LIGHT-INDUCED STRUCTURAL-CHANGES IN PHOTOSYNTHETIC REACTION-CENTER - IMPLICATIONS FOR MECHANISM OF ELECTRON-PROTON TRANSFER

High resolution x-ray diffraction data from crystals of the Rhodobacte r sphaeroides photosynthetic reaction center (RC) have been collected at cryogenic temperature in the dark and under illumination, and the s tructures were refined at 2.2 and 2.6 angstrom resolution, respectivel y. In the charge-separated D(+)Q(A)Q(B)(-) state (where D is the prima ry electron donor (a bacteriochlorophyll dimer), and Q(A) and Q(B) are the primary and secondary quinone accepters, respectively), Q(B)(-) i s located approximately 5 angstroms from the Q(B) position in the char ge-neutral (DQ(A)Q(B)) slate, and has undergone a 180 degrees propelle r twist around the isoprene chain. A model based on the difference bet ween the two structures is proposed to explain the observed kinetics o f electron transfer from Q(A)-Q(B) to Q(A)Q(B)(-) and the relative bin ding affinities of the different ubiquinone species in the Q(B) pocket . In addition, several water channels (putative proton pathways) leadi ng from the Q(B) pocket to the surface of the RC were delineated, one of which leads directly to the membrane surface.