title: Formation of the iron-oxo hydroxylating species in the catalytic cycle of aromatic amino acid hydroxylases.
abstract: The first part of the catalytic cycle of the pterin-dependent, dioxygen-using nonheme-iron aromatic amino acid hydroxylases, leading to the Fe(IV)=O hydroxylating intermediate, has been investigated by means of density functional theory. The starting structure in the present investigation is the water-free Fe-O(2) complex cluster model that represents the catalytically competent form of the enzymes. A model for this structure was obtained in a previous study of water-ligand dissociation from the hexacoordinate model complex of the X-ray crystal structure of the catalytic domain of phenylalanine hydroxylase in complex with the cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)) (PAH-Fe(II)-BH(4)). The O-O bond rupture and two-electron oxidation of the cofactor are found to take place via a Fe-O-O-BH(4) bridge structure that is formed in consecutive radical reactions involving a superoxide ion, O(2)(-). The overall effective free-energy barrier to formation of the Fe(IV)=O species is calculated to be 13.9 kcal  mol(-1), less than 2 kcal  mol(-1) lower than that derived from experiment. The rate-limiting step is associated with a one-electron transfer from the cofactor to dioxygen, whereas the spin inversion needed to arrive at the quintet state in which the O-O bond cleavage is finalized, essentially proceeds without activation.
authors.affiliation: Department of Chemistry, University of Bergen, Allégaten 41, 5007 Bergen, Norway.
authors.forename: Elaine
authors.initials: E
authors.lastname: Olsson
authors.forename: Aurora
authors.initials: A
authors.lastname: Martinez
authors.forename: Knut
authors.initials: K
authors.lastname: Teigen
authors.forename: Vidar R
authors.initials: VR
authors.lastname: Jensen
mesh_terms: D024322:Amino Acids, Aromatic / Q000737:chemistry* / Q000378:metabolism; D001708:Biopterins / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D002384:Catalysis; D003067:Coenzymes / Q000737:chemistry* / Q000378:metabolism; D018360:Crystallography, X-Ray; D005296:Ferrous Compounds / Q000737:chemistry* / Q000378:metabolism; D006900:Hydroxylation; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D006899:Mixed Function Oxygenases / Q000737:chemistry* / Q000378:metabolism; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011622:Pterins / Q000737:chemistry*
chemical_list: D024322:Amino Acids, Aromatic; D003067:Coenzymes; D005296:Ferrous Compounds; D008024:Ligands; D011622:Pterins; D001708:Biopterins; D007501:Iron; D006899:Mixed Function Oxygenases; C003402:sapropterin