This family of domains generally have a regulatory role. ACT domains are linked to a wide range of metabolic enzymes that are regulated by amino acid concentration. Pairs of ACT domains bind specifically to a particular amino acid leading to regulation of the linked enzyme. The ACT domain is found in: D-3-phosphoglycerate dehydrogenase EC:220.127.116.11, which is inhibited by serine. Aspartokinase EC:18.104.22.168, which is regulated by lysine. Acetolactate synthase small regulatory subunit, which is inhibited by valine. Phenylalanine-4-hydroxylase EC:22.214.171.124, which is regulated by phenylalanine. Prephenate dehydrogenase EC:126.96.36.199. formyltetrahydrofolate deformylase EC:188.8.131.52, which is activated by methionine and inhibited by glycine. GTP pyrophosphokinase EC:184.108.40.206.