Encyclopedia of Biophysics

Living Edition
| Editors: Gordon Roberts, Anthony Watts, European Biophysical Societies

Bacterial Polysaccharide Structure and Biosynthesis

  • Yuriy A. KnirelEmail author
  • Miguel A. Valvano
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-35943-9_91-1
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Synonyms

Definition

Polysaccharides are linear or branched polymers built up exclusively or mainly of monosaccharides. In glycoconjugates, a polysaccharide chain(s) is linked to a protein/peptide (glycoprotein, proteoglycan, and peptidoglycan) or a lipid (lipoglycans, glycolipids). The bacterial glycome consists of a wide repertoire of cell surface polysaccharides and glycoconjugates, including lipopolysaccharides (LPS, endotoxins) of Gram-negative bacteria, cell-wall anionic polysaccharides of Gram-positive bacteria, mycobacterial lipoglycans, capsular and extracellular polysaccharides (EPS), and S-layer glycoproteins (Seltmann and Holst 2002). A rigid protective peptidoglycan layer surrounds the cytoplasmic membrane in all bacterial groups. Specific surface polysaccharides play an important role in bacterial life and, particularly, are implicated in recognition and virulence of pathogens. They determine the immunospecificity of bacteria, and various immunogenic forms of polysaccharides and their fragments are used as vaccine components. Bacteria often produce also nonspecific polysaccharides, such as glycogen, dextran, other glucans, and fructans. The biosynthesis of bacterial polysaccharides is a complex process that involves membrane and soluble enzymes, as well as a mandatory transmembrane translocation step in all cases studied to date.

Basic Characteristics

Peptidoglycan or murein has a polysaccharide backbone of alternating residues of N-acetylglucosamine and N-acetylmuramic acid bearing a peptide chain of three to five amino acids. The peptide chain can be cross-linked to that of another strand forming a three-dimensional mesh-like layer (Heidrich and Vollmer 2002) (Fig. 1). Some archaea have a similar layer of pseudopeptidoglycan or pseudomurein, where the sugar residues are N-acetylglucosamine and N-acetyltalosaminuronic acid.
Fig. 1

Schematic representation of the cell wall of Gram-negative and Gram-positive bacteria

Gram-negative bacteria are distinguished by the presence of an outer membrane with an outer leaflet composed mainly of LPS. Three structurally different domains can be distinguished in the LPS molecules: a polysaccharide portion (O-chain, O-antigen) attached to a large oligosaccharide called core, which in turn is linked to the lipid A typically made of a disaccharide of acylated glucosamine residues (Fig. 2). Lipid A anchors the LPS into the membrane and is responsible for its biological (endotoxic) activities. The O-chain is a homoglycan or a heteroglycan built up of oligosaccharide repeats (O-units) that range from di- to octa-saccharides. Heteropolysaccharides are more widespread and are extraordinarily diverse in composition and structure. Typical components of the O-antigens are hexoses, pentoses, hexosamines, their deoxy derivatives, and sugar acids (3,6-dideoxyhexoses, diaminohexoses, aldulosonic acids, amino- and diamino-hexuronic acids, and amino- and diamino-aldulosonic acids), as well as some uncommon branched monosaccharides (Knirel 2009). The O-antigens consisting of two repetitive homo- or heteropolysaccharide domains are known. In LPS with a homopolysaccharide O-chain or a heteropolysaccharide having a disaccharide O-unit, a non-repetitive domain may occur at the reducing end and/or between the O-chain and the core. Some bacteria produce a truncated LPS that is devoid of any O-antigen or have a single O-unit linked to the core-lipid A moiety.
Fig. 2

Structure of the lipopolysaccharide of Pseudomonas aeruginosa O10a,10b

Gram-positive bacteria lack the outer membrane and have a much thicker peptidoglycan layer. Their cell-wall polysaccharides are linked to peptidoglycan (teichoic acids) or to a lipid anchor in the cytoplasmic membrane (lipoteichoic acids). These anionic glycans are involved in ion exchange, permeability of the cell wall to nutrients and antibiotics, and control of the activity of autolytic enzymes and adhesins. Teichoic acids include a polyol (usually glycerol or ribitol) phosphate and often also glycosyl moieties either in the main chain or as lateral groups (Lazarevic et al. 2002) (Fig. 3). Teichuronic acids or other regular non-phosphorylated anionic polysaccharides may replace teichoic acids in some Gram-positive bacteria or under certain environmental conditions.
Fig. 3

Structure of the linkage region between glycerol teichoic acid and peptidoglycan in Bacillus subtilis. R stands for H, α-, or β-d-glucopyranosyl

Although mycobacteria stain slightly Gram-positive, they possess Gram-negative rather than Gram-positive cell envelope features, i.e., a thin peptidoglycan layer and a lipid bilayer outer membrane. Important components of their cell wall are two major lipoglycans, the mycolyl-arabinogalactan complex linked to peptidoglycan and lipoarabinomannan attached to the membrane via a mannosyl-phosphatidyl-myo-inositol anchor (Seltmann and Holst 2002). They are characterized by various non-repetitive motifs and show little structural variability. Arabinogalactan has a main linear galactan chain of about 30 d-galactose residues, to which arabinan chains are attached. The nonreducing end of the latter has a branched domain of six d-arabinose residues, which bears long-chain α-alkyl-β-hydroxyacyl (mycolyl) groups in clusters of four (Fig. 4). The main chain of lipoarabinomannan consists of d-mannose residues and has branched regions. A branched arabinan domain including 50–70 d-arabinose residues is attached close to the nonreducing end of the mannan core. The nonreducing end of the arabinan chains may be capped with mannose, mannobiose, or mannotriose.
Fig. 4

Structural model of the mycolyl-arabinogalactan complex of Mycobacterium tuberculosis. MurAc N-acetylmuramic acid, Myc mycolyl, P phosphate

Extracellular glycans are found in both Gram-negative and Gram-positive bacteria. Some of them, such as K-antigens, are bound to the cell surface (e.g., with the aid of a phosphatidic acid anchor) and form a protective capsule, whereas others are released to the environment as a slime or a component of biofilm, a matrix of polymeric substances encapsulating bacteria and adherent to a living or inert surface (Cescutti 2009). Like cell-wall polysaccharides, exoglycans are built up of oligosaccharide repeating units, which sometimes have the same structure as O-units in Gram-negative bacteria. They usually have anionic character, often including phosphate groups. Some zwitterionic exopolysaccharides exhibit peculiar biological activities and, particularly, induce immunomodulatory T cell response.

Bacterial cells may be surrounded by an S-layer, an envelope of paracrystalline two-dimensional lattices composed of glycoprotein molecules self-aggregated by electrostatic or hydrophobic interactions. S-layer glycan chains are repetitive long polysaccharides analogous to the O-chains or oligosaccharides resembling the LPS core (Messner et al. 2009). Typically, they are O-linked to serine, threonine, or, less common, tyrosine and are much more diverse in composition and structure than glycan chains of eukaryotic glycoproteins. S-layers are characteristic also of archaea, but in contrast to those of bacteria, they have predominantly short-chain glycans that are N-linked to asparagine.

The biosynthesis of O-antigens (Fig. 5) (and also of other bacterial surface polysaccharides, exopolysaccharides, and cell-wall peptidoglycan, as well as glycans for protein glycosylation) starts at the cytosolic face of the plasma membrane by the formation of an undecaprenyl diphosphate (Und-PP)-linked saccharide using a sugar delivered as a nucleoside diphosphate (Raetz and Whitfield 2002). Two different families of integral membrane proteins designated as polyisoprenyl-phosphate N-acetylaminosugar-1-phosphate and hexose-1-phosphate transferases (PNPT and PHPT, respectively) catalyze the initiation reaction (Valvano 2003). The subsequent steps in the glucan biosynthesis depend on the mode of export of the phosphoisoprenol-linked saccharide across the lipid membrane, which can be distinguished into the polymerase (Wzy)-dependent and ATP-binding cassette (ABC)-dependent pathways.
Fig. 5

Schematics of O-antigen biosynthesis

In the Wzy-dependent pathway, the O-unit is assembled from reactions catalyzed by specific glycosyltransferases, which are typically peripheral membrane proteins associated with the plasma membrane by ionic interactions. Individual O-units are translocated across the membrane by an ATP hydrolysis-independent mechanism mediated by the protein Wzx (O-unit flippase). On the periplasmic side of the membrane, the translocated O-unit is polymerized to a certain length, unique to each O-antigen, by the concerted functions of Wzy (O-antigen polymerase) and Wzz (O-antigen chain length regulator). In the ABC-dependent pathway, the O-antigen is elongated in the cytosol by processive glycosyltransferases, and the polymer is translocated across the membrane by an ABC transporter (Wzt/Wzm or Wzk). ABC transporters are also used in the synthesis of capsular polysaccharides (Whitfield 2006) and S-layers (Messner et al. 2009). In some cases, a termination signal that ceases the chain elongation is provided by methylation or other terminal sugar modifications that couple the polymerization with the ABC transporter.

The LPS core oligosaccharide is assembled in a separate pathway on preformed lipid A at the cytosolic face of the plasma membrane, and the resultant lipooligosaccharide is translocated to the periplasmic face of the membrane by a specialized ABC transporter (MsbA). The ligation of the O-antigen “en bloc” onto lipid A-core (Fig. 6) is catalyzed by an oligosaccharyltransferase (WaaL, O-antigen ligase), resulting in the release of Und-PP. WaaL has similarities to oligosaccharyltransferases involved in the synthesis of O-linked glycoproteins.
Fig. 6

Schematics of LPS biosynthesis and export to the outer membrane

The complete LPS molecule is transported to the outer membrane by the LptABCDEFG protein complex (Fig. 6) (Ruiz et al. 2009). The export of capsules to the outer membrane also requires a distinct complex of proteins, which includes an outer membrane secreting protein (Whitfield 2006).

Cross-References

References

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Copyright information

© European Biophysical Societies' Association (EBSA) 2018

Authors and Affiliations

  1. 1.N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of SciencesMoscowRussia
  2. 2.Department of Microbiology and ImmunologyUniversity of Western OntarioLondonCanada
  3. 3.Wellcome-Wolfson Institute of Experimental MedicineQueen’s University BelfastBelfastUK

Section editors and affiliations

  • Elizabeth Hounsell
    • 1
  1. 1.School of Biological and Chemical SciencesBirkbeck College, University of LondonLondonUK