Energy transduction function of the quinone reactions in cytochrome bc complexes

Cytochrome bc1, a multi-subunit integral membrane protein complex found in mammalian mitochondria, plays a central role in the transfer of electrons and protons generated by the oxidation of ubiquinol. According to the classical chemiosmotic theory, quinones shuttle protons across the hydrophobic membrane bilayer with the net result of H+ transfer to the aqueous side and generation of an electrochemical proton gradient. Recently, high-resolution structures of the mitochondrial bc1 complex showed quinone binding sites at one of the transmembrane helices of cytochrome b, and two potentially protonatable histidine residues on the Rieske iron-sulfur protein. The modern biochemical refinements of the original chemiosmotic theory require electron and proton transfer from quinones to particular residues/redox centers of integral membrane proteins and subsequent transfer of H+ to the bulk aqueous phase outside the membrane.