Different mechanisms of the binding of soluble electron donors to the photosynthetic reaction center of Rubrivivax gelatinosus and Blastochloris viridis.

The tetraheme cytochrome subunits of the photosynthetic reaction centers (RCs) in two species of purple bacteria, Rubrivivax gelatinosus and Blastochloris (Rhodopseudomonas) viridis, were compared in terms of their capabilities to bind different electron-donor proteins. The wild-type RCs from both species and mutated forms of R. gelatinosus RCs (with amino acid substitutions introduced to the binding domain for electron-donor proteins) were tested for their reactivity with soluble cytochromes and high potential iron-sulfur protein. Cytochromes from both species were good electron donors to the B. viridis RC and the R. gelatinosus RC. The reactivity in the R. gelatinosus RC showed a clear dependence on the polarity of the charges introduced to the binding domain, indicating the importance of the electrostatic interactions. In contrast, high potential iron-sulfur protein, presumed to operate according to the hydrophobic mechanism of binding, reacted significantly only with the R. gelatinosus RC. Evolutionary substitution of amino acids in a region of the binding domain on the cytochrome subunit surface probably caused the change in the principal mode of protein-protein interactions in the electron-transfer chains.