Aspartate kinase
class of enzymes
Aspartate kinase or aspartokinase (AK) is an enzyme that catalyzes the phosphorylation of the amino acid aspartate. This reaction is the first step in the biosynthesis of three other amino acids: methionine, lysine, and threonine, known as the "aspartate family". Aspartokinases are present only in microorganisms and plants, but not in animals, which must obtain aspartate-family amino acids from their diet. Consequently, methionine, lysine and threonine are essential amino acids in animals.
Function
Aspartate kinase uses the cofactor, adenosine triphosphate (ATP), to transfer a phosphate group to the amino acid L-aspartic acid, giving phosphoaspartate. Adenosine diphosphate (ADP) is a byproduct.
Nomenclature
The generic abbreviation for aspartokinases is AK. However, the nomenclature for aspartokinase genes and proteins varies considerable among species. The main aspatokinases are lysC (Bacillus subtilis, Escherichia coli and many other bacteria), ask (Mycobacterium bovis, Thermus thermophilus), AK1–AK3 (Arabidopsis thaliana), FUB3 (Fusarium and Gibberella) and HOM3 (Saccharomyces cerevisiae). Additionally, apk is a synonym for lysC.
Enzymatic regulation
Aspartokinases may use the morpheein model of allosteric regulation.
In Escherichia coli, aspartokinase is present as three independently regulated isozymes (thrA, metL and lysC), each of which is specific to one of the three downstream biochemical pathways.
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