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

Abstract

The CuA 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 CuA 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 CuA electron transfer invivo. These findings may also prove useful for the development of molecular electronics.