Aspartokinase is an enzyme of amino acid biosynthesis that catalyzes the reaction below:

Aspartate + ATP <=> -Aspartyl-Phosphate + ADP

Aspartokinase is a major point of regulation of the biosynthetic pathways leading to threonine, lysine, and methionine. In bacteria, there are three isoenzymes of aspartokinase. Activity of one form is inhibited specifically by threonine and that of another form by lysine. Synthesis of the third form is inhibited by methionine.