Zein Storage Proteins
The seeds of angiosperms accumulate large amounts of nitrogen in the form of storage proteins that are hydrolyzed and utilized by the embryo during germination. Besides their importance for establishing the seedling, storage proteins are also a valuable source of amino acids for the animals that consume them. Storage proteins are the most abundant proteins in the seed, accounting for 50–70% of the total protein. As a result, their amino acid composition has important nutritional consequences, especially for monogastric animals such as humans and certain livestock. Usually one or more of the essential amino acids is deficient in seeds. As a result of this, a great deal of research has been devoted to finding ways of increasing the level of these amino acids. Aside from their nutritional value, the structure and solubility properties of storage proteins are also important, because they confer important functional characteristics to flours made from seeds. These properties affect food processing and manufacturing, and they also make these proteins useful for manufacturing a variety of industrial products. Therefore, it is not surprising that the structure and synthesis of seed storage proteins has been of interest for many years.
In angiosperms, seed storage proteins are found in the cotyledons of the embryo, where they form accretions, or protein bodies, within specialized protein storage vacuoles (Hermann and Larkins 1999). The most common forms of embryo storage proteins are saline-soluble globulins of three (7S or vicillin-type) or six (11S or legumin-type) subunits (Casey 1999). These proteins have been crystallized and the molecular details of their structure are being increasingly understood (Adachi et al. 2003). Embryos also contain significant amounts of other proteins, including protease inhibitors (Domoney 1999; Shewry 1999), α-amylase inhibitors (Carbonero and García-Olmedo 1999), lectins (Peumans and Van Damme 1999) and ribosome inactivating proteins (RIPs) (Nielsen and Boston 2001), which presumably are synthesized as anti-nutritional factors that protect the seed from pathogens, insects and bird predators. However, these proteins also affect the food and feed value of certain types of seeds.
KeywordsStarch Granule Protein Body Seed Storage Protein Quality Protein Maize Maize Endosperm
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