title: Transient kinetics of subunit-III-depleted cytochrome c oxidase.
abstract: Cytochrome c oxidase from ox heart was depleted of subunit III and its transient kinetic properties studied by stopped-flow and flash photolysis. It was found that the overall mechanism of electron transfer is very similar for subunit-III-depleted and native oxidase, although significant differences in some kinetic parameters have been detected. These include the second-order rate constant for cytochrome c oxidation and the rate-limiting step of the overall process. Moreover, at low cytochrome c/oxidase ratios (where the number of reducing equivalents is insufficient), the rate of reoxidation of cytochrome a was found to be very slow, even in air, and in fact for the subunit-III-depleted enzyme is even slower than for the native oxidase. The stability of reduced cytochrome a excludes the likelihood that removal of subunit III leads to a new O2-binding site, and the result may be relevant to the lowered vectorial H+/e- stoichiometry. The subunit-III-depleted oxidase can be pulsed under appropriate conditions and its combination with CO is unchanged, as shown by kinetic experiments and difference spectroscopy.
mesh_terms: D000818:Animals; D002248:Carbon Monoxide / Q000378:metabolism; D002417:Cattle; D017936:Cytochrome a Group; D003574:Cytochrome c Group / Q000378:metabolism; D003580:Cytochromes / Q000378:metabolism; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000378:metabolism*; D007700:Kinetics; D009206:Myocardium / Q000201:enzymology; D010084:Oxidation-Reduction; D010782:Photolysis
chemical_list: D017936:Cytochrome a Group; D003574:Cytochrome c Group; D003580:Cytochromes; D002248:Carbon Monoxide; D003576:Electron Transport Complex IV