Electronic properties of pentacoordinated heme complexes in cytochrome P450 enzymes: search for an Fe(I) oxidation state

The cytochrome P450s are versatile enzymes that catalyze a range of monoxygenation reactions. Their catalytic cycle includes molecular oxygen binding and two reduction and protonation steps to create the active species, Compound I. In an anaerobic environment, however, only two reduction steps starting from the resting state can take place. Currently, very little information is known on this doubly-reduced species; therefore, we have performed a detailed density functional theory (DFT) and combined quantum mechanics/molecular mechanics (QM/MM) study on this complex. In principle, the doubly-reduced pentacoordinated heme can exist in two possible electronic configurations, namely an Fe(I) with closed-shell heme or an Fe(II) coupled to a heme anion radical [Fe^II Por⁻˙]. Our calculations show that there are several close-lying spin states with a [Fe^II Por⁻˙] configuration and these states are much lower in energy than the alternative [Fe^I Por⁰] situation. We have calculated spectroscopic parameters of the lowest lying sextet spin state, including IR spectra and EPR parameters.