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 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.

Trapped Conformational States of Semiquinone (D+•QB-•) Formed by B-Branch Electron Transfer at Low Temperature in Rhodobacter sphaeroides Reaction Centers

“…Furthermore, our results show that a significant protein response is required to stabilize (Q A Q B −• ). Although our results do not imply that the movement of quinone from the distal to proximal sites is ratelimiting, they do support the idea that the movement to the proximal site is a necessary prerequisite for its stabilization (27). EPR signal monitored before, during and following illumination of dark frozen samples for the native RC (left panels) and the Quintuple B-branch RCs (right panels).…”

“…where k C is the rate constant of the rate-limiting conformational gate (9) the necessity for Q B to move into the proximal position for stabilization (27). Furthermore, our results show that a significant protein response is required to stabilize (Q A Q B −• ).…”

“…Since binding to these two sites is nearly isoenergetic, small changes in the binding free energy resulting from temperature or external solvent could lead to a change in the relatively occupancy. This may be one reason for the reported differences in the binding position of Q B (26)(27)(28)(29)(30)69) and the FTIR observation that Q B prefers to bind at the proximal position at room temperature (46). …”

“…Furthermore, the preferred binding position may be a function of the cryoprotectant (if the crystal is frozen) and temperature (30). In contrast, Q B −• , the reduced anionic semiquinone, has always been found to be bound in the proximal site (27,28) consistent with the stabilization of the negative charge by H-bonds. This binding position is supported by EPR and ENDOR (31,32) and FTIR (33,34) spectroscopic studies.…”

EPR, ENDOR, and Special TRIPLE measurements of P•+ in wild type and modified reaction centers from Rb. sphaeroides

“…In purple anoxygenic bacteria, reaction centers (RCs) embedded in the membrane perform the primary photochemistry (Blankenship et al 1995). The RC from Rhodobacter sphaeroides consists of three protein subunits and several cofactors (see e.g., Allen et al 1987;Yeates et al 1988;Ermler et al 1994;Stowell et al 1997;Camara-Artigas et al 2002). The core L and M subunits surround the cofactors that are divided into two distinct branches related by an approximate two-fold symmetry axis that runs from the center of P to the non-heme iron (Fig.…”