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Abstract
In all photosynthetic bacteria and plants, the primary photochemistry occurs in an integral-membrane protein-cofactor complex called the photosynthetic reaction center. During photosynthesis, light energy is funnelled to the reaction center, where the energy is used to create a charge-separation, followed by electron transfer across the photosynthetic membrane. We have determined the three-dimensional crystal structure of the photosynthetic reaction center of the purple bacterium Rhodobacter sphaeroides at a resolution of 2.8 A.
The complex consists of three protein subunits (L, M, and H) and nine cofactors (four bacteriochlorophyll a, two bacteriopheophytin a, two ubiquinones, and a non-heme iron) in the R-26 mutant. The reaction center of the wild type strain (2.4.1) contains an additional carotenoid cofactor whose location in the complex has also been determined by crystallographic analysis of the wild type reaction center at 3.5 A resolution. Together, the three subunits have 11 transmembrane helices which form the architecture for binding the cofactors in the proper configuration for electron transfer across the membrane. The three-dimensional structure of this reaction center is similar to the one from the related species, Rps. viridis.
The structural details of the protein subunits, the cofactors, and their interactions are discussed with respect to the function of the reaction center. Some of the important topics addressed are the probable arrangement of the reaction center with respect to the membrane and the energetics of their interaction, the observation that electron transfer occurs down only one of the two nearly equivalent cofactor branches, and the possible roles for several amino acids in the electron transfer and protonation events.
The crystallographic techniques used to determine the structure of the reaction center are described. In addition, new crystallographic techniques involving crystal twinning, symmetry effects in rotation functions, and rigid body and unit cell refinement are described in the Appendices.
