Biopterin-dependent aromatic amino acid hydroxylase

In molecular biology, the biopterin-dependent aromatic amino acid hydroxylases (abbreviated AAAH) constitute a family of aromatic amino acid hydroxylases, including phenylalanine 4-hydroxylase (EC1.14.16.1), tyrosine 3-hydroxylase (EC1.14.16.2), and tryptophan 5-hydroxylase (EC1.14.16.4). These enzymes primarily hydroxylate phenylalanine, tyrosine, and tryptophan, respectively. These enzymes are all rate-limiting catalysts for important metabolic pathways. The proteins are structurally and functionally related, each containing iron, and catalysing ring hydroxylation of aromatic amino acids, using tetrahydrobiopterin (BH4) as a substrate. All are regulated by phosphorylation at serines in their N-termini. It has been suggested that the proteins each contain a conserved C-terminal catalytic (C) domain and an unrelated N-terminal regulatory (R) domain. It is possible that the R domains arose from genes that were recruited from different sources to combine with the common gene for the catalytic core. Thus, by combining with the same C domain, the proteins acquired the unique regulatory properties of the separate R domains.