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Trihaem cytochromec3 (also known as cytochrome c551.5 and cytochromec7 ) is isolated fromthe periplasmic space of Desulfuromonas acetoxidans, a sulfur-reducing bacterium. Thermodynamic and kinetic data for the trihaem cyto-chromec3are presented and discussed in the context of the possible physiological implications of its functional proper-ties with respect to the natural habitat of D. acetoxidans, namelyas a symbiontwithgreensulfurbacteriaworkingas a mini-sulfuretum. | Eur. J. Biochem. 269 5722-5730 2002 FEBS 2002 doi 10.1046 j.1432-1033.2002.03286.x Thermodynamic and kinetic characterization of trihaem cytochrome c3 from Desulfuromonas acetoxidans Ilidio J. Correia1 Catarina M. Paquete1 Ricardo O. Louro1 Teresa Catarino1 2 David L. Turner1 3 and Antonio V. Xavier1 1Instituto de Tecnologia Quimica e Biologica and 2Departamento de Quimica da Faculdade de Ciencias e Tecnologia Universidade Nova de Lisboa Portugal 3Department of Chemistry University of Southampton England Trihaem cytochrome c3 also known as cytochrome c551.5 and cytochrome c7 is isolated from the periplasmic space of Desufuromonas acetoxidans a sulfur-reducing bacterium. Thermodynamic and kinetic data for the trihaem cytochrome c3 are presented and discussed in the context of the possible physiological implications of its functional properties with respect to the natural habitat of D. acetoxidans namely as a symbiont with green sulfur bacteria working as a mini-sulfuretum. The thermodynamic properties were determined through the fit of redox titration data followed by NMR and visible spectroscopy to a model of four functional centres that describes the network of cooperativities between the three haems and one protolytic centre. The kinetics of trihaem cytochrome c3 reduction by sodium dithionite were studied using the stopped-flow technique and the data were fitted to a kinetic model that makes use of the thermodynamic properties to obtain the rate constants of the individual haems. This analysis indicates that the electrons enter the cytochrome mainly via haem I. The reduction potentials of the haems in this cytochrome show little variation with pH within the physiological range and the kinetic studies show that the rates of reduction are also independent of pH in the range studied. Thus although the trihaem cytochrome c3 is readily reduced by hydrogenases from Desulfovibrio sp. and its haem core is similar to that of the homologous tetrahaem cytochromes c3 its .
