Understanding the cytochrome c oxidase proton pump: thermodynamics of redox linkage.

The cytochrome c oxidase complex (CcO) catalyzes the four-electron reduction of dioxygen to water by using electrons from ferrocytochrome c. Redox free energy released in this highly exergonic process is utilized to drive the translocation of protons across a transmembrane electrochemical gradient. Although numerous chemical models of proton pumping have been developed, few attempts have been made to explain the stepwise transfer of energy in the context of proposed protein conformational changes. A model is described that seeks to clarify the thermodynamics of the proton pumping function of CcO and that illustrates the importance of electron and proton gating to prevent the occurrence of the more exergonic electron leak and proton slip reactions. The redox energy of the CcO-membrane system is formulated in terms of a multidimensional energy surface projected into two dimensions, a nuclear coordinate associated with electron transfer and a nuclear coordinate associated with elements of the proton pump. This model provides an understanding of how a transmembrane electrochemical gradient affects the efficiency of the proton pumping process. Specifically, electron leak and proton slip reactions become kinetically viable as a result of the greater energy barriers that develop for the desired reactions in the presence of a transmembrane potential.