Maize lysine synthesis

Seven enzyme catalyzed reactions are required for the synthesis of lysine from pyruvate and aspartate semialdehyde as illustrated in Fig. 3. However, enzymes catalyzing only the first and last of these reactions have been isolated from plants. Dihydropicolinate synthase facilitates the condensation of the precursors during a reaction which presumably proceeds in two steps. A double bond between the C-4 of the semialdehyde and the methyl carbon of pyruvate would be formed, with the loss of water, followed by spontaneous ring closure and the loss of a second molecule of water. Catalysis of this reaction in plant extracts was first demonstrated by Cheshire and Miflin (1975) using maize seedlings as the source of the enzyme. Mazelis et al. (1977) detected the enzyme in extracts obtained from six different taxonomic families of plants and partially purified the enzyme from wheat germ. Only the L isomer of aspartate semialdehyde was active as a substrate of this enzyme and strong cooperativity was noted when the concentration of pyruvate was varied. A dihydropicolinate synthase has also been isolated from carrot cells (Matthews and Widholm, 1978). 

An understanding of the mode and sequence of production of the components and factors affecting protein synthesis could be of considerable use to plant breeders and geneticists. Proteins of some seeds form an important part of the human diet in many countries, but are often deficient in certain amino acids, and if these are not provided from alternative food sources serious deficiency symptoms ensue. In normal maize, zein (the major protein body constituent) is poor in lysine and tryptophan, and is the protein which is synthesized late during maturation. Glutelins (the non-sequestered proteins) are synthesized... 

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