title: Role of substrate positioning in the catalytic reaction of 4-hydroxyphenylpyruvate dioxygenase-A QM/MM Study.
abstract: Ring hydroxylation and coupled rearrangement reactions catalyzed by 4-hydroxyphenylpyruvate dioxygenase were studied with the QM/MM method ONIOM(B3LYP:AMBER). For electrophilic attack of the ferryl species on the aromatic ring, five channels were considered: attacks on the three ring atoms closest to the oxo ligand (C1, C2, C6) and insertion of oxygen across two bonds formed by them (C1-C2, C1-C6). For the subsequent migration of the carboxymethyl substituent, two possible directions were tested (C1→C2, C1→C6), and two different mechanisms were sought (stepwise radical, single-step heterolytic). In addition, formation of an epoxide (side)product and benzylic hydroxylation, as catalyzed by the closely related hydroxymandelate synthase, were investigated. From the computed reaction free energy profiles it follows that the most likely mechanism of 4-hydroxyphenylpyruvate dioxygenase involves electrophilic attack on the C1 carbon of the ring and subsequent single-step heterolytic migration of the substituent. Computed values of the kinetic isotope effect for this step are inverse, consistent with available experimental data. Electronic structure arguments for the preferred mechanism of attack on the ring are also presented.
authors.affiliation: Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences , ul. Niezapominajek 8, 30-239 Cracow, Poland.
authors.forename: Anna
authors.initials: A
authors.lastname: Wójcik
authors.forename: Ewa
authors.initials: E
authors.lastname: Broclawik
authors.forename: Per E M
authors.initials: PE
authors.lastname: Siegbahn
authors.forename: Marcus
authors.initials: M
authors.lastname: Lundberg
authors.forename: Graham
authors.initials: G
authors.lastname: Moran
authors.forename: Tomasz
authors.initials: T
authors.lastname: Borowski
mesh_terms: D015111:4-Hydroxyphenylpyruvate Dioxygenase / Q000737:chemistry / Q000378:metabolism*; D055162:Biocatalysis; D018360:Crystallography, X-Ray; D005290:Ferric Compounds / Q000737:chemistry / Q000378:metabolism*; D005296:Ferrous Compounds / Q000737:chemistry / Q000378:metabolism*; D007501:Iron / Q000737:chemistry / Q000378:metabolism*; D008958:Models, Molecular; D015394:Molecular Structure; D011551:Pseudomonas fluorescens / Q000201:enzymology; D011789:Quantum Theory; D013379:Substrate Specificity
chemical_list: D005290:Ferric Compounds; D005296:Ferrous Compounds; C066317:ferryl iron; D007501:Iron; D015111:4-Hydroxyphenylpyruvate Dioxygenase