Structure of the reaction center from Rhodobacter sphaeroides R-26: the protein subunits.

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Abstract
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 .ANG. 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 .beta.-sheets. The structured 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.