Aromatic amino acid hydroxylase (AAAH)

Aromatic amino acid hydroxylases (AAAHs) are a small family of non-heme iron enzymes responsible for the hydroxylation of aromatic amino acids, specifically L-phenylalanine, L-tyrosine, and L-tryptophan[1][2][4]. The major members are phenylalanine hydroxylase (EC 1.14.16.1), tyrosine hydroxylase (EC 1.14.16.2), and tryptophan hydroxylase (EC 1.14.16.4), each catalyzing the initial and often rate-limiting steps in the synthesis of key neurotransmitters—dopamine, norepinephrine (via tyrosine), and serotonin (via tryptophan)[2][8]. They require tetrahydrobiopterin (BH4) as a cofactor and molecular oxygen for activity[1][7][4]. Deficiencies or mutations in these enzymes are associated with severe metabolic and neurological diseases, including phenylketonuria (PAH deficiency) and neurotransmitter deficiencies (TH or TPH deficiencies)[2][5][8]. AAAHs have also emerged as off-targets of some drugs, most notably certain histone deacetylase (HDAC) inhibitors such as panobinostat, which can inhibit their enzymatic function at therapeutically relevant concentrations[3]. These enzymes are regulated by phosphorylation, allosteric effectors, substrate availability, and cofactor levels, and share a conserved catalytic domain with divergent N-terminal regulatory regions that contribute to tissue- and enzyme-specific regulation[1][7][4]. Safety concerns for therapeutically targeting or inadvertently inhibiting these enzymes include the risk of neurotransmitter imbalance and accumulation of neurotoxic metabolites.

Other names

Biopterin-dependent aromatic amino acid hydroxylaseAromatic amino acid monooxygenasePhenylalanine hydroxylase (PAH)Tyrosine hydroxylase (TH)Tryptophan hydroxylase (TPH)

Enzyme inhibition (competitive, allosteric), Iron chelation, Cofactor (tetrahydrobiopterin/BH4) modulation