Thermodynamic and kinetic characterization of trihaem cytochrome c(3) from Desulfuromonas acetoxidans

Correia, Ilídio J., Paquete, Catarina M., Louro, Ricardo O., Catarino, Teresa, Turner, David L. and Xavier, António V. (2002) Thermodynamic and kinetic characterization of trihaem cytochrome c(3) from Desulfuromonas acetoxidans. European Journal of Biochemistry, 269 (22), 5722-5730.

Abstract

Trihaem cytochrome c(3) (also known as cytochrome c(551.5) and cytochrome c(7)) is isolated from he periplasmic space of Desulfuromonas acetoxidans, a sulfur-reducing bacterium. Thermodynamic and kinetic data for the rihaem cytochrome c(3) are presented and discussed in he context of he 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 rihaem cytochrome c(3) reduction by sodium dithionite were studied using the stopped-flow technique and the data were fitted to a kinetic model that makes use of he thermodynamic properties to obtain the rate constants of he individual haems. This analysis indicates that the electrons enter he cytochrome mainly via haem I. The reduction potentials of he haems in his cytochrome show little variation with pH within the physiological range, and he kinetic studies show that the rates of reduction are also independent of pH in the range studied. Thus, although he trihaem cytochrome c(3) is readily reduced by hydrogenases from Desulfovibrio sp. and its haem core is similar to hat of he homologous tetrahaem cytochromes c(3), its physico-chemical properties are quite different, which suggests that these multihaem cytochromes with similar structures perform different functions.

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