ISSN:
1573-6881
Keywords:
iron–sulfur protein
;
midpoint potential
;
cytochrome bc 1 complex
;
ubiquinol
;
cytochrome c 1
;
hydrogen bonds
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
,
Chemistry and Pharmacology
,
Physics
Notes:
Abstract The midpoint potential of the [2Fe–2S] cluster of the Rieske iron–sulfurprotein (E m 7 = +280mV) is the primary determinant of the rate of electron transfer from ubiquinol to cytochromec catalyzed by the cytochrome bc 1 complex. As the midpoint potential of the Rieske clusteris lowered by altering the electronic environment surrounding the cluster, theubiquinol-cytochrome c reductase activity of the bc 1 complex decreases; between 220 and 280 mV therate changes 2.5-fold. The midpoint potential of the Rieske cluster also affects thepresteady-state kinetics of cytochrome b and c 1 reduction. When the midpoint potential of the Rieskecluster is more positive than that of the heme of cytochrome c 1, reduction of cytochrome bis biphasic. The fast phase of b reduction is linked to the optically invisible reduction of theRieske center, while the rate of the second, slow phase matches that of c 1 reduction. The ratesof b and c 1 reduction become slower as the potential of the Rieske cluster decreases andchange from biphasic to monophasic as the Rieske potential approaches that of theubiquinone/ubiquinol couple. Reduction of b and c 1 remain kinetically linked as the midpoint potentialof the Rieske cluster is varied by 180 mV and under conditions where the presteady statereduction is biphasic or monophasic. The persistent linkage of the rates of b and c 1 reduction isaccounted for by the bifurcated oxidation of ubiquinol that is unique to the Q-cycle mechanism.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1005419712731
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