Abstract
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.
Highlights
In purple bacterial photosynthesis, periplasmic, soluble electron carrier proteins mediate the electron transfer from the cytochrome bc1 complex to the reaction center (RC)1 complex
In comparison to the wild-type RCs, which reacted with cytochrome c2 with the second-order rate constant of 3.1 ϫ 105 MϪ1 sϪ1 (Fig. 1a), all mutants shown in this figure considerably accelerated the reaction rate
Horse cytochrome c reacted with B. viridis RC slower than B. viridis cytochrome c2 but to its reactivity with the wild-type R. gelatinosus RC [9]
Summary
Periplasmic, soluble electron carrier proteins mediate the electron transfer from the cytochrome bc1 complex to the reaction center (RC)1 complex. In the case of cytochrome c, the electrostatic interactions between the oppositely charged residues of the binding domains of cytochrome c (lysines) and the RC (aspartates and glutamates) are of primary importance [9], whereas the binding of HiPIP seems to be controlled mainly by the hydrophobic coupling of the encounter surfaces of the two proteins [10, 11].
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