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

Science  02 May 1997:
Vol. 276, Issue 5313, pp. 812-816
DOI: 10.1126/science.276.5313.812

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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+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° 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 QBpocket. 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.

  • * Present address: Medical Research Council, Laboratory of Molecular Biology, Cambridge CB2 2QH, UK.

  • Present address: Stanford Synchrotron Radiation Laboratory, P.O. Box 4349, Bin 69, Stanford, CA 94309, USA.

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