title: Ionic strength dependence of the kinetics of electron transfer from bovine mitochondrial cytochrome c to bovine cytochrome c oxidase.
abstract: The effect of ionic strength on the one-electron reduction of oxidized bovine cytochrome c oxidase by reduced bovine cytochrome c has been studied by using flavin semiquinone reductants generated in situ by laser flash photolysis. In the absence of cytochrome c, direct reduction of the heme a prosthetic group of the oxidase by the one-electron reductant 5-deazariboflavin semiquinone occurred slowly, despite a driving force of approximately +1 V. This is consistent with a sterically inaccessible heme a center. This reduction process was independent of ionic strength from 10 to 100 mM. Addition of cytochrome c resulted in a marked increase in the amount of reduced oxidase generated per laser flash. Reduction of the oxidase at the heme a site was monophasic, whereas oxidation of cytochrome c was multiphasic, the fastest phase corresponding in rate constant to the reduction of the heme a. During the fast kinetic phase, 2 equiv of cytochrome c was oxidized per heme a reduced. We presume that the second equivalent was used to reduce the Cua center, although this was not directly measured. The first-order rate-limiting process which controls electron transfer to the heme a showed a marked ionic strength effect, with a maximum rate constant occurring at mu = 110 mM (1470 s-1), whereas the rate constant obtained at mu = 10 mM was 630 s-1 and at mu = 510 mM was 45 s-1. There was no effect of "pulsing" the enzyme on this rate-limiting one-electron transfer process. These results suggest that there are structural differences in the complex(es) formed between mitochondrial cytochrome c and cytochrome c oxidase at very low and more physiologically relevant ionic strengths, which lead to differences in electron-transfer rate constants.
authors.affiliation: Department of Biochemistry, University of Arizona, Tucson 85721.
authors.forename: J T
authors.initials: JT
authors.lastname: Hazzard
authors.forename: S Y
authors.initials: SY
authors.lastname: Rong
authors.forename: G
authors.initials: G
authors.lastname: Tollin
mesh_terms: D000818:Animals; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism*; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D008928:Mitochondria / Q000378:metabolism*; D009994:Osmolar Concentration; D010084:Oxidation-Reduction; D010782:Photolysis; D012256:Riboflavin / Q000031:analogs & derivatives / Q000378:metabolism; D013053:Spectrophotometry
chemical_list: D003574:Cytochrome c Group; C007228:5-deazariboflavin; D003576:Electron Transport Complex IV; D012256:Riboflavin