Pigment-protein interactions in Rhodobacter sphaeroides Y photochemical reaction center; comparison with other reaction center structures

Structural characteristics of pigments and cofactors are analyzed in the X-ray structure of the Rhodobacter sphaeroides (Y strain) photochemical reaction center, recently refined at 3 Å resolution (Arnoux B, Gaucher JF, Ducruix A and Reiss-Husson F (1995) Acta Cryst D51: 368–379). As several structures are now available for these pigment-protein complexes from various Rhodobacter sphaeroides strains and for Rhodopseudomonas viridis, a detailed comparison was done for highlighting converging structural results as well as for pointing to incidental differences. Comparison of mean plane orientations and distances, and also direct superposition of the pigment arrays, indicated that the best agreement between all the structures concerned the dimer and the bacteriopheophytin of the A branch. In the Y reaction center structure the pentacoordination of the Mg⁺⁺ atoms of the bacteriochlorophylls, and the H bonding pattern of the porphyrin conjugated carbonyls are consistent with the better resolved Rhodobacter sphaeroides recently published structure (Ermler U, Fritzsch G, Buchanan SK and Michel H (1995) Structure 2:925–936). Discrepancies between the various Rhodobacter sphaeroides structures are larger for the quinones, particularly the secondary one. In the Y reaction center structure the phytyl and isoprenoid chains of the cofactors are defined and their local mobility was evaluated by analyzing the temperature factor and the density of neighbouring atoms. Significant differences were observed between the A and B branches, and, within each branch, from the dimer to the quinone molecules.Correspondence to: F. Reiss-Husson