The dimeric structure of the cytochrome bc1 complex prevents center P inhibition by reverse reactions at center N

Energy transduction in the cytochrome bc1 complex is achieved by catalyzing opposite oxido-reduction reactions at two different quinone binding sites. We have determined the pre-steady state kinetics of cytochrome b and c1 reduction at varying quinol/quinone ratios in the isolated yeast bc1 complex to investigate the mechanisms that minimize inhibition of quinol oxidation at center P by reduction of the bH heme through center N. The faster rate of initial cytochrome b reduction as well as its lower sensitivity to quinone concentrations with respect to cytochrome c1 reduction indicated that the bH hemes equilibrated with the quinone pool through center N before significant catalysis at center P occurred. The extent of this initial cytochrome b reduction corresponded to a level of bH heme reduction of 33%-55% depending on the quinol/quinone ratio. The extent of initial cytochrome c1 reduction remained constant as long as the fast electron equilibration through center N reduced no more than 50% of the bH hemes. Using kinetic modeling, the resilience of center P catalysis to inhibition caused by partial pre-reduction of the bH hemes was explained using kinetics in terms of the dimeric structure of the bc1 complex which allows electrons to equilibrate between monomers.