Protonation of Compound 0 in the catalytic cycle of cytochrome P450cam may lead to the formation of either the reactive Compound I (coupling) or the ferric resting state (Uncoupling). In this work, we investigate the effect of the D251N mutation oil the coupling and uncoupling reaction by combined quantum mechanics/ molecular mechanics (QM/MM) calculations. The mutated Asn251 residue has two possible orientations, i.e. directed toward the active site (no flip) or away from the active site (flip), with the latter one being preferred in classical molecular dynamics (MD) simulations. The possible proton transfer mechanisms in the Coupling and uncoupling reaction were studied for three models of the D251N mutant, i.e. no flip (model I), flip (model II), and flip with an extra water (model III). According to the QM/MM calculations, the uncoupling reaction is always less favorable than the coupling reaction. The coupling reaction in the D251N mutant follows the same mechanism as in the wild-type enzyme, with initial O-O cleavage followed by proton transfer. The barrier for the initial step is similar in all D251N models, but the proton transfer is most facile in model III. The hydroxide anion formed in model III is not reprotonated easily by neighboring residues, while proton delivery from bulk solvent seems possible via a water network that remains intact during 2 ns classical MD simulation. The computational results are consistent with the experimental findings that the Coupling reaction dominates the consumption of dioxygen in the D251N mutant, but with lower activity than in the wild-type enzyme.