Structure and Mechanism of the Quinol-Oxidation Site of the Mitochondrial bc1 Complex

Abstract

It has been widely accepted that the protonmotive Q-cycle describes the general mechanism of electron and proton flow of mitochondrial and bacterial bc 1 complexes. The key reaction of this mechanism is a bifurcation of electron flow at the quinol-oxidation site (center Q p or Q o located on the positive side of the energized membrane. This unique reaction involves reduction of two different redox centers upon oxidation of ubiquinol, the ‘Rieske’ iron-sulfur cluster and heme b L . Inhibitor binding studies show that domains of both proteins are directly involved in forming the reaction pocket. Using yeast, bacterial and mouse cytochrome b mutants and revertants, domains have been assigned to the quinol oxidation center. Mutations at amino acids 137 and 256 in yeast cytochrome b resulted in a markedly weakened association of the iron-sulfur protein, suggesting that these cytochrome b domains are part of the interface between the two proteins. Mitochondrial bc 1 complexes contain seven (yeast) or eight (mammals) so called ‘supernumerary’ subunits. Genetic and biochemical evidence indicates that the 6.4 kDa subunit of bovine and the recently discovered 8.5 kDa subunit of yeast bc 1 complex are involved in the association of the iron-sulfur protein. The subunit of mammalian bc 1 complex not identified in yeast bc 1 complex so far is the targeting presequence of the iron-sulfur protein which remains in the complex after processing. Based on a recent study indicating that the quinol-oxidation center binds two ubiquinone molecules at a time, a detailed functional model for the ubiquinol-oxidation site is presented. The proposed mechanism is a base catalyzed, concerted two-electron process involving the combined action of one ubiquinol and one ubiquinone symproportionating via a charge-transfer complex similar to a quinhydrone.