title: Multiple Q-cycle bypass reactions at the Qo site of the cytochrome bc1 complex.
abstract: 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 ( approximately 25%), 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 O(2) this species can reduce cyt c by some other pathway. We suggest as one possibility that a semiquinone escapes from the Q(o) 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).
authors.affiliation: Institute of Biological Chemistry, 289 Clark Hall, Washington State University, Pullman, WA 99164-6340, USA.
authors.forename: Florian
authors.initials: F
authors.lastname: Muller
authors.forename: Antony R
authors.initials: AR
authors.lastname: Crofts
authors.forename: David M
authors.initials: DM
authors.lastname: Kramer
mesh_terms: D000332:Aerobiosis; D000693:Anaerobiosis; D000968:Antimycin A / Q000737:chemistry; D003574:Cytochrome c Group / Q000037:antagonists & inhibitors / Q000378:metabolism; D004579:Electron Transport; D014450:Electron Transport Complex III / Q000737:chemistry* / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000737:chemistry; D008689:Methacrylates; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology; D013481:Superoxides / Q000378:metabolism; D013844:Thiazoles / Q000737:chemistry; D014451:Ubiquinone / Q000031:analogs & derivatives / Q000737:chemistry* / Q000378:metabolism
chemical_list: D003574:Cytochrome c Group; D004791:Enzyme Inhibitors; D008689:Methacrylates; D013844:Thiazoles; D013481:Superoxides; D014451:Ubiquinone; D000968:Antimycin A; C030517:myxothiazol; D010088:Oxidoreductases; C020094:duroquinol oxidase; D014450:Electron Transport Complex III; C003741:ubiquinol