Common system setup for the entire catalytic cycle of cytochrome P450 _cam in quantum mechanical/molecular mechanical studies

We describe a system setup that is applicable to all species in the catalytic cycle of cytochrome P450_cam. The chosen procedure starts from the X-ray coordinates of the ferrous dioxygen complex and follows a protocol that includes the careful assignment of protonation states, comparison between different conceivable hydration schemes, and system preparation through a series of classical minimizations and molecular dynamics (MD) simulations. The resulting setup was validated by quantum mechanical/molecular mechanical (QM/MM) calculations on the resting state, the pentacoordinated ferric and ferrous complexes, Compound I, the transition state and hydroxo intermediate of the C-H hydroxylation reaction, and the product complex. The present QM/MM results are generally consistent with those obtained previously with individual setups. Concerning hydration, we find that saturating the protein interior with water is detrimental and leads to higher structural flexibility and catalytically inefficient active-site geometries. The MD simulations favor a low water density around Asp251 that facilitates side chain rotation of protonated Asp251 during the conversion of Compound 0 to Compound I. The QM/MM results for the two preferred hydration schemes (labeled SE-1 and SE-4) are similar, indicating that slight differences in the solvation close to the active site are not critical as long as camphor and the crystallographic water molecules preserve their positions in the experimental X-ray structures.