How O2 Binds to Heme

We have used density functional methods to calculate fully relaxed potential energy curves of the seven lowest electronic states during the binding of O2 to a realistic model of ferrous deoxyheme. Beyond a Fe–O distance of ∼2.5 Å, we find a broad crossing region with five electronic states within 15 kJ/mol. The almost parallel surfaces strongly facilitate spin inversion, which is necessary in the reaction of O2 with heme (deoxyheme is a quintet and O2 a triplet, whereas oxyheme is a singlet). Thus, despite a small spin-orbit coupling in heme, the transition probability approaches unity. Using reasonable parameters, we estimate a transition probability of 0.06–1, which is at least 15 times larger than for the nonbiological Fe–O+ system. Spin crossing is anticipated between the singlet ground state of bound oxyheme, the triplet and septet dissociation states, and a quintet intermediate state. The fact that the quintet state is close in energy to the dissociation couple is of biological importance, because it explains how both spin states of O2 may bind to heme, thereby increasing the overall efficiency of oxygen binding. The activation barrier is estimated to be <15 kJ/mol based on our results and Mössbauer experiments. Our results indicate that both the activation energy and the spin-transition probability are tuned by the porphyrin as well as by the choice of the proximal heme ligand, which is a histidine in the globins. Together, they may accelerate O2 binding to iron by ∼1011 compared with the Fe–O+ system. A similar near degeneracy between spin states is observed in a ferric deoxyheme model with the histidine ligand hydrogen bonded to a carboxylate group, i.e. a model of heme peroxidases, which bind H2O2 in this oxidation state.