STRUCTURE OF THE PHOTOCHEMICAL-REACTION CENTER OF A SPHEROIDENE-CONTAINING PURPLE BACTERIUM, RHODOBACTER-SPHAEROIDES-Y, AT 3 ANGSTROM RESOLUTION

The crystal structure of the photochemical reaction centre from Rhodob acter sphaeroides Y, a carotenoid-containing wild-type purple bacteriu m, has been determined at 3 Angstrom resolution. This membrane complex consists of three subunits (281, 307 and 260 residues, respectively) and ten cofactors. It was crystallized in presence of beta-D-octylgluc oside. The crystals are orthorhombic with unit-cell dimensions, a = 14 3.7, b = 139.8, c = 78.7 Angstrom, space group P2(1)2(1)2(1) with four molecules in the unit cell. Refinement of the structure by X-PLOR and manual reconstructions yielded an R value of 22.1% for 19 630 reflect ions between 7 and 3 Angstrom. The secondary structure is highly homol ogous to those determined for Rhodopseudomonas viridis (Protein Data B ank entry 1PRC) and Rhodobacter sphaeroides R26 (Protein Data Bank ent ry 4RCR) reaction centres. In the latter two structures one Fe2+ ion l ocated between the two quinones is coordinated by four histidines and one glutamic acid. In the Rhodobacter sphaeroides Y structure, Mn2+ oc cupies the same position with identical ligands and geometry. The caro tenoid conformation which is a non-planar 15-15'-cis spheroidene molec ule in our structure differs from the 13-14-cis 2,4-dihydroneusporene in the Rhodopseudomonas viridis structure.