Comparison of the binding sites for high-potential iron-sulfur protein andcytochrome c on the tetraheme cytochrome subunit bound to the bacterial photosynthetic reaction center

A tetraheme cytochrome subunit bound to the photosynthetic reaction center (RC) of purple bacterium, Rubrivivax gelatinosus, interacts with two types of soluble electron donors, cytochromes c and high-potential iron-sulfur pr otein (HiPIP), at a binding domain in the vicinity of low-potential heme 1, the fourth heme from the special pair of bacteriochlorophyll. To clarify t he mechanism of the interaction, the domain around heme 1 was examined usin g site-directed mutants that changed the surface charge in the region withi n 20 Angstrom from the heme edge. In the case of the interaction with solub le cytochrome c, a strong dependence on the sign of the introduced charge w as observed in all mutants: positive charge inhibited the reaction rate, wh ereas additional negative charge accelerated it. This confirmed the electro static nature of the binding. Interaction with HiPIP was inhibited by a lim ited number of mutations at the close vicinity of heme 1, and no accelerati on was observed (the effects of some mutations were independent of the sign of the introduced charge). The acidic residues which were critically impor tant for the binding of cytochrome c showed much less contribution to the b inding of HiPIP. The binding site for HiPIP appears to be mostly formed by uncharged and hydrophobic residues, occupying a significantly smaller area than the cytochrome-c-binding site. It is proposed that the docking of HiPI P to the RC in Rvi, gelatinosus is primarily controlled by hydrophobic cont acts between protein surfaces, thus differing from the electrostatic mode o f the RC-cytochrome c interaction.