STRUCTURE OF THE PHOTOSYNTHETIC REACTION-CENTER FROM RHODOBACTER-SPHAEROIDES AT 2.65-ANGSTROM RESOLUTION - COFACTORS AND PROTEIN-COFACTOR INTERACTIONS

Background: Photosynthetic reaction centres (RCs) catalyze light-drive n electron transport across photosynthetic membranes. The photosynthet ic bacterium Rhodobacter sphaeroides is often used for studies of RCs, and three groups have determined the structure of its reaction centre . There are discrepancies between these structures, however, and to re solve these we have determined the structure to higher resolution than before, using a new crystal form. Results: The new structure provides a more detailed description of the Rb. sphaeroides RC, and allows us to compare it with the structure of the RC from Rhodopseudomonas virid is. We find no evidence to support most of the published differences i n cofactor binding between the RCs from Rps. viridis and Rb. sphaeroid es. Generally, the mode of cofactor binding is conserved, particularly along the electron transfer pathway. Substantial differences are only found at ring V of one bacteriochlorophyll of the 'special pair' and for the secondary quinone, Q(B). A water chain with a length of about 23 Angstrom including 14 water molecules extends from the Q(B) to the cytoplasmic side of the RC. Conclusions: The cofactor arrangement and the mode of binding to the protein seem to be very similar among the n on-sulphur bacterial photosynthetic RCs. The functional role of the di splaced Q(B) molecule, which might be present as quinol, rather than q uinone, is not yet clear. The newly discovered water chain to the Q(B) binding site suggests a pathway for the protonation of the secondary quinone Q(B).