Laser flash photolysis studies of electron transfer in complex III from yeast mitochondria.
The kinetics of reduction of cytochrome b and cytochrome c1 of yeast Complex III by 5-deazariboflavin semiquinone, generated by laser flash photolysis under anaerobic conditions, have been investigated. The reduction of cytochrome b occurs in two phases with first-order rate constants of 1300 and 670 s-1, whereas the reduction of cytochrome c1 appears as a unique exponential phase with an intermediate value of 800 s-1. Under these experimental conditions, about 50% of cytochrome b is reduced in comparison with cytochrome c1. After photoreduction, the re-oxidation of the cytochromes by internal re-equilibrium occurs in both cases, following pseudo-first-order kinetics at a rate constant of 43 s-1 for cytochrome b and 39 s-1 for cytochrome c1. These results, which agree with the data from the rapid mixing technique (A.-L. Tsai, J.S. Olson, G. Palmer, J. Biol. Chem. 262 (1987) 8677-8684), have implications for the mechanistic understanding of inner Complex III electron transfer. One of the goals of the investigation reported here is to provide direct evidence for the hypothesis of a proton-motive Q cycle for the mechanism of electron transfer in Complex III. Moreover, these results demonstrate the usefulness of laser flash photolysis in studying the redox kinetic properties of mitochondrial Complex III.
de la Rosa
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Departamento de Bioquímica Vegetal y Biología Molecular, Universidad de Sevilla, Spain.
D003573:Cytochrome b Group / Q000378:metabolism; D003575:Cytochromes c1 / Q000378:metabolism; D004579:Electron Transport; D014450:Electron Transport Complex III / Q000378:metabolism* / Q000528:radiation effects; D007834:Lasers; D008928:Mitochondria / Q000201:enzymology* / Q000528:radiation effects; D010782:Photolysis; D012441:Saccharomyces cerevisiae; D013054:Spectrophotometry, Atomic
D003573:Cytochrome b Group; D003575:Cytochromes c1; D014450:Electron Transport Complex III