New Insights into the X-Ray Structure of the Reaction Center from Rhodopseudomonas viridis

Light-induced electron transfer in the reaction center (RC) is coupled to the uptake of protons from the cytoplasm at the binding site of the secondary quinone (QB). With quinone reconstitution, a higher QB occupancy has been obtained in the Rhodopseudomonas (Rp.) viridis RC crystals, which, after X-ray analysis, has resulted in a well-defined QB-site model. On the basis of this new model and the structure of a stigmatellin-RC complex, a new understanding of QB binding, particularly concerning the role of Ser L223, has emerged. Furthermore, with high quality data collected on RC complexes with atrazine and two chiral atrazine derivatives, it has been possible to describe the exact nature of triazine binding and its effect on the structure of the protein. The new data are of sufficient quality to improve the original model also in other parts, e.g. regarding the asymmetry of binding of the primary quinone (QA), the structure of the carotenoid, and additional tightly-bound water molecules