Kinetics of the c-cytochromes in chromatophores from Rhodopseudomonas sphaeroides as a function of the concentration of cytochrome c2. Influence of this concentration on the oscillation of the secondary acceptor of the reaction centers Q B

Abstract

The oxidation kinetics of Cyt c 1 and c 2 have been measured in normal chromatophores and in chromatophores fused with liposomes in order to increase the internal volume. The kinetics of Cyt c 1 oxidation were found to be dependent on Cyt c 2 concentration. The initial rate of Cyt c 1 oxidation decreased after fusion by a factor of about two, indicating a process dependent on diffusion. The results do not allow a clear distinction between a diffusion of Cyt c 2 along the inner membrane surface or through the inner volume of the vesicle; two- and three-dimensional models are discussed. In contrast to Cyt c 1, the kinetics of oxidation of Cyt c 2 were not influenced by changes in concentration. It is concluded that reduced Cyt c 2 is preferentially bound to the reaction centers. A binary pattern as a function of flash number from the dark-adapted state was measured in the turn-over of the two-electron gate of the reaction center. In chromatophores with more than 0.5 cytochrome c 2 molecules per reaction center, this binary pattern titrated out with a midpoint around 340 mV on reduction of the suspension. In experiments with chromatophores with a low Cyt c 2 content, or with spheroplast-derived vesicles which had lost Cyt c 2, the binary oscillation in the two-electron gate could be observed at much lower potentials. The results suggest that the binding of reduced cytochrome c 2 modifies the behavior of the two-electron gate. A model in which reaction center dimers are stabilized by Cyt c 2 is proposed to explain the effect.