Quantum yield of a two-site proton diffusion model: Suggestions for dominant uncoupling processes in cytochrome oxidase

In this paper, we propose a two-redox-site diffusion model to examine the salient features of proton diffusion in the redox-driven proton pump cytochrome oxidase. The key problems addressed are proton diffusion in the presence of an electrostatic bias and the role of proton-uncoupling pathways. The proton quantum yield per electron transferred (H⁺/e^-) is analyzed in the presence of the external bias with two spatially distinct forms of redox-proton uncoupling. Analysis of the proton quantum yield shows that the competition between proton diffusion and proton uncoupling results in the electron-transfer step being rate limiting in our model. In addition, the second redox site is utilized to control the access of the escape of the proton to solution. This feature may be ultimately important in controlling the proton backflows expected when the redox cycle is completed.