Peripheral heme substituents control the hydrogen-atom abstraction chemistry in cytochromes P450.

We elucidate the hydroxylation of camphor by cytochrome P450 with the use of density functional and mixed quantum mechanics/molecular mechanics methods. Our results reveal that the enzyme catalyzes the hydrogen-atom abstraction step with a remarkably low free-energy barrier. This result provides a satisfactory explanation for the experimental failure to trap the proposed catalytically competent high-valent heme Fe(IV) oxo (oxyferryl) species responsible for this hydroxylation chemistry. The primary and previously unappreciated contribution to stabilization of the transition state is the interaction of positively charged residues in the active-site cavity with carboxylate groups on the heme periphery. A similar stabilization found in dioxygen binding to hemerythrin, albeit with reversed polarity, suggests that this mechanism for controlling the relative energetics of redox-active intermediates and transition states in metalloproteins may be widespread in nature.