and thousands more may occur in nature or be synthesized in the laboratory (Goodwin and Mercer 1983; Ambrogelly et al. 2007). Dietary exposure to the nonstandard amino acid beta-amino-l-alanine (BMAA) has been linked to human neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) (Holtcamp 2012; Cox et al. 2016), Parkinson’s disease (PD), etc. However, nonprotein amino acids may have important roles as metabolic intermediates in humans such as in the biosynthesis of the neurotransmitter gamma-amino-butyric acid (GABA).

Many NPAAs are plant secondary metabolites and because of their similarity or resemblance in chemical structure, size, shape, and charge to protein amino acids (PAAs) and can be mistakenly used in protein synthesis, interfere in biochemical pathways, overstimulate receptors, or chelate metal ions (Rodgers et al. 2015). Because of structural similarity, nonprotein amino acid can be included in the composition of the protein, e.g., introduction of azetidine-2-carboxylic acid by tissues of Phaseolus aureus as a replacement of proline moieties in protein (Fowder and Lea 1979). NPAAs may be formed due to posttranslational modification of PAAs (5-hydrxytryptophan from tryptophan in legumes), modification of the synthetic pathways of PAAs (synthesis of β-parazylol-1-yl-alanine in cucurbits), or independent biosynthetic pathway (formation of lathyrine in Lathyrus spp.) (Goodwin and Mercer 1983). Many nonprotein amino acids are incorporated in non-ribosomal peptides.

Some examples of NPAAs are citrulline, ornithine, arginosuccinic acid, theanine, thyroxine, triiodothyronine, trimethylglycine, taurine, homocysteine, DOPA, creatinine, β-alanine, γ-aminobutyric acid, etc. Many NPAAs are important because they are intermediates in biosynthesis, posttranslationally formed in proteins, possess a physiological role (e.g., components of bacterial cell walls, neurotransmitters, and toxins), natural or man-made pharmacological compounds, and are present in meteorites and in prebiotic experiments. About 700 amino acids are known from natural sources and at least 300 from plants. They are found mostly in a small number of families such as Fabaceae, seeds of Cucurbitaceae, Sapindaceae, Aceraceae, Hippocastanaceae microbes, seaweeds, from fungi, etc.

The nonprotein amino acids may play role in protecting plants against predators including insects, pathogens, and competing plant species (allelopathy). Nonprotein amino acids play an important role against the insect pests and high levels of nonprotein amino acids have been identified in certain plant groups (e.g., legumes and grasses) where they have been associated with resistance to insect herbivory, nitrogen storage (Huang et al. 2011). Generally, many of them found in food and fodder plants are found toxic to man and domestic animals (e.g., Lathyrus). They inhibit protein synthesis, disrupt urea synthesis and neurotransmission and are also incorporated into proteins with toxic effects. Specialized insect herbivores often possess specific mechanisms to avoid, detoxify nonprotein amino acids from their host plants or have evolved advanced tRNA synthetases that are able to discriminate between protein and nonprotein amino acids (Dunlop et al. 2015). In spite of these, some of them may have the potential of either as drugs or as leads to drugs in human and veterinary medicine (Bell 2003) (Fig. 2.81).
Fig. 2.81

Structure of nonprotein amino acids (NPAAs)

Nonprotein amino acids are common in plants and are present in widely consumed animal feeds and human foods, e.g., alfalfa (Medicago sativa) contains canavanine, lentil (Lens culinaris) contains homoarginine and Lathyrus species contain neurotoxic oxalyl-amino acid. Some occur in wild species that are inadvertently harvested with crop species. They may be passed along a food chain via animal intermediates (Nunn et al. 2010). Citrulline (in water melon), ornithine (in coconut, oats, soybean, wheat germs, gelatin, meat), theanine (in tea leaf), trimethylglycine (in beet, spinach, broccoli, whole grains, shellfish), taurine (in brewer’s yeast, egg, fish and animal protein) are some of the examples of therapeutically important NPAAs. Citrulline is recommended for therapeutics against erectile dysfunction, ornithine acts as a detoxification agent in liver, theanine is a relaxant and aids in stress reduction, trimethylglycine assists in the formation of S-adenosylmethionine (SAM), an amino acid required for brain as antidepressant and taurine lowers cortisol levels, prevents diabetes, and fights against inflammation.

Vitamin B3 (pantothenic acid), which contains a fragment of the nonprotein amino acid β-alanine (3-aminopropionic acid) is used in polyneurites, dermatoses, bronchitis, and venous ulcers. Nonprotein γ-aminobutyric acid acts as a mediator in the transmission of nerve impulses. γ-Aminobutyric acid (GABA) (aminolon, gammalon) is used to treat nervous system disorders, speech disorders, memory loss, cerebral vascular atherosclerosis, and mental retardation in children. 6-Aminohexanoic acid (ε-aminocaproic acid) is used in medicine to stop severe bleeding, as it helps in effective blood clotting.

Several oligomers, made up of α-amino acids, play an important role in body functions, and some of them are used in medical practice, e.g., methyl ether of l-asparagyl-l-phenylalanine dipeptide (aspartate, aspartame) is used for diabetes as low-calorie sugar substitute (150 times sweeter than glucose); a natural antibiotic Gramicidin, S-cyclic decapeptide—[Val-Orn-Leu-(D)-Phe-Pro] 2, produced by Bacillus brevis, has bacteriostatic and bactericidal action and is used to treat wounds, burns, and inflammatory diseases. Gramicidin is a polypeptide made up from mixture of d- and l-amino acids (Ketchem et al. 1993). This antimicrobial peptide includes a d-form of phenylalanine and small peptides from several natural sources (e.g., leather tree frogs, snail’s ganglion, poison spiders, etc.) were found to contain one or two d-amino acids. d-isomers are uncommon in live organisms but d-form of the amino acid moiety in such peptides greatly increases their resistance to hydrolytic action of exo- and endoproteases and this prolongates the action of oligopeptide drug substances (Soldatenkov et al. 2001). Tyrocidine and valinomycin also contain d-amino acids and these compounds disrupt bacterial cell walls, particularly in Gram-positive bacteria. Only 837 d-amino acids were found in Swiss-Prot database (187 million amino acids analyzed) (Khoury et al. 2011).

Proteins

Proteins are large polymeric molecules of 20 l-α-amino acids joined with each other by peptide bonds. Proteins are optically active, colloidal in nature, soluble in water or salt solution, show amphoteric property, and undergo denaturation on stress. Proteins in cell exist as enzymes, structural proteins and storage proteins. In cereal seeds, ~70% of the protein is gluten (consisting of equal amounts of prolamin and glutelin) and the rest consists of albumin and globulin almost in equal proportion. On the other hand, globulin dominates in oats. Albumin-globulin fraction dominates (>80%) in legume seeds. The proportion of amino acids in different protein fractions is also different, e.g., in barley seeds, albumin contains high proportion of glutamic acid, aspartic acid leucine and low amide-N, globulin contains high proportion of glutamic acid, glycine, arginine and low amide-N, porlamine contains high proportion of amide-N, proline and low lysine and glutelin contains high proportion of amide-N and proline. Major part of the leaf protein is located in chloroplast. Chloroplast protein in association with pigments forms chromoprotein and other proteins of the chloroplasts are enzymes. Enzyme proteins are also present in other cell organelles and cytoplasm of the cell (Goodwin and Mercer 1983). Proteins perform a vast array of functions in living organisms, e.g., provide cellular structure, cytoskeleton, and function as enzyme in metabolic reactions, DNA replication, signal transduction, antibody, storage, messenger or growth hormone and membrane transport.

Some plant protein sources do not always contain all the essential amino acids in required proportions, e.g., low or lacks one or more of the essential amino acids such aslysine, methionine, threonine, etc., rendering those incomplete proteins (Young and Pellett 1994). On the other hand, soybean, chia, seeds, etc. do contain all the essential amino acids while legumes (e.g., beans, lentils, peanuts, etc.) combined with cereal grains (e.g., wheat, rice, corn, etc.) yield a complete protein. An animal protein gelatin is incomplete because it lacks amino acid tryptophan. Foods of animal origin (e.g., meat, poultry, fish, whey, eggs, milk, cheese, yogurt, etc.) are considered complete protein sources. An incomplete protein source is one that is low in one or more of the essential amino acids, such as legumes, which lacks methionine. Many plant proteins are incomplete protein sources. Vegetarian meals may supply complete protein by the practice of protein combining which raises the amino acid profile through plant variety.

  • Classification of proteins based on composition and structure

(a) Based on composition, proteins classified as (i) Simple proteins, (ii) Conjugated proteins and (iii) Derived proteins.

Simple Proteins are again classified according to solubility into Albumins, Globulins, Glutelins, Histories, Protamine, prolamines, and Scleroproteins. Conjugated proteins (polypeptide chain of amino acid + prosthetic group) are further divided into Glycoproteins, Chromoproteins, Lipoproteins, Nucleoproteins, and Phosphoprotein based on the nature of their prosthetic group. Derived proteins are the derivatives of proteins due to action of heat, enzymes, or chemical reagents and are grouped as primary derived and secondary derived proteins.

Structurally proteins are grouped as fibrous and globular while on the basis of function proteins are classified as storage, transport, structural material, metabolic growth regulator, control of physiological functions, catalytic activity, hormonal, and toxicity producing foreign proteins.

Proteins are divided into three main classes, e.g., globular proteins, fibrous proteins, and membrane proteins. Almost all globular proteins are soluble and many are enzymes. Fibrous proteins are often structural (collagen, elastin), the major component of connective tissue (cartilage), keratin (protein component of hair and nails). Membrane proteins often serve as receptors, carrier or provide channels for polar or charged molecules to pass through the cell membrane.

Animal sources of protein tend to deliver all the amino acids that body needs. Plant protein sources such as fruits, vegetables, grains, nuts and seeds, lack one or more essential amino acids. Nine out of the 20 amino acids are essential (e.g., phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine, i.e., F V T W M L I K H; histidine is essential only for infants). Of the remaining 11 amino acids, 6 are conditional amino acids (e.g., arginine, cysteine, glycine, glutamine, proline and tyrosine (i.e., R C G Q P Y) because sick body or body under significant pathogenic and stresses may not be able to produce enough of these amino acids to meet needs. The remaining 5 (e.g., alanine, aspartic acid, asparagine, glutamic acid, and serine, i.e., A D N E S) are nonessential. An essential amino acid (or indispensable amino acid) cannot be synthesized de novo by the organism, and thus must be supplied in its diet (Young 1994). Conditionally essential amino acid synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. Nonessential or dispensable amino acids can be synthesized in the body. Two or more amino acids may join with each other by peptide bonds (–CO–NH–) and thus develop di- or polypeptide chain with amino end (N-terminus) and carboxyl end (C-terminus) (Fig. 2.82).
Fig. 2.82

Structure of peptide chain with amino end (N-terminus) and carboxyl end (C-terminus)

Biologically active proteins fold into one or more specific structural (spatial) conformations driven by a number of non-covalent interactions such as hydrogen bonding, ionic interactions, van der Waals forces, and hydrophobic packing. There are four structural levels of proteins such as primary, secondary, tertiary, and quaternary and a fully functional protein is assembled through these four levels of hierarchy (Fig. 2.83). Primary structure is the linear chain of amino acid sequence, a polypeptide or a polyamide chain. The sequence of amino acids determines the basic structure of the protein.
Fig. 2.83

Structure of different structure levels of proteins

Primary (linear chain of amino acid sequence), secondary (alpha helix and beta pleated sheets—folded version of the linear polypeptide stabilized by hydrogen bonding), tertiary (several secondary structures are assembled together to develop tertiary structures in which, in addition to hydrogen bond, participate hydrophobic interactions, ionic interactions, salt bridges and disulfide bonds), and quaternary (several tertiary structures come together to develop a quaternary protein structure. The same forces of interactions operate in a quaternary structure as operate in a tertiary structure. The same forces of interactions operate in a quaternary structure as operate in a tertiary structure) structures of protein.

Unlike the rigid peptide bond, the other two bonds (bonds linking the amino group to the alpha carbon and the bond linking the alpha carbon to the carbonyl carbon) are free to rotate about the amide (peptide) bonds and allow the amino acids in the polypeptide chain to take on a variety of orientations. The enhanced freedom of rotation with regards to these two bonds allows proteins to fold into a variety of shapes to develop secondary protein structures (the folded version of the linear polypeptide stabilized by hydrogen bonding), which are essentially of two types—alpha helix and beta pleated sheets. These folded secondary structures are stabilized by the formation of hydrogen bonds between the amino acids. In an α helix, the amino acids get oriented in such a manner that the carbonyl, C=O, group of the nth amino acid can form a hydrogen bond with the amido, N−H, group of the (n + 4)th amino acid. This results in a strong hydrogen bond that has an optimum hydrogen to oxygen, H…. O, distance of 2.8 Å. The hydrogen bonds between the amino acids stabilize the α-helix structure. Unlike an α helix (where bonding occurs within the same polypeptide), in β sheets, hydrogen bonding occurs between neighboring polypeptide chains antiparallel (hydrogen-bonded chains extend in the opposite direction) and parallel (hydrogen-bonded chains extend in the same direction) manner to develop the antiparallel β sheet and the parallel β sheet, respectively.

Several secondary structures are assembled together to develop tertiary structures. In addition to hydrogen bond, amino acid side chains of the various secondary structures start interactions with each other including hydrophobic interactions, ionic interactions, and disulfide bonds. Nonlocal interactions generally stabilize tertiary structure, most commonly the formation of a hydrophobic core, but also through salt bridges, hydrogen bonds, disulfide bonds, and even posttranslational modifications. The tertiary structure is what controls the basic function of the protein.

Several tertiary structures come together to develop a quaternary protein structure, e.g., hemoglobin is a functional quaternary protein formed by the coming together of four tertiary structures, called globin proteins. The same forces of interactions operate in a quaternary structure as operate in a tertiary structure. Developmental assemblage of different structural levels of proteins from amino acids to hemoglobin, for example, may be comparable with that of a paragraph writing of a book (primary structure-alphabet, secondary structure-word, tertiary structure-sentence and quaternary structure-paragraph). Covalent bond (amide/peptide bond) is the lone bond of primary structure; in addition to covalent bond, hydrogen bonds are present in secondary structure; in addition to these two bonds, ionic bonds, disulfide bonds, and hydrophobic interactions (van der Waals force) are operative in tertiary and quaternary structures of proteins.

Protein performs a wide array of important functions in human bodies, e.g., store amino acids, function as enzymes, antibodies, hormones (insulin), structural component (keratin and collagen connective tissue), and transporter (hemoglobin and myoglobin transport oxygen), etc. The human insulin is composed of 51 amino acids (5808 da), a dimer of an A-chain and a B-chain linked together by disulfide bonds (Fig. 2.84). Proteins are important for nutritional as well as for therapeutic purposes. The idea of protein therapy is similar to gene therapy, but unlike that, protein therapy delivers healing protein to a person in illness in specific amounts that would ordinarily be present absent in amount, to help repair illnesses, treat pain, or remake structures. It has wide-reaching healing possibilities in many fields such as diabetes, brain disease, and cancer (Frankel et al. 2002).
Fig. 2.84

Structure of human insulin, consisted of peptide chains A and B containing 21 and 30 amino acids, respectively. Chains A and B are linked together by two disulfide bonds and another disulfide bond is present within the A-chain

( source Shutterstock)

Until recently, pharmaceutical drugs were largely used based on relatively small organic molecules such as antibiotics, analgesics, hormones, and other pharmaceuticals synthesized by microbes, plants, animals, or by organic chemistry. But now protein-based large molecule drugs are the fastest growing class of drugs. These are used for the treatment of various diseases including infectious, diabetic, inflammatory, cardiovascular, etc. diseases in humans. Traditionally used protein-based pharmaceuticals generally were manufactured through the expression of protein in bacterial, fungal, and mammalian cell cultures. More than 95 therapeutic proteins or peptides (biologics) have been licensed for production using bacterial, fungal, and mammalian cells grown in sterile cultures since 1982 and hundreds of additional therapeutic proteins are currently being developed and tested but it is anticipated that the capacity of cell culture facilities will fall far short of demand in the near future. Now, the production of more diverse pharmaceutical proteins, biologics or plant-made pharmaceuticals (PMPs), can be produced in transgenic plants developed through genetic engineering, i.e., plants engineered to produce specific proteins (Suslow et al. 2002; Thomas et al. 2002; Shama and Peterson 2004).

The use of transgenic plants will lower cost of production and easier expansion (simply by growing and harvesting additional plants) for large-volume production than cell culture systems (which require a large capital investment). About 50% of the total cost of production is in extraction and purification of the proteins common in both the systems. Plant expression systems can potentially produce hundreds of kilograms per year of a purified protein with tolerable investment while the cost of a similar production capacity using mammalian cell cultures may be simply prohibitive. In addition, unlike mammalian cells and bacterial cells, plant cells offer several advantages such as posttranslational modification, glycosylation, etc. in the production of diverse bioactive proteins for pharmaceutical purposes and to date, a large number of plants including tobacco, potato, tomato, corn, soybeans, alfalfa, rice, wheat are now available biofactories (Fischer et al. 2003; Ma et al. 2003; Streatfield et al. 2003; Goldstein and Thomas 2004).  Plant-made pharmaceuticals like vaccines have several advantages over the conventional such as they are free from the risk of viral contamination, heat stable, edible, storable in seeds (Walmsley and Arntzen 2000; Daniell et al. 2001; Sala et al. 2003).

Recombinant protein therapies have many advantages compared to chemically synthesized drugs, e.g., high specificity, correct function, non-interpretation with other biological reactions, and non-induction of immunological responses (Leader et al. 2008). Plasma protein therapies are unique, biologic medicines that are either infused or injected to treat a variety of rare, life-threatening, chronic and genetic diseases including bleeding disorders, immune deficiencies, pulmonary disorders, neurological disorders, shock and trauma, liver cirrhosis and infectious diseases such as tetanus, hepatitis, and rabies. Protein-based therapeutics are highly successful in clinic and more than 100 genuine and similar number of modified therapeutic proteins are approved for clinical use in the European Union and the USA with 2010 sales of US$108 bln. (Dimitrov 2012).

The protein therapeutics can be enzymes (digestion enzymes or other enzymes like urokinase), clotting factors (recombinant proteins produced in bacteria or in cell cultures), therapeutic antibodies (trastuzumab, or herceptin-antibody-dependent cell-mediated cytotoxicity, ADCC, therapeutic proteins are targeted against the tumor cells), antibody mimetics (Ecallantide, a 60-amino acid polypeptide), vaccines (vaccines against human papilloma virus (HPV) and hepatitis B), and small peptide-based drugs (peptide hormones like insulin, glucagon).

Based on their pharmacological activity, they can be divided into five groups: (a) replacing a protein that is deficient or abnormal; (b) augmenting an existing pathway; (c) providing a novel function or activity; (d) interfering with a molecule or organism; and (e) delivering other compounds or proteins, such as a radionuclide, cytotoxic drug, or effector proteins (Dimitrov 2012).

2.3.1.6 Nucleic Acids (C, H, O, N & P)

Nucleic acids, polymeric molecules of nucleotides, are of two types, e.g., deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is usually double stranded, while RNA is usually single stranded. DNA consists of four types of nucleotides, e.g., adenine (A), guanine (G), thymine (T), and cytosine (C) while RNA, instead of thymine, contains uracil (U). Besides this, RNA contains ribose structure but 2-deoxy ribose is present in DNA molecule. DNA is found in the nucleus and mitochondria and chloroplasts and stores genetic information used for the synthesis of proteins (including enzymes). RNA is found in the nucleus, cytosol and mitochondria and they are of several types and performs different functions, e.g., messenger RNA (mRNA) carries genetic information obtained from DNA to sites that translate the information into a protein, transfer RNA (tRNA) carries activated amino acids to sites where the amino acids are linked together to form polypeptides, ribosomal RNA (rRNA) is a structural component of ribosomes (sites for protein synthesis), small nuclear RNA (snRNA) is a component of small nuclear ribonucleoprotein particles, which process heterogeneous RNA (hnRNA, the immature form of mRNA) into mature mRNA. MicroRNAs (miRNAs) are small RNAs that help to regulate gene expression. In some viruses, HIV, influenza, polio, RNA functions as the storage house of genetic information. The flow of genetic information involves a unidirectional flow from DNA to RNA to Protein (Fig. 2.85).
Fig. 2.85

The unidirectional flow of genetic information from DNA to mRNA to protein

They are composed of nucleotides monomers, a nucleotide is made of three components: a 5-carbon sugar (ribose/2-deoxyribose sugar), a phosphate group, and a nitrogenous base (1 nitrogen base/nucleotide out of 5 bases). Nucleic acids are found in abundance in all life forms including eukaryotic and prokaryotic cells, cell organelles like mitochondria, chloroplasts and acellular viruses, and viroids, where they function in encoding, transmitting, and expressing genetic information. The encoded information is contained and conveyed via the nucleic acid sequence, which provides the “ladder-step” ordering of nucleotides within the molecules of RNA and DNA. Strings of nucleotides strung together in a specific sequence are the mechanism for storing and transmitting hereditary or genetic information and they (nucleotides) are bonded to form helical backbones (one for RNA and two for DNA) and assembled into chains of base pairs selected from the five primary, or canonical, nucleobases, (e.g., adenine, cytosine, guanine, thymine, and uracil; thymine occurs only in DNA and uracil only in RNA) (Fig. 2.86). Using amino acids and the process known as protein synthesis, the specific sequencing in DNA of these nucleobase pairs enables storing and transmitting coded instructions as genes. In RNA, base pair sequencing provides for manufacturing new proteins that determine the frames and parts and most chemical processes of all life forms.
Fig. 2.86

Structure of different nitrogen bases

Nucleic acids are macromolecules and DNA molecules are perhaps the largest individual molecules ever known. Nucleic acid molecules may range in size from 21 nucleotides (e.g., interfering RNA) to as large as 247 million base pairs constituting a single molecule of DNA (e.g., in human chromosome 1) (Gregory et al. 2006).

Naturally occurring DNA molecules are double-stranded (dsDNA) but may also be single-stranded DNA (ssDNA) while the RNA molecules are single-stranded (ssRNA). However, dsRNA viruses are known, e.g., rotaviruses in human, bluetongue virus in cattle and sheep. Only a few human and animal pathogenic viruses are known that have a single-strandedDNA (ssDNA) genome (e.g., in Anelloviridae, Circoviridae). A circular ssDNA virus (e.g., torque teno virus) of the family Anneloviridae was first isolated from human in 1997.

Nucleic acids are linear polymers (chains) of nucleotides. Each nucleotide consists of three components: a purine or pyrimidine nucleobase (sometimes termed nitrogenous base or simply base), a pentose sugar, and a phosphate group. The substructure consisting of a nucleobase plus sugar is termed a nucleoside. Nucleic acid types differ in the structure of the sugar in their nucleotides–DNA contains 2′-deoxyribose while RNA contains ribose (where the only difference is the presence of a hydroxyl group). Also, the nucleobases found in the two nucleic acid types are different: adenine, cytosine, and guanine are found in both RNA and DNA, while thymine occurs in DNA and uracil occurs in RNA.

The sugars and phosphates in nucleic acids are connected to each other in an alternating chain (sugar-phosphate backbone) through phosphodiester linkages (Stryer et al. 2007). In conventional nomenclature, the carbons to which the phosphate groups attach are the 3′-end and the 5′-end carbons of the sugar. This gives nucleic acids directionality, and the ends of nucleic acid molecules are referred to as 5′-end and 3′-end. The nucleobases are joined to the sugars via an N-glycosidic linkage involving nucleobase ring nitrogen (N-1 for pyrimidines and N-9 for purines) and the 1′ carbon of the pentose sugar ring.

Nonstandard nucleosides are also found in both RNA and DNA and usually arise from modification of the standard nucleosides within the DNA molecule or the primary (initial) RNA transcript. Transfer RNA (tRNA) molecules contain a particularly large number of modified nucleosides (Rich and Raj Bhandary 1976).

Adenine and guanine are double-ring purine bases; thymine, cytosic, and uracil are single ring pyrimidine bases.

In nucleic acids chain, ribose (or 2-deoxy ribose) sugar and phosphoric acid form the backbone and phosphoric acids connect sugars by joining at its 3C or 5C position (3→5 or 5→3 direction) and each ribose sugar of the chain holds a nitrogen base at its 1C position. Sugar and nitrogen constitute nucleoside while phosphoric acid, sugar and nitrogen constitute nucleotide of the nucleic acid (Fig. 2.87, Table 2.12).
Fig. 2.87

Structure of different nucleotides

Table 2.12

Different nucleosides and nucleotides based on nitrogen base composition

Base

Ribonucleoside

Ribonucleotide

N-base

(Base + Ribose)

(Base + Ribose + Phos.)

Adenine (A)

Adenosine

Adenosine 5′-monophosphate (AMP)

Guanine (G)

Guanosine

Guanosine 5′-monophosphate (GMP)

Cytosine (C)

Cytidine

Cytidine 5′-monophosphate (CMP)

Thiamine

Thymidine

Thymidine 5′-monophosphate (TMP)

Uracil (U)

Uridine

Uridine 5′-monophosphate (UMP)

Nucleotides structure

  • Primary structure of nucleic acids

The sequence or order of the nucleotides defines the primary structure of DNA and RNA. When nucleotides polymerize to form nucleic acids, they follow a definite sequence such as the hydroxyl group attached to the 3′ carbon of a sugar of one nucleotide forms an ester bond to the phosphate of another nucleotide with the elimination of a molecule of water (Fig. 2.88).
Fig. 2.88

End-to-end chemical orientation of nucleic acid strand: the 5end has a free hydroxyl or phosphate group on the 5′ carbon of its terminal sugar; the 3end has a free hydroxyl group on the 3′ carbon of its terminal sugar

This condensation reaction may be comparable with peptide bond of protein formed between two amino acids. A single nucleic acid strand is a phosphate-pentose polymer (a polyester) with purine and pyrimidine bases as side groups. The ester bonds between the nucleotides are called phosphodiester bonds. A nucleic acid strand (similar to peptide chain) has an end-to-end chemical orientation: the 5end has a free hydroxyl or phosphate group on the 5′ carbon of its terminal sugar; the 3end has a free hydroxyl group on the 3′ carbon of its terminal sugar (Fig. 2.89). This is why polynucleotide sequences are written and read in the 5′→3′ direction (from left to right), e.g., the sequence AUG is assumed to be (5′) AUG (3′). The 5′→3′ directionality of a nucleic acid strand is an extremely important property of the molecule. The oxygen and nitrogen atoms in the backbone give DNA and RNA polarity.
Fig. 2.89

Structure of a single strand of DNA a a trinucleotide containing only three bases: cytosine (C), adenine (A), and guanine (G) with free hydroxyl group at the 3′ end and free phosphate group at the 5′ end; b two common simplified methods of representing polynucleotides. Left—in the “stick” diagram, the sugars are indicated as vertical lines and the phosphodiester bonds as slanting lines; the bases are denoted by their single-letter abbreviations. Right in the simplest representation, the bases are indicated by single letters. By convention, a polynucleotide sequence is always written in the 5′→3′ direction (left to right)

Secondary structure of nucleic acids

Two nucleotides on opposite complementary DNA or RNA strands that are connected via hydrogen bonds are called a base pair (bp) and following the Watson-Crick base pairing model, a purine base always pairs with a pyrimidine base such as adenine (A) forms a base pair with thymine (T) and guanine (G) forms one with cytosine (C) in DNA and in RNA, thymine is replaced by uracil (U) (Fig. 2.90).
Fig. 2.90

Structure of complementary base pair of DNA or RNA strands

A purine base always pairs with a pyrimidine base or more specifically Guanosine (G) with cytosine (C) and adenine (A) with thymine (T) or uracil (U).

The G-C pair has three hydrogen bonds while the A-T pair has two hydrogen bonds. Alternate hydrogen bonding patterns (wobble base pair and Hoogsteen base pair) may also occur in RNA giving rise to complex and functional tertiary structures.

The secondary structure of DNA consists of two polynucleotide chains wrapped around one another to form a double helix. The orientation of the helix is usually right handed with the two chains running antiparallel to one another (Fig. 2.91). Structure of tRNA represents the secondary structure of RNA (Fig. 2.92).
Fig. 2.91

The secondary structure of DNA

Fig. 2.92

Secondary structure of RNA—tRNA represents the secondary structure of RNA

The completion of human genome sequencing and elucidation of many molecular pathways of gene function related to diseases have provided unprecedented opportunities for the development of nucleic acid therapeutics, which include DNA therapeutics, oligonucleotide therapeutics, etc. The nucleic acid therapeutics has created an opportunity to handle many diseases that were generally considered undruggable by small molecule or protein therapeutics. The nucleic acid-based drugs have emerged in recent years and they are in early stages of clinical trials but they appeared to be extremely promising candidates for drug therapy to a wide range of diseases, including cancer, infectious diseases, diabetes, cardiovascular, inflammatory, and neurodegenerative diseases, cystic fibrosis, hemophilia, and other genetic disorders (Alvarez-Salas 2008; Pushpendra et al. 2012).

Therapeutic nucleic acids (TNAs) and its precursors are applied in medical treatment. TNA-based therapy can be classified into three main groups: (i) Therapeutic nucleotides and nucleosides; (ii) Therapeutic oligonucleotides; and (iii) Therapeutic polynucleotides. Therapeutic nucleotides and nucleosides that interfere with nucleic acid metabolism and DNA polymerization have been successfully used as anticancer and antiviral drugs, but they often produce toxic secondary effects related to dosage and continuous use. The use of oligonucleotides such as ribozyme and antisense oligodeoxynucleotides (AS-ODNs) promised as therapeutic moieties but faced several issues such as nuclease sensitivity, off-target effects and efficient delivery. Immuno stimulatory oligodeoxynucleotides and AS-ODNs represent the most successful group of therapeutic oligonucleotides in the clinic. A newer group of therapeutic oligonucleotides, the aptamers, is rapidly advancing towards early detection and treatment alternatives have reached the commercial interest.

DNA-based therapeutics include plasmids, oligonucleotides for antisense and antigene applications, deoxyribonucleic acid aptamers, and deoxyribonucleic acidzymes, while RNA based therapeutics includes ribonucleic acid aptamers, ribonucleic acid decoys, antisense ribonucleic acid, ribozymes, small interfering ribonucleic acid, and micro-ribonucleic acid. Constructed double-stranded DNA of plasmids containing transgenes may be used to biosynthesize therapeutic protein (Uherek and Wels 2000) and plasmids can be used as DNA vaccines for genetic immunization (Johnston et al. 2002). Plasmid-based gene therapy protocol in human was initiated in 1990 for the treatment of adenosine deaminase deficiency (Anderson 1998) and since then, >500 gene therapy protocols have been approved or implemented including severe combined immunodeficiency (SCID), head and neck squamous carcinoma (Vorburger and Hunt 2002; Otsu and Candotti 2002). Diseases with complex etiologies such as cancer, Alzheimer’s and Parkinson’s diseases (i.e., neurodegenerative diseases) are also considered for DNA-based therapeutics (Baekelandt et al. 2000; Galanis and Russell 2001; Mulherkar 2001). DNA vaccines for malaria, AIDS, allergic response, etc. are in development (Bunnel and Morgan 1996; Horner et al. 2001). DNA aptamers are double-stranded nucleic acid segments that can directly interact with proteins (Stull and Szoka 1995), interfere with the molecular functions of disease-implicated proteins or those that participate in the transcription or translation processes and are preferred over antibodies in protein inhibition owing to their specificity, nonimmunogenicity, and stability of pharmaceutical formulation (Jayasena 1999). DNA aptamers have demonstrated promise in intervention of pathogenic protein biosynthesis against HIV-1 integrase enzyme (de Soultrait et al. 2002).

Oligonucleotides, short single-stranded segments of DNA, are used for antisense and antigene applications as they interact and form a duplex with the mRNA or the pre-mRNA and inhibit its translation and protein synthesis finally. Antisense oligonucleotides are single-stranded nucleic acid analogs (14–20 nucleotides in length) that also silence genes by targeting individual mRNAs. An effective antisense RNA must bind to a specific mRNA and prevent translation of the protein. Phosphorothioate antisense oligonucleotides (S-oligos) are the first generation agents, alkyl modifications at the 2′ position of the ribose (2′-O-methyl ribose) are second generation agents while the third generation antisense oligonucleotides contain a variety of modifications within ribose and phosphate backbone. Oligonucleotides, in therapy, can be used to selectively block the expression of proteins that are implicated in diseases (Akhtar et al. 2000), e.g., the first oligonucleotides antisense drug, fomivirsen sodium, was approved for the treatment of cytomegalovirus retinitis in AIDS patients in 1998 (Crooke 1998); MG98 and ISIS 5132 are in human clinical trials for cancer (Mardan et al. 2002).

Ribozymes are naturally occurring catalytic RNA molecules (~40 to 50 nucleotides in length) and maintain separate catalytic and substrate-binding domains and can be engineered to specifically cleave any mRNA sequence. Ribozymes can inhibit the expression of a variety of viral genes and the proliferation of organisms. DNAzymes are analogs of ribozymes with greater biological stability (Akhtar et al. 2000). The RNA backbone chemistry is replaced by the DNA motifs that confer improved biological stability. DNAzyme directed against vascular endothelial growth factor receptor 2 was confirmed to be capable of tumor suppression by blocking angiogenesis upon intratumoral injections in mice (Zhang et al. 2002).

Aptamers are oligonucleotide that binds well to proteins, amino acids, etc. and RNA aptamers (single-stranded nucleic acid segments) can directly interact with proteins (Stull and Szoka 1995) and recognize their targets on the basis of shape complementarity (Kaur and Roy 2008) and have demonstrated promise in the intervention of pathogenic protein biosynthesis against HIV-1 transcriptase (Chaloin et al. 2002). The RNA decoys can prevent translation or induce instability and, ultimately, destruction of the mRNA. Antisense RNA and ribozymes can selectively bind to target mRNAs and form a duplex having highly distorted confirmation that is easily hydrolyzed, and this hydrolysis of mRNA may be used for targeted suppression of specific gene (Mardan et al. 2002). Ribozymes can be used for knockout gene therapy by targeting overexpressed oncogenes such as the human epidermal growth factor receptor type-2 gene implicated in breast cancer (Aigner et al. 2001) and human papilloma virus infection. RNA interference (RNAi) is a method to achieve gene silencing by double-stranded RNA (dsRNA) segment. It is a posttranscriptional mechanism of gene silencing through chromatin remodeling, inhibition of protein translation, or direct mRNA degradation (Caplen 2004; Dorsett and Tuschl 2004; Shankar et al. 2005). Small interfering RNAs (siRNAs) are double-stranded RNA analogs (two 22–27 nucleotide strands) and can be used for downregulation of disease-causing genes through RNA interference. Short hairpin RNA (shRNA) expressed by plasmid shows RNAi effect. MicroRNAs (miRNAs) are a class of naturally occurring, single-stranded small noncoding RNA molecules 21–25 nucleotides in length. These molecules are partially complementary to messenger RNA (mRNA) molecules, and their main function is downregulation of gene expression via translational repression, mRNA cleavage, and deadenylation.

Gene therapy is a technique for correcting defective genes responsible for disease development and theoretically, introduction of a normal gene following gene therapy into a cell with a defective gene may correct the disorder. Nucleic acid-based molecules (DNA, cDNA, complete genes, RNA, and oligonucleotides) are utilized as research tools within the broad borders of gene therapy and the emerging field of molecular medicine. Gene therapy may be classified into two types: somatic and germ line gene therapy. It is an extremely promising field of therapy to a wide range of diseases, including cancer, infectious diseases, diabetes, cardiovascular, inflammatory, and neurodegenerative diseases, cystic fibrosis, hemophilia, and other genetic disorders. The first human gene therapy trial was initiated in two adenosine deaminase (ADA)-deficient patient (a genetic disease of a 4-year-old girl made her defenseless against infections) on 14 September 1990 at the NIH Clinical Center. The patient received large dose of her own cells that had been engineered to carry a functional ADA gene. Ornithine transcarbamylase (OTC) deficient 18-year-old boy, for gene therapy at the University of Pennsylvania in 1999, was given a large dose of an adenoviral vector carrying the OTC gene but the case was fatal after 4 days due to a massive immune response. Now, various nonviral gene transfer systems as well as pure DNA constructs have been devised. Current gene therapy is experimental and has not proven very successful in clinical trials.

2.4 Sources, Chemistry, and Health Effects of the Bioactive Compounds of Primary Metabolic Origin

Higher plants produce a large number of diverse group of chemical compounds (>200,000 different structures). These compounds can be classified as belonging to primary or secondary metabolites, also called natural products. Primary metabolites are ubiquitous in all plants and fulfill essential metabolic roles. Primary metabolism governs all basic physiological processes that allow a plant to grow and set seeds, by translating the genetic code into proteins, carbohydrates, and amino acids, while secondary metabolism is connected to primary metabolism by using building blocks and biosynthetic enzymes derived from primary metabolism. Primary plant metabolites are simple molecules or polymers of simple molecules (viz., chlorophyll, sugar, starch, total protein, ascorbic acid, organic acids, etc.), generally do not possess therapeutic property as such but they are essential for life activity of plants and they contain high-energy bonds. Primary metabolites determine the nutritional potential of plants and also serve as precursors (used up) for the synthesis of secondary metabolites (Tatsuta and Hosokawa 2006; Vijayvergia and Kumar 2007; Harada and Fukusaki 2009). Plants are the sources of many bioactive compounds containing many primary metabolites like, carbohydrates—starch and sugar, energy-providing carbohydrates, proteins, lipids, essential amino acids, fatty acids, ascorbic acid, chlorophyll, etc., are useful ingredients in medicine, nutraceuticals, pharmaceutical intermediates, bioactive principles and lead compounds in synthetic drugs. Therapeutic application of folate is important because its deficiency induces neural tube defects during early pregnancy; some fatty acids are critical in metabolism, cardiovascular health, inflammatory responses, and blood pressure regulation. The health benefits of green tea and its pleasant taste are due to the presence of bioactive compounds predominantly derived from secondary metabolic pathway, however, the composition of primary metabolite and secondary metabolites determines the ultimate quality of green tea. Among the tea quality parameters, tea polyphenol, free amino acid, and theanine concentrations increased, while the caffeine concentration decreased after CO2 enrichment; CO2 enrichment on photosynthesis and respiration in tea plants eventually modulated the biosynthesis of key secondary metabolites towards the production of a quality green tea.

In microorganisms, primary metabolites are typically formed during the growth phase and include glucides, lipids, alcohol (ethanol), organic acids (acetic acid, lactic acid, citric acid), nucleotides (5′guanylic acid), antioxidants (isoascorbic acid), amino acids (aspartic acid, l-glutamate and l-lysine) and amino acid derivatives, vitamins (B2), and polyols (glycerol). Many of these metabolites can be used in industrial microbiology to obtain amino acids, develop vaccines and antibiotics, and isolate chemicals necessary for organic synthesis.

Microalgae, autotrophic prokaryotic, and eukaryotic microorganisms (~3–10 μm in length/dia), in freshwater or marine with different morphological, physiological, and genetic traits, are able to produce different biologically active metabolites. Microalgae including Arthrospira (Spirulina), B. braunii, Chlorella vulgaris, Dunaliella salina, Haematococcus pluvialis, Nostoc, etc., have been investigated for bioactive compounds (Mendes et al. 2006; Nobre et al. 2006; Palavra et al. 2011). Bioactive compounds of microalgal origin can be sourced directly from primary metabolism, such as proteins, fatty acids, vitamins, and pigments, or can be synthesized from secondary metabolism and such compounds can present antifungal, antiviral, antialgal, antienzymatic, or antibiotic actions (Volk 2008). Many compounds including cyanovirin, oleic acid, linolenic acid, palmitoleic acid, vitamin E, B12, β-carotene, phycocyanin, phycobilins, polysaccharides, sterols, lutein, zeaxanthin, etc., have antimicrobial antioxidant, and anti-inflammatory capacities, with the potential for the reduction and prevention of diseases (Smee et al. 2008; Ibañez and Cifuentes 2013; Markou and Nerantzis 2013). Bioactive metabolites of microalgal origin are of special interest in the development of new products for medical, pharmaceutical, cosmetic, and food industries. Microalgae are photosynthetic organisms that play a key role in aquatic ecosystems and ~40% of global photosynthesis is due to these microorganisms (Moreno-Garrido 2008). As microalgal metabolism reacts to changes in the external environment with changes in its intracellular environment, the manipulation of the culture conditions, or the presence or absence of certain nutrients, stimulates the biosynthesis of specific compounds.

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© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of BotanyChittagong UniversityChittagongBangladesh

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