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Globulins are the most abundant storage proteins
Storage globulins occur in varying amounts in practically all plants. The most important globulins are legumin and vicilin, both of which are encoded by a multigene family. These multigene families descend from a common ancestor. Legumin is the main storage protein of leguminous seeds. In broad bean, for instance, 75% of the total storage protein consists of legumin.
Legumin is a hexamer with a molecular mass of 300 to 400 kDa. The mono mers contain two different peptide chains (α ,β ), which are linked by a disulfide bridge. The large α-chain usually has a molecular mass of about 35 to 40 kDa, and the small β-chain has a molecular mass of about 20 kDa. Hexamers can be composed of different (α ,β ) monomers some of which contain methionine. In the hexamer, the protein molecules are arranged in a very regular package and can be deposited in this form in the protein bodies. Protein molecules, in which some of the protein chains are not properly folded, do not fit into this package and are degraded by peptidases. Although it is easy nowadays to exchange amino acids in a protein by genetic engineering, it turned out to be difficult for storage proteins, most likely because the three-dimensional structure of the molecule was altered by such exchanges. Recent progress was made in obtaining protein crystals which enabled the analysis of the three-dimensional protein structure of the precursor trimers as well as of the mature storage proteins. These studies revealed that the stability of the storage proteins towards the proteases in the storage vacuoles is due to possible cleavage sites being hidden within the protein structure and in this way protected against proteolysis.
Vicilin shows similarities in its amino acid sequence to legumin, and primarily forms trimers, of which the monomers consist of only one pep-tide chain. Due to the lack of cysteine, the vicilin monomers are unable to form S-S bridges. In contrast to legumins, vicilins are often glycosylated; they contain carbohydrate residues, such as mannose, glucose, and N-acetylglucosamine.
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