Amino Acid Synthesis in Plastids

Plants have the capacity to assimilate inorganic nitrogen and synthesize the twenty amino acids required for normal protein synthesis, unlike animals that need at least nine essential amino acids in their diet. The major metabolic reactions involved in the synthesis of essential amino acids, and the enzymes that catalyse them, are located in plastids. In the chloroplast, ATP and reductant required for amino acid synthesis are derived directly from light energy via photosystems I and II. Non-green plastids are also able to synthesize amino acids, using reductant derived from the oxidative pentose-phosphate pathway. Ammonia produced either from the reduction of nitrite, or a variety of other reactions including the decarboxylation of glycine during photorespiration, is rapidly assimilated into glutamine by glutamine synthetase (GS) and transferred to the ?-amino position of glutamate by glutamate synthase (GOGAT). The amino group can then be used for the synthesis of lysine, threonine, methionine, isoleucine, leucine, valine, tryptophan, phenylalanine, tyrosine and histidine inside the plastid, through strictly regulated pathways. The two major mechanisms by which regulation of amino acid biosynthesis occurs in plastids, (1) feedback inhibition of the enzymes by end-product amino acids and (2) regulation of transcription of the multigene families that encode the protein(s) that make up the individual enzymes, are described in detail.