The Cytochrome‐c Binding Surface of Reaction Centers from Rhodobacter sphaeroides

In this paper, we describe the water‐exposed, periplasmic surface of the bacterial photosynthetic reaction center from Rhodobacter sphaeroides R‐26. This surface contains binding sites for c ‐type cytochromes. The approximate two‐fold structural symmetry is evident in the folding of the L and M subunits of the reaction center on this surface. In spite of this, the distribution of charged residues shows a marked deviation from symmetry. One group of charged residues is distributed symmetrically. For this group, equivalently charged residues are located at symmetry‐related positions on both L and M subunits. However, charged residues are also found at positions which do not follow the two‐fold symmetry. Seven charged residues appear on the L and M subunits at positions which do not have a charged residue at the corresponding position on the other subunit. These symmetry‐breaking charged residues appear in a single region, or “patch”, on the surface of the reaction center. We propose that this patch may function as a site for cytochrome c docking. A model of the Rb. sphaeroides cytochrome c 2 ‐reaction center electron transfer complex has been built. Preliminary energy calculations show that this configuration is at least a local energy minimum.