The Structure of Reaction Centers from Purple Bacteria

Abstract

SummaryThe function of photosynthetic bacterial reaction centers (RCs) is closely related to their structure. In the last 15 years a wealth of structural data has been accumulated on bacterial RCs, mainly through X-ray structure analysis of three-dimensional RC crystals. In this chapter, the arrangement of protein subunits and cofactors in the RC complexes of the non-sulfur purple bacteria Rhodobacter (Rb.) sphaeroides and Rhodopseudomonas (Rp.) viridis are delineated. A prominent feature of the bacterial RCs is their location in the photosynthetic membrane. Inside the RC complex, a finely tuned arrangement of amino acid residues and cofactors maintains a highly ordered system. The positions and likely functions of hydrogen bonds are described, since they play a key role in protein-cofactor interactions. Special emphasis is placed on the symmetry relations in the RC and on the functional asymmetry of electron and proton transfer that contradicts the observed pseudo two-fold structural symmetry.The structures of the RCs from Rb. sphaeroides and Rp. viridis show a striking identity, apart from the cytochrome-c subunit found only in the latter RC. The core regions around the bacteriochlorophylls and bacteriopheophytins, including the carotenoid, are particularly similar. New observations of water clusters close to the primary and secondary quinones are described and their impact on proton transfer processes is discussed. These findings help elucidate the intermeshed processes of electron-proton coupling in the RC.