Bovine cytochrome c oxidase structures enable O₂ reduction with minimization of reactive oxygens and provide a proton-pumping gate

The O₂ reduction site of cytochrome c oxidase (CcO), comprising iron (Fea₃) and copper (CuB) ions, is probed by x-ray structural analyses of CO, NO, and CN⁻ derivatives to investigate the mechanism of the complete reduction of O₂. Formation of the Formula derivative contributes to the trigonal planar coordination of Formula and displaces one of its three coordinated imidazole groups while a water molecule becomes hydrogen bonded to both the CN⁻ ligand and the hydroxyl group of Tyr244. When O₂ is bound to Formula , it is negatively polarized (Formula ), and expected to induce the same structural change induced by CN⁻. This structural change allows Formula to receive three electron equivalents nonsequentially from Formula , Formula , and Tyr-OH, providing complete reduction of O₂ with minimization of production of active oxygen species. The proton-pumping pathway of bovine CcO comprises a hydrogen-bond network and a water channel which extend to the positive and negative side surfaces, respectively. Protons transferred through the water channel are pumped through the hydrogen-bond network electrostatically with positive charge created at the Fea center by electron donation to the O₂ reduction site. Binding of CO or NO to Formula induces significant narrowing of a section of the water channel near the hydrogen-bond network junction, which prevents access of water molecules to the network. In a similar manner, O₂ binding to Formula is expected to prevent access of water molecules to the hydrogen-bond network. This blocks proton back-leak from the network and provides an efficient gate for proton-pumping.