In molecular biology, the
biopterin-dependent aromatic amino acid hydroxylases (abbreviated
AAAH) constitute a family of aromatic
amino acid hydroxylases, including
phenylalanine 4-hydroxylase ,
tyrosine 3-hydroxylase , and
tryptophan 5-hydroxylase . 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.