Light-Induced Structural Changes in Photosynthetic Reaction Center: Implications for Mechanism of Electron-Proton Transfer

Abstract

High resolution x-ray diffraction data from crystals of theRhodobacter 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+Q_AQ_B− state (where D is the primary electron donor (a bacteriochlorophyll dimer), and Q_A and Q_B are the primary and secondary quinone acceptors, respectively), Q_B− is located approximately 5 angstroms from the Q_B position in the charge-neutral (DQ_AQ_B) state, and has undergone a 180° 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 Q_A–Q_B to Q_AQ_B− and the relative binding affinities of the different ubiquinone species in the Q_B pocket. In addition, several water channels (putative proton pathways) leading from the Q_B pocket to the surface of the RC were delineated, one of which leads directly to the membrane surface.