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 t he X-ray structure of the Rhodobacter sphaeroides (Y strain) photochem ical reaction center, recently refined at 3 Angstrom resolution (Arnou x B, Gaucher JF, Ducruix A and Reiss-Husson F (1995) Acta Cryst D51: 3 68-379). As several structures are now available for these pigment-pro tein complexes from various Rhodobacter sphaeroides strains and for Rh odopseudomonas viridis, a detailed comparison was done for highlightin g 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 be st agreement between all the structures concerned the dimer and the ba cteriopheophytin of the A branch. In the Y reaction center structure t he pentacoordination of the Mg++ atoms of the bacteriochlorophylls, an d the H bonding pattern of the porphyrin conjugated carbonyls are cons istent with the better resolved Rhodobacter sphaeroides recently publi shed 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 isopr enoid chains of the cofactors are defined and their local mobility was evaluated by analyzing the temperature factor and the density of neig hbouring atoms. Significant differences were observed between the A an d B branches, and, within each branch, from the dimer to the quinone m olecules.