The rate-limiting step in P450 hydroxylation of hydrocarbons a direct comparison of the "somersault" versus the "consensus" mechanism involving compound I.
Model theoretical quantum mechanical (QM) calculations are described for the P-450 hydroxylation of methane, isobutane, and camphor that compare the concerted somersault H-abstraction mechanism with the oxidation step involving Cpd I. Special emphasis has been placed on maintaining a balanced basis set in the oxidation step. QM calculations, employing the 6-311+G(d,p) basis set on the Fe atom and all of the key surrounding atoms involved in the C-H abstraction step, reaffirm a mechanism involving rearrangement of the iron hydroperoxide group (FeO-OH --> FeO...HO(*)) in concert with hydrogen abstraction from the C-H bond of the substrate by the incipient bound hydroxyl radical HO(*). The barrier for the somersault rearrangement of model Cpd 0 (FeO-OH) is calculated to be 21.4 kcal/mol in the absence of substrate. The overall activation energy for the oxidation of camphor involving the somersault motion of the FeO-OH group of P450 model porphyrin iron(III) hydroperoxide [Por(SH)Fe(III)-OOH(-)] --> [Por(SH)Fe(III)-O....HO(-)] in concert with hydrogen abstraction is DeltaE(++) = 12.4 kcal/mol. The corresponding abstraction of the hydrogen atom from the C-H bond of camphor by Cpd I has an activation barrier of 17.6 kcal/mol. Arguments are presented that the somersault rearrangement is induced by steric compression at the active site. Kinetic isotope effect data are discussed that provides compelling evidence for a rate-limiting step involving C-H bond cleavage.
Bach
Robert D
RD
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, USA. rbach@udel.edu
D003577:Cytochrome P-450 Enzyme System / Q000737:chemistry* / Q000378:metabolism; D006838:Hydrocarbons / Q000737:chemistry* / Q000378:metabolism; D006900:Hydroxylation; D007700:Kinetics; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D011789:Quantum Theory; D013816:Thermodynamics
D006838:Hydrocarbons; D003577:Cytochrome P-450 Enzyme System