Quantum and molecular mechanical study of the first proton transfer in the catalytic cycle of cytochrome P450cam and its mutant D251N

In the catalytic cycle of cytochrome P450cam, the hydroperoxo intermediate (Cpd 0) is formed by proton transfer from a reduced oxyheme complex (S5), This process is drastically slowed down when Asp251 is mutated to Asn (D251N). We report quantum mechanical/molecular mechanical (QM/MM) calculations that address this proton delivery in the doublet state through a hydrogen-bond network in the Asp251 channel, both for the wild-type enzyme and the D251N mutant, using four different active-site models. For the wildtype, we find a facile concerted mechanism for proton transfer from protonated Asp251 via Wat901 and Thr252 to the FeOO moiety, with a barrier of about 1 kcal/mol and a high exothermicity of more than 20 kcal/mol. In the D251N mutant with a neutral Asn251 residue, the proton transfer is almost thermoneutral or slightly exothermic in the three models considered. It is still very facile when the Asn251 residue adopts a conformation analogous to Asp251 in the wild-type enzyme, but the barrier increases significantly when the Asn251 side chain flips (as indicated by classical molecular dynamics simulations). This flip disrupts the hydrogen-bond network and hence the proton-transfer pathway, which causes a longer lifetime of S5 in the D251N mutant (consistent with experimental observations). The entry of an additional water molecule into the active site of D251N with flipped Asn251 regenerates the hydrogen-bond network and provides a viable mechanism for proton delivery in the mutant, with a moderate barrier of about 7 kcal/mol.