Phenylalanine hydroxylase

Phenylalanine hydroxylase (PAH) (EC 1.14.16.1) is an enzyme that catalyzes the hydroxylation of the aromatic side-chain of phenylalanine to generate tyrosine. PAH is one of three members of the biopterin-dependent aromatic amino acid hydroxylases, a class of monooxygenase that uses tetrahydrobiopterin (BH4, a pteridine cofactor) and a non-heme iron for catalysis. During the reaction, molecular oxygen is heterolytically cleaved with sequential incorporation of one oxygen atom into BH4 and phenylalanine substrate. In humans, mutations in its encoding gene, PAH, can lead to the metabolic disorder phenylketonuria.

Enzyme mechanism

The reaction is thought to proceed through the following steps:

formation of a Fe(II)-O-O-BH4 bridge.

heterolytic cleavage of the O-O bond to yield the ferryl oxo hydroxylating intermediate Fe(IV)=O

attack on Fe(IV)=O to hydroxylate phenylalanine substrate to tyrosine.

Formation and cleavage of the iron-peroxypterin bridge. Although evidence strongly supports Fe(IV)=O as the hydroxylating intermediate, the mechanistic details underlying the formation of the Fe(II)-O-O-BH4 bridge prior to heterolytic cleavage remain controversial. Two pathways have been proposed based on models that differ in the proximity of the iron to the pterin cofactor and the number of water molecules assumed to be iron-coordinated during catalysis. According to one model, an iron dioxygen complex is initially formed and stabilized as a resonance hybrid of Fe2+O2 and Fe3+O2−.