Role of the Rieske iron-sulfur protein midpoint potential in the protonmotive Q-cycle mechanism of the cytochrome bc1 complex.

The midpoint potential of the [2Fe-2S] cluster of the Rieske iron-sulfur protein (Em7 = +280 mV) is the primary determinant of the rate of electron transfer from ubiquinol to cytochrome c catalyzed by the cytochrome bc1 complex. As the midpoint potential of the Rieske cluster is lowered by altering the electronic environment surrounding the cluster, the ubiquinol-cytochrome c reductase activity of the bc1 complex decreases; between 220 and 280 mV the rate changes 2.5-fold. The midpoint potential of the Rieske cluster also affects the presteady-state kinetics of cytochrome b and c1 reduction. When the midpoint potential of the Rieske cluster is more positive than that of the heme of cytochrome c1, reduction of cytochrome b is biphasic. The fast phase of b reduction is linked to the optically invisible reduction of the Rieske center, while the rate of the second, slow phase matches that of c1 reduction. The rates of b and c1 reduction become slower as the potential of the Rieske cluster decreases and change from biphasic to monophasic as the Rieske potential approaches that of the ubiquinone/ubiquinol couple. Reduction of b and c1 remain kinetically linked as the midpoint potential of the Rieske cluster is varied by 180 mV and under conditions where the presteady state reduction is biphasic or monophasic. The persistent linkage of the rates of b and c1 reduction is accounted for by the bifurcated oxidation of ubiquinol that is unique to the Q-cycle mechanism.