Bovine Cytochrome Oxidase Structures Enable Molecular Oxygen Reduction without Formation of Active Oxygen Species, Providing a Proton Pumping Gate

Abstract

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 Fea32+-CN- derivative contributes to the trigonal planar coordination of CuB1+ 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 Fea32+, it is negatively polarized (O2-), and expected to induce the same structural change induced by CN-. This allows O2- to receive three electron equivalents non-sequentially from CuB1+, Fea33+ 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 Fea32+ 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 Fea32+ 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.