The Role of Heme in Monooxygenation Reaction Mechanism by Cytochrome P-450

Abstract

Structures of reaction intermediate that appeared in the monooxygenation reaction cycle by cytochrome P-450 were determined by density functional theory method. Model structures for the calculation were constructed by extracting heme and 5th and 6th ligands of the heme from X-ray crystallographic structure registered in the Protein Data Bank (PDB). Stable structure of oxy-ferrous hemerevealed that there was no bond cleavage of an oxygen molecule at the 6th ligand in reduction by a single electron. The negative charge of the 6th ligand was reduced to only -0.32 e. Subsequently, a ultimate species for the monooxygenation reaction by P-450, which was proposed in the previous paper [3], was produced in the 6th ligand of heme. The stable structure revealed that no O-O bond cleavage occurred, though distance of the 0-0 bond of the 6th ligand increased from that of the oxy ferrous heme. The total charge of the 6th ligand was approximately neutral. A set of structural changes of the 6th ligand of heme in the reaction process until the production of the ultimate active species by the introduction of two electrons and the binding of two protons was the same as those of the 6th ligand alone. For this reason, we concluded that the monooxygenation reaction mechanism by P-450 might be the reaction between the 6th ligand of heme and the substrate. The role of heme in themonooxygenation reaction mechanism by P-450 might be to transfer electrical charge to the bindingoxygen of the 6th ligand, keeping interaction between an oxygen atom that is not directly binding with heme Fe atom and the oxygenating site of the substrate.