Chapter 11. Structures of Reaction Centers in Anoxygenic Bacteria

Abstract

Photosynthetic reaction centers from anoxygenic bacteria are the best-characterized membrane protein complexes. This chapter compares over 50 X-ray crystal structures of reaction centers from Rhodopseudomonas(Blastochloris)viridis, Rhodobacter sphaeroides, and Thermochromatium tepidum on the basis of data quality and quantity, maximum resolution limits, and structural features. Not only the overall architecture of the reaction centers and the relevant positions and orientations of the prosthetic groups, but also specific structural features are conserved. Small structural differences might provide a basis for the explanation of the observed spectral and functional discrepancies between the three species. Particular points of focus in this chapter are, first, the site of binding of the secondary quinone (QB) where electron transfer is coupled to the uptake of protons from the cytoplasm; second, the respective binding sites of the electron donor proteins; third the increasing number of structures of variant reaction centers; and, fourth, the binding of phospholipids to these membrane protein complexes. Finally, recent progress in the structure determination of Photosystem II allows a comparison of the structures of bacterial RCs to that of Photosystem II.