Reversible Switching of Redox-Active Molecular Orbitals and Electron Transfer Pathways in CuA Sites of Cytochrome c Oxidase

The Cu-A site of cytochromec oxidase is a redox hub that participates in rapid electron transfer at low driving forces with two redox cofactors in nearly perpendicular orientations. Spectroscopic and electrochemical characterizations performed on first and second-sphere mutants have allowed us to experimentally detect the reversible switching between two alternative electronic states that confer different directionalities to the redox reaction. Specifically, the M160H variant of a native Cu-A shows a reversible pH transition that allows to functionally probe both states in the same protein species. Alternation between states exerts a dramatic impact on the kinetic redox parameters, thereby suggesting this effect as the mechanism underlying the efficiency and directionality of Cu-A electron transfer invivo. These findings may also prove useful for the development of molecular electronics.