Structure of the Reaction Center from Rhodobacter sphaeroides R-26: The Protein Subunits

The three-dimensional structure of the protein subunits of the reaction center (RC) of Rhodobacter sphaeroides has been determined by x-ray diffraction at a resolution of 2.8 angstrom with an R factor of 26%. The L and M subunits each contain five transmembrane helices and several helices that do not span the membrane. The L and M subunits are related to each other by a 2-fold rotational symmetry axis that is approximately the same as that determined for the cofactors. The H subunit has one transmembrane helix and a globular domain on the cytoplasmic side, which contains a helix that does not span the membrane and several β -sheets. The structural homology with RCs from other purple bacteria is discussed. A structure of the complex formed between the water soluble cytochrome c₂ and the RC from Rb. sphaeroides is proposed.