Multiple Q-cycle bypass reactions at the Qo site of the cytochiome bc1 complex

The cytochrome (cyt) bc(1) complex is central to energy transduction in many species. Most investigators now accept a modified Q-cycle as the catalytic mechanism of this enzyme. Several thermodynamically favorable side reactions must be minimized for efficient functioning of the Q-cycle. Among these, reduction of oxygen by the Q(o) site semiquinone to produce superoxide is of special pathobiological interest. These superoxide-producing bypass reactions are most notably observed as the antimycin A- or myxothiazol-resistant reduction of cyt c. In this work, we demonstrate that these inhibitor-resistant cyt c reductase activities are largely unaffected by removal of O-2 in the isolated yeast cyt bc(1) complex. further, increasing O-2 tension 5-fold stimulated the antimycin A-resistant reduction by a small amount (similar to25%), while leaving the myxothiazol-resistant reduction unchanged. This most likely indicates that the rate-limiting step in superoxide production is the formation of a reactive species (probably a semiquinone), capable of rapid O-2 reduction, and that in the absence Of 02 this species can reduce cyt c by some other pathway. We suggest as one possibility that a semiquinone escapes from the Qo site and reduces either O-2 or cyt c directly. The small increase in antimycin A-resistant cyt c reduction rate at high O-2 can be explained by the accumulation of a low concentration of a semiquinone inside the Q(o) site. Under aerobic conditions, addition of saturating levels of superoxide dismutase (SOD) inhibited 50% of cyt c reduction in the presence of myxothiazol, implying that essentially all bypass reactions occur with the production of superoxide. However, SOD inhibited only 35% of antimycin A-resistant cyt c reduction, suggesting the presence of a second, slower bypass reaction that does not reduce O-2. Given that myxothiazol blocks cyt b reduction whereas antimycin A promotes it, we propose that this second bypass occurs by reduction of the Q(o) site semiquinone by prereduced cyt b(L).