The oxidation of ubiquinol by the isolated Rieske iron-sulfur protein in solution.
The pre-steady-state redox reactions of the Rieske iron-sulfur protein isolated from beef heart mitochondria have been characterized. The rates of oxidation by c-type cytochromes is much faster than the rate of reduction by ubiquinols. This enables the monitoring of the oxidation of ubiquinols by the Rieske protein through the steady-state electron transfer to cytochrome c in solution. The pH and ionic strength dependence of this reaction indicate that the ubiquinol anion is the direct reductant of the oxidized cluster of the iron-sulfur protein. The second electron from ubiquinol is diverted to oxygen by the isolated Rieske protein, and forms oxygen radicals that contribute to the steady-state reduction of cytochrome c. Under anaerobic conditions, however, the reduction of cytochrome c catalyzed by the protein becomes mechanicistically identical to the chemical reduction by ubiquinols. The present kinetic work outlines that: (i) the electron transfer between the ubiquinol anion and the Rieske cluster has a comparable rate when the protein is isolated or inserted into the parent cytochrome c reductase enzyme; (ii) the Rieske protein may be a relevant generator of oxygen radicals during mitochondrial respiration.
DegliEsposti
M
M
Department of Biology, University of Bologna, Italy.
Ballester
F
F
Timoneda
J
J
Crimi
M
M
Lenaz
G
G
D000818:Animals; D002417:Cattle; D003580:Cytochromes / Q000378:metabolism; D014450:Electron Transport Complex III / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007506:Iron-Sulfur Proteins / Q000378:metabolism*; D007700:Kinetics; D008929:Mitochondria, Heart / Q000378:metabolism*; D010084:Oxidation-Reduction; D012996:Solutions; D014451:Ubiquinone / Q000378:metabolism* / Q000031:analogs & derivatives
D003580:Cytochromes; D007506:Iron-Sulfur Proteins; C037989:Rieske iron-sulfur protein; D012996:Solutions; D014451:Ubiquinone; D014450:Electron Transport Complex III; C003741:ubiquinol