Aspartokinase

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Aspartokinase (or Aspartate kinase) is an enzyme that catalyzes the phosphorylation of the amino acid aspartate. This reaction is the first step in the biosynthesis of three essential amino acids: methionine, lysine, and serine, known as the "aspartate family". The gene for aspartkinase is present only in microorganisms and plants; it is not present in animals, who must obtain aspartate-family amino acids in their diet.

In E. coli, aspartokinase is present as three independently regulated isozymes, each of which is specific to one of the three downstream biochemical pathways. This allows the independent regulation of the rates of methionine, lysine, and serine production. The forms that produce threonine and lysine are subject to feedback inhibition and all three can be repressed at the level of gene expression by high concentrations of their end products.