QM/MM Study of Mechanisms for Compound I Formation in the Catalytic Cycle of Cytochrome P450cam

Abstract

In the catalytic cycle of cytochrome P450cam, after molecular oxygen binds as a ligand to the heme iron atom to yield a ferrous dioxygen complex, there are fast proton transfers that lead to the formation of the active species, Compound I (Cpd I), which are not well understood because they occur so rapidly. In the present work, the conversion of the ferric hydroperoxo complex (Cpd 0) to Cpd I has been investigated by combined quantum-mechanical/molecular-mechanical (QM/MM) calculations. The residues Asp₂₅₁ and Glu₃₆₆ are considered as proton sources. In mechanism I, a proton is transported to the distal oxygen atom of the hydroperoxo group via a hydrogen bonding network to form protonated Cpd 0 (prot-Cpd0: FeOOH₂), followed by heterolytic O−O bond cleavage that generates Cpd I and water. Although a local minimum is found for prot-Cpd0 in the Glu₃₆₆ channel, it is very high in energy (more than 20 kcal/mol above Cpd 0) and the barriers for its decay are only 3−4 kcal/mol (both toward Cpd 0 and Cpd I). In mechanism II, an initial O−O bond cleavage followed by a concomitant proton and electron transfer yields Cpd I and water. The rate-limiting step in mechanism II is O−O cleavage with a barrier of about 13−14 kcal/mol. According to the QM/MM calculations, the favored low-energy pathway to Cpd I is provided by mechanism II in the Asp₂₅₁ channel. Cpd 0 and Cpd I are of similar energies, with a slight preference for Cpd I.