Structural model of the photosynthetic reaction center of Rhodobacter capsulatus.

The reaction center (RC) from the photosynthetic bacterium Rhodobacter (Rb.) capsulatus has been the subject of a considerable amount of molecular biological and spectroscopic work aimed at improving our understanding of the primary steps of photosynthesis. However, no three-dimensional structure is available for this protein. We present here a model obtained by combining information from the structure of the highly homologous RC from Rhodopseudomonas (Rps.) viridis with molecular mechanics and simulated annealing calculations. In the Rb. capsulatus model the orientations of the bacteriochlorophyll monomer and the bacteriopheophytin on the branch inactive in electron transfer differ significantly from those in the RCs of Rps. viridis and Rb. sphaeroides. The bacteriopheophytin orientational difference is in good accord with previous linear dichroism measurements. A comparison is made of interactions between the pigments and the protein environment that may be of functional significance in Rps. viridis, Rb. sphaeroides, and Rb. capsulatus.