Water Dissociation and Dioxygen Binding in Phenylalanine Hydroxylase

Abstract

AbstractPhenylalanine hydroxylase (PAH) catalyzes the hydroxylation of L‐phenylalanine to L‐tyrosine and is dependent on the contribution of electrons from the reduced cofactor tetrahydrobiopterin (BH4). Whereas three ligating water molecules are bound to the central iron atom in the existing crystal structures of binary complexes of the catalytic domain of both the inactive, PAH–FeIII–BH2, and active, PAH–FeII–BH4, forms of the human enzyme, previous spectroscopic studies show that the water molecules dissociate prior to the onset of the catalytic reaction. In the present study, starting from a cluster model of the active FeII center of PAH, three successive water ligand dissociations followed by dioxygen coordination have been investigated by using density functional theory. The calculations show that the formation of the active, water‐free O2 complex from the water‐ligated complex of the PAH–FeII–BH4 crystal structure is remarkably facile (ΔG = 1.5 kcal/mol). Moreover, the initial water dissociation is accompanied by entrance of the cofactor into the first coordination sphere of iron.