CORRELATION BETWEEN PROTEIN FLEXIBILITY AND ELECTRON-TRANSFER FROM Q(A)(-CENTER-DOT) TO Q(B) IN PSII MEMBRANE-FRAGMENTS FROM SPINACH

TO analyze a possible correlation between the extent of Q(A)(-.) reoxi dation and protein dynamics, fluorometric and Mossbauer spectroscopic measurements were performed in photosystem II membrane fragments from spinach. Numerical evaluation of the flash-induced change of the norma lized fluorescence quantum yield revealed that the extent of reoxidati on starts to decrease below 275 K and is almost completely suppressed at 230 K. Detailed analyses of Mossbauer spectra measured at different temperatures in Fe-57-enriched material indicate that the onset of fl uctuations between conformational substates of the protein matrix occu rs also at around 230 K. Based on this correspondence, protein flexibi lity is inferred to play a key role for Q(A)(-.) reoxidation in photos ystem II. Taking into account the striking similarities with purple ba cteria and the latest structural information on these reaction centers [Stowell, M. H. B., McPhillips, T. M., Rees, D. C., Soltis, S. M., Ab resch, E., and Feher, G. (1997) Science 276, 812-816], it appears most plausible that also the headgroup of plastoquinone-9 bound to the QB- site in PSII requires a structural reorientation for its reduction to the semiquinone.