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Abstract

Photosynthetic reaction centers from purple bacteria are the best known membrane protein complexes. In this review, X-ray crystal structures of the reaction centers from Rhodopseudomonas (Rp.) viridis and Rhodobacter (Rb.) sphaeroides are compared on the basis of data quality and quantity, maximum resolution limits, and structural features. In contrast to earlier comparisons and on the basis of the most recent, best defined Rb. sphaeroides structure, a number of the reported differences between the two species cannot be confirmed. Not only the overall architecture of the reaction centers and the relative positions and orientations of the cofactors, but also specific structural features are well conserved. For example, the hydrogen-bonding pattern between the protein and the monomeric bacteriochlorophylls, the bacteriopheophytins, and the primary quinone, Q_A^x are identical. Therefore, the asymmetry between the A- and B- branch is maintained. However, there are small conformational differences which might provide a basis for the explanation of observed spectral and functional discrepancies between the two species.
A particular focus in this review is on the binding site of the secondary quinone (Q_b), where electron transfer is coupled to the uptake of protons from the cytoplasm. For the description of the Q_b-binding site, an improved coordinate set for the Rp. viridis reaction center has been included. In particular, a new understanding of the role of Ser L223 in the Q_B-binding site has become apparent. In addition, chains of ordered water molecules are found leading from the cytoplasm to the Q_Bsite in the best defined structures of both Rp. viridis and Rb. sphaeroides reaction centers. New insights concerning the binding of triazine-type electron transfer inhibitors to the Rp. viridis Q_B site include the identification of additional hydrogen bonds and of two tightly bound water molecules. Finally, recent developments regarding the structures of mutant reaction centers are discussed.