ACT domain of the nonheme iron-dependent, aromatic amino acid hydroxylases (AAAH)ACT domain of the nonheme iron-dependent, aromatic amino acid hydroxylases (AAAH): Phenylalanine hydroxylases (PAH), tyrosine hydroxylases (TH) and tryptophan hydroxylases (TPH), both peripheral (TPH1) and neuronal (TPH2) enzymes. This family of enzymes shares a common catalytic mechanism, in which dioxygen is used by an active site containing a single, reduced iron atom to hydroxylate an unactivated aromatic substrate, concomitant with a two-electron oxidation of tetrahydropterin (BH4) cofactor to its quinonoid dihydropterin form. PAH catalyzes the hydroxylation of L-Phe to L-Tyr, the first step in the catabolic degradation of L-Phe; TH catalyses the hydroxylation of L-Tyr to 3,4-dihydroxyphenylalanine, the rate limiting step in the biosynthesis of catecholamines; and TPH catalyses the hydroxylation of L-Trp to 5-hydroxytryptophan, the rate limiting step in the biosynthesis of 5-hydroxytryptamine (serotonin) and the first reaction in the synthesis of melatonin. Eukaryotic AAAHs have an N-terminal ACT (regulatory) domain, a middle catalytic domain and a C-terminal domain which is responsible for the oligomeric state of the enzyme forming a domain-swapped tetrameric coiled-coil. The PAH, TH, and TPH enzymes contain highly conserved catalytic domains but distinct N-terminal ACT domains (this CD) and differ in their mechanisms of regulation. One commonality is that all three eukaryotic enzymes are regulated in part by the phosphorylation of serine residues N-terminal of the ACT domain. Members of this CD belong to the superfamily of ACT regulatory domains.