ACT domains are commonly involved in specifically binding an amino acid or other small ligand leading to regulation of the enzymeMembers of this CD belong to the superfamily of ACT regulatory domains. Pairs of ACT domains are commonly involved in specifically binding an amino acid or other small ligand leading to regulation of the enzyme. The ACT domain has been detected in a number of diverse proteins; some of these proteins are involved in amino acid and purine biosynthesis, phenylalanine hydroxylation, regulation of bacterial metabolism and transcription, and many remain to be characterized. ACT domain-containing enzymes involved in amino acid and purine synthesis are in many cases allosteric enzymes with complex regulation enforced by the binding of ligands. The ACT domain is commonly involved in the binding of a small regulatory molecule, such as the amino acids L-Ser and L-Phe in the case of D-3-phosphoglycerate dehydrogenase and the bifunctional chorismate mutase-prephenate dehydratase enzyme (P-protein), respectively. Aspartokinases typically consist of two C-terminal ACT domains in a tandem repeat, but the second ACT domain is inserted within the first, resulting in, what is normally the terminal beta strand of ACT2, formed from a region N-terminal of ACT1. ACT domain repeats have been shown to have nonequivalent ligand-binding sites with complex regulatory patterns such as those seen in the bifunctional enzyme, aspartokinase-homoserine dehydrogenase (ThrA). In other enzymes, such as phenylalanine hydroxylases, the ACT domain appears to function as a flexible small module providing allosteric regulation via transmission of conformational changes, these conformational changes are not necessarily initiated by regulatory ligand binding at the ACT domain itself. ACT domains are present either singularly, N- or C-terminal, or in pairs present C-terminal or between two catalytic domains. Unique to cyanobacteria are four ACT domains C-terminal to an aspartokinase domain. A few proteins are composed almost entirely of ACT domain repeats as seen in the four ACT domain protein, the ACR protein, found in higher plants; and the two ACT domain protein, the glycine cleavage system transcriptional repressor (GcvR) protein, found in some bacteria. Also seen are single ACT domain proteins similar to the Streptococcus pneumoniae ACT domain protein (uncharacterized pdb structure 1ZPV) found in both bacteria and archaea. Purportedly, the ACT domain is an evolutionarily mobile ligand binding regulatory module that has been fused to different enzymes at various times.