title: Characterization of the interaction of cytochrome c and mitochondrial ubiquinol-cytochrome c reductase.
abstract: Characterization of the steady state kinetics of reduction of horse ferricytochrome c by purified beef ubiquinol-cytochrome c reductase, employing 2,3-dimethoxy-5-methyl-6-decylbenzoquinol as reductant, has shown that: 1) the dependence of the reaction on quinol and on ferricytochrome c concentration is consistent with a ping-pong mechanism; 2) the pH optimum of the reaction is near 8.0; 3) the effect of ionic strength on the apparent Km and the TNmax of the reaction for the native cytochrome c is small, and at higher cytochrome c concentrations substrate inhibition is observed; 4) the effect of ionic strength on the kinetic parameters for the reaction of 4-carboxy-2,6-dinitrophenyllysine 27 horse cytochrome c is much larger than for the native protein; and 5) competitive product inhibition is also observed with a Ki consistent with the binding affinity of ferrocytochrome c for Complex III, as determined by gel filtration. In addition, direct binding measurements demonstrated that ferricytochrome c binds more tightly than the reduced protein to Complex III under low ionic strength conditions and that under these conditions more than one molecule of cytochrome c is bound per molecule of Complex III. Exchange of Complex III into a nonionic detergent decreases this excess nonspecific binding. Measurement of the rates of dissociation of the oxidized and reduced 1:1 complexes of cytochrome c and Complex III by stopped flow was consistent with the disparity of binding affinities, the dissociation rate constant for ferrocytochrome c being about 5-fold higher than that for the ferric protein. A model which accounts for the properties of this system is described, assuming that cytochrome c bound to noncatalytic sites on the respiratory complex decreases the catalytic site binding constant for the substrate.
mesh_terms: D000818:Animals; D003574:Cytochrome c Group / Q000378:metabolism*; D014450:Electron Transport Complex III; D006736:Horses; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008928:Mitochondria / Q000201:enzymology*; D008956:Models, Chemical; D009097:Multienzyme Complexes / Q000378:metabolism*; D009247:NADH, NADPH Oxidoreductases / Q000378:metabolism*; D009994:Osmolar Concentration; D011808:Quinone Reductases / Q000378:metabolism*
chemical_list: D003574:Cytochrome c Group; D009097:Multienzyme Complexes; D009247:NADH, NADPH Oxidoreductases; D011808:Quinone Reductases; D014450:Electron Transport Complex III