Functional and dysfunctional enzymatic pathways of cytochrome P450s after formation of the reduced ferrous dioxygen species have been investigated using nonlocal density functional quantum chemical methods, employing a methyl mercapto iron porphine model of the cytochrome P450 heme complex. The goal of this study was to assess the validity of proposed pathways to both compound I and peroxide involving protonation of the distal and proximal oxygen atoms of the reduced ferrous dioxygen species. Optimized geometries, energies, and electrostatic and electronic properties of each putative heme intermediate in these pathways were calculated and these properties examined for consistency with the proposed role of the intermediate in compound I or peroxide formation. Single protonation of the distal oxygen resulted in significant weakening of the O?O bond. Addition of a second proton to the distal oxygen and energy optimization led directly to compound I and water products, without any apparent activation barrier or formation of a diprotonated intermediate. These results provide direct robust support for the proton-assisted mechanism of dioxygen bond cleavage to form compound I. The dysfunctional pathway to the formation of peroxide was explored by examining the properties of the distal and proximal singly protonated species. The proximal tautomer is thermodynamically less favorable than the distal species by 18.4 kcal/mol. Electrostatic features of both singly protonated species suggest preferred proton delivery to the remaining unprotonated oxygen in each case, favoring peroxide formation. Moreover, addition of a second proton to either of these singly protonated species results in formation of a stable hydrogen peroxide heme complex. These results, taken together, suggest that the simultaneous availability of two protons on the distal oxygen is a requirement for P450 enzymatic efficacy, while asynchronous delivery of protons to the dioxygen site favors decoupling.
Theoretical investigation of the proton assisted pathway to formation of cytochrome P450 compound I