Correlation between protein flexibility and electron transfer from Q(A)/(-·) to Q(B) in PSII membrane fragments from spinach

To analyze a possible correlation between the extent of Q(A)/(-·) reoxidation 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 normalized fluorescence quantum yield revealed that the extent of reoxidation starts to decrease below 275 K and is almost completely suppressed at 230 K. Detailed analyses of Mossbauer spectra measured at different temperatures in ⁵⁷Fe- enriched material indicate that the onset of fluctuations between conformational substates of the protein matrix occurs also at around 230 K. Based on this correspondence, protein flexibility is inferred to play a key role for Q(A)/(-·) reoxidation in photosystem II. Taking into account the striking similarities with purple bacteria and the latest structural information on these reaction centers [Stowell, M. H. B., McPhillips, T. M., Rees, D. C, Soltis, S. M., Abresch, E., and Feher, G. (1997) Science 276, 812-816], it appears most plausible that also the headgroup of plastoquinone- 9 bound to the Q(B)-site in PSII requires a structural reorientation for its reduction to the semiquinone.