A structural model for across membrane coupling between the Qo and Qi active sites of cytochrome bc1

The two spatially distant quinone-binding sites of the ubihydroquinone: cytochrome c oxidoreductase (cyt bc1) complex have been shown to influence one another in some fashion. This transmembrane communication alters cofactor and redox partner binding interactions and could potentially influence the timing or ‘concerted’ steps involved in the steady-state turnover of the homodimeric enzymes. Yet, despite several lines of evidence corroborating the coupling of the quinone binding active sites to one another, little to no testable hypothesis has been offered to explain how such a “signal” might be transmitted across the presumably rigid hydrophobic domain of the enzyme. Recently, it has been shown that this interquinone binding sites communication influences the steady-state position of the mobile [2Fe–2S] cluster containing iron sulfur protein (Sarewicz M., Dutka M., Froncisz W., Osyczka A. (2009) Biochemistry 48, 5708–5720) as mediated by at least one transmembrane helix of the b-type cyt containing subunit (Cooley, J. W., Lee, D. W., and Daldal, F. (2009) Biochemistry 48, 1988–1999). Here we provide an overview of the evidence supporting the structural coupling of these sites and provide a theoretical framework for how the redox state of a quinone at one cofactor binding site might influence the cofactor–, inhibitor–, and/or protein–protein interactions at the structurally distant opposing Q binding site.