The oxidation-reduction kinetics of cytochromes b, c1 and c in initially fully reduced mitochondrial membranes are in agreement with the Q-cycle hypothesis.
Stopped-flow experiments were performed to distinguish between two hypotheses, the Q-cycle and the SQ-cycle, each describing the pathway of electron transfer in the QH2:cytochrome c oxidoreductases. It was observed that, when mitochondrial membranes from the yeast Saccharomyces cerevisiae were poised at a low redox potential with appropriate amounts of sodium dithionite to completely reduce cytochrome b, the kinetics of oxidation of cytochrome b showed a lag period of maximally 100 ms. Under the same experimental conditions, the oxidation-reduction kinetics of cytochromes c + c1 showed transient behaviour. These results do not support the presence of a mobile species of semiquinone in the QH2:cytochrome c oxidoreductases, as envisaged in the SQ-cycle, but are consistent with a Q-cycle mechanism in which the two quinone-binding domains do not exchange electrons directly on the timescale of turnover of the enzyme.
de Vries
S
S
Laboratory of Biochemistry, University of Amsterdam, The Netherlands.
van Hoek
A N
AN
Berden
J A
JA
D000968:Antimycin A / Q000031:analogs & derivatives / Q000494:pharmacology; D016227:Benzoquinones; D003573:Cytochrome b Group / Q000037:antagonists & inhibitors / Q000378:metabolism*; D003574:Cytochrome c Group / Q000031:analogs & derivatives* / Q000037:antagonists & inhibitors / Q000378:metabolism*; D003575:Cytochromes c1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004227:Dithionite / Q000494:pharmacology; D006873:Hydroquinones / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D007700:Kinetics; D008689:Methacrylates; D008928:Mitochondria / Q000378:metabolism*; D008954:Models, Biological; D010084:Oxidation-Reduction; D011809:Quinones / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000648:ultrastructure*; D013386:Succinates / Q000494:pharmacology; D019802:Succinic Acid; D013844:Thiazoles / Q000494:pharmacology
D016227:Benzoquinones; D003573:Cytochrome b Group; D003574:Cytochrome c Group; D006873:Hydroquinones; D008689:Methacrylates; D011809:Quinones; D013386:Succinates; D013844:Thiazoles; C032456:antimycin; D004227:Dithionite; C004532:quinone; D000968:Antimycin A; C030517:myxothiazol; D003575:Cytochromes c1; D019802:Succinic Acid