Indirect evidence for structural changes coupled with QB-. formation in photosystem II.
The thermal blockage of QA-. oxidation was analysed in PS II membrane fragments by monitoring flash-induced changes of the relative fluorescence quantum yield as a function of temperature. The results obtained reveal: (a) in dark-adapted samples the fraction of QA-. that is not reoxidised within a time domain of 10 s after the actinic flash increases with lowering the temperature (half-maximum effect at 250-260 K), (b) at low temperatures where QA-. generated in dark-adapted samples remains almost completely reduced, a significant extent of QA-. reoxidation arises when samples are used that were preilluminated at room temperature by one saturating flash followed by rapid freezing before performing the experiment, and (c) the extent of QA-. that is reoxidised at 258 K exhibits a characteristic binary oscillation as a function of the number of preillumination flashes given at room temperature. Based on these data it is inferred that QB and QB-. are located at different equilibrium positions in the QB site. As a consequence the formation of QB-. is coupled with significant structural changes that require sufficient flexibility of the protein matrix. This general feature corresponds with a recently proposed model for the acceptor side reactions of anoxygenic bacteria [Stowell, M.H.B., McPhillips, T.M., Rees, D.C., Soltis, S.M., Abresch, E. and Feher, G., Science 276 (1997) 812-816].
Reifarth
F
F
Max-Volmer-Institute for Biophysical Chemistry and Biochemistry, Technical University Berlin, Germany.
Renger
G
G
D003624:Darkness; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D018724:Spinacia oleracea / Q000378:metabolism; D013816:Thermodynamics
D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex