High resolution x-ray diffraction data from crystals of the Rhodobacter sphaeroides photosynthetic reaction center (RC) have been collected at cryogenic temperature in the dark and under illumination, and the structures were refined at 2.2 and 2.6 angstrom resolution, respectively. In the charge-separated D+QAQB- state (where D is the primary electron donor (a bacteriochlorophyll dimer), and QA and QB are the primary and secondary quinone acceptors, respectively), QB- is located approximately 5 angstroms from the QB position in the charge-neutral (DQAQB) state, and has undergone a 180 degrees propeller twist around the isoprene chain. A model based on the difference between the two structures is proposed to explain the observed kinetics of electron transfer from QA-QB to QAQB- and the relative binding affinities of the different ubiquinone species in the QB pocket. In addition, several water channels (putative proton pathways) leading from the QB pocket to the surface of the RC were delineated, one of which leads directly to the membrane surface.

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The three-dimensional structure of the cofactors of the reaction center of Rhodobacter sphaeroides R-26 has been determined by x-ray diffraction and refined at a resolution of 2.8 A with an R value of 26%. The main features of the structure are similar to the ones determined for Rhodopseudomonas viridis [Michel, H., Epp, 0. & Deisenhofer, J. (1986) EMBO J. 5, 2445EMBO J. 5, -2451. The cofactors are arranged along two branches, which are approximately related to each other by a 2-fold symmetry axis. The structure is well suited to produce light-induced charge separation across the membrane. Most of the structural features predicted from physical and biochemical measurements are confirmed by the x-ray structure.The reaction center (RC) is an integral membrane proteinpigment complex that mediates the primary processes of photosynthesis-i.e., the light-induced electron transfers from a donor to a series of acceptor species. The three-dimensional structure of the RC from the photosynthetic bacterium Rhodopseudomonas viridis has recently been determined by x-ray diffraction at a resolution of 2.9 A (1-3). In this paper, we report the structure analysis of the RC from another purple bacterium, the carotenoidless mutant R-26 of Rhodobacter sphaeroides (previously called Rhodopseudomonas sphaeroides). The motivation for undertaking the structure determination of the RC of a second bacterial species was 2-fold. (i) The RC from Rb. sphaeroides has been investigated for the past two decades and, consequently, is the best characterized RC (for reviews see refs. 4 and 5); in addition, the methodologies for manipulating its structure (e.g., exchanging cofactors, dissociating and reassociating the subunits) have been worked out in detail (4-10). (ii) The availability of structures from two organisms may help in elucidating structure-function relationships by correlating differences in structure with differences in function.The RC from Rb. sphaeroides is composed of three protein subunits-L, M, and H-and the following cofactors: four bacteriochlorophylls (Bchls), two bacteriopheophytins (Bphes), two ubiquinones, and one nonheme iron. The RC from R. viridis has an additional subunit, a cytochrome with four c-type hemes; its Bchls and Bphes are of the "b" type instead of the "a" type found in Rb. sphaeroides, and its primary quinone is a menaquinone. Notwithstanding these differences, the two structures were found to be very similar. This made it possible to use the method of molecular replacement (11) to solve the phase problem in the x-ray analysis (12)(13)(14). The crystals of Rb. sphaeroides diffract at least to a resolution of 2.6 A and retain the ability to perform the primary photochemistry (15). We have solved the structure of the protein and the cofactors to a resolution of 2.8 A with an R factor of 26%. In this paper, we report the structure of the cofactors. The structure of the protein, the relation of the RC protein to the membrane, and the interaction of the cofactors with the protein will be reported in ...