tailieunhanh - Báo cáo Y học: Mutations in the docking site for cytochrome c on the Paracoccus heme aa3 oxidase Electron entry and kinetic phases of the reaction

Introducing site-directed mutations in surface-exposed residues of subunit II of the heme aa3 cytochrome c oxidase of Paracoccus denitrificans, we analyze the kinetic parameters of electron transfer from reduced horse heart cytochrome c. Specifically we address the following issues: (a) which residues on oxidase contribute to the docking site for cytochrome c, (b) is an aromatic side chain required for electron entry from cytochrome c, and (c) what is the molecular basis for the previously observed biphasic reaction kinetics | Eur. J. Biochem. 269 2980-2988 2002 FEBS 2002 doi Mutations in the docking site for cytochrome c on the Paracoccus heme aa3 oxidase Electron entry and kinetic phases of the reaction Viktoria Drosou1 Francesco Malatesta2 and Bernd Ludwig1 1 Molecular Genetics Institute of Biochemistry Johann-Wolfgang-Goethe Universitat Frankfurt Germany 2Department of Basic and Applied Biology University of L Aquila Italy Introducing site-directed mutations in surface-exposed residues of subunit II of the heme aa3 cytochrome c oxidase of Paracoccus denitrificans we analyze the kinetic parameters of electron transfer from reduced horse heart cytochrome c. Specifically we address the following issues a which residues on oxidase contribute to the docking site for cytochrome c b is an aromatic side chain required for electron entry from cytochrome c and c what is the molecular basis for the previously observed biphasic reaction kinetics. From our data we conclude that tryptophan 121 on subunit II is the sole entry point for electrons on their way to the CuA center and that its precise spatial arrangement but not its aromatic nature is a prerequisite for efficient electron transfer. With different reaction partners and experimental conditions biphasicity can always be induced and is critically dependent on the ionic strength during the reaction. For an alternative explanation to account for this phenomenon we find no evidence for a second cytochrome c binding site on oxidase. Keywords Paracoccus denitrificans cytochrome c oxidase docking site electron transfer biphasic kinetics. Cytochrome c oxidase is the terminal complex of the respiratory chains of mitochondria and many bacteria 1-4 . It catalyzes the reduction of oxygen to water coupling the free energy of this reaction to the generation of a proton gradient across the membrane. During the redox reaction an electron delivered from cytochrome c is first transferred to CuA a binuclear copper center .

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