Mechanisms of O-O Bond Cleavage and Transfer of an Oxygen Atom to a Substrate in Monooxygenation Reaction by Cytochrome P-450

Abstract

Mechanisms of 0-0 bond cleavage and transfer of an oxygen atom to a substrate, from a proposed ultimate species for the monooxygenation reaction by P-450 [4] were investigated using the first principle calculation with density functional theory (DFT) and PM3 method. If the substrate does not exist in the active site, cleavage of oxygen molecule binding at the 6th ligand site does not occur by reduction of two electrons and subsequent reaction with a couple of protons. The monooxygenation reaction occurs spontaneously by the reduction of two electrons and the subsequent reaction with two protons when there is an interaction between the oxygen molecule at the 6th ligand of heme and substrate. Application of this result to selective hydroxylation of C5 atom of d-camphor by P-450cam corresponds to experimental results; i.e., C5 atom hydroxylation occurred in one step with an activa-tion energy of 5.6 kcal/mol and is a possible reaction at body temperature. The oxygen atom transfer occurred at C9-H bond in unusual conditions in which the transfer did not occur at C5-H bond; e.g., replacement of hydrogen atoms binding C5 atom with fluorine atoms. Reaction with C4 atom which is near C5 atom did not occur because of large activation energy (9.8 kcal/mol).