Water Molecules Coupled to the Redox-Active Tyrosine Y_D in Photosystem II as Detected by FTIR Spectroscopy

摘要

The redox-active tyrosine Y_D (D2-Tyrl60) in photosystem II (PSII) serves as a side-path electron donor to P680.When Y_D is oxidized,a proton is released from phenolic OH,and a neutral radical Y_D~· is formed.A hydrogen bond network around Y_D must be deeply involved in the mechanism of the Y_D reaction.In this study,we have detected water molecules structurally coupled to Y_D by means of Fourier transform infrared (FTIR) spectroscopy.Light-induced Y_D~·/Y_D FTIR difference spectrum of a hydrated film of the PSII core complexes from Thermosynechococcus elongatus showed major signals at 3636(-)/3617(+) and 3594(+)/3585(-) cm-1 in the weakly hydrogen bonded OH stretching region.These peaks downshifted by 11-12 cm-1 upon H218O substitution and almost disappeared upon H/D exchange,and hence,they were definitely assigned to the water OH vibrations.Small intramolecular couplings of 3-6 cm-1 estimated from the OH frequencies of residual HOD species in a deuterated film indicate that these OH signals arise from two different water molecules that have significantly asymmetric hydrogen bond structures.Similar OH signals were observed in PSII-enriched membranes from spinach,suggesting that two water molecules commonly exist near Y_D irrespective of biological species.These water molecules are coupled to Y_D most probably through a hydrogen bond network or one of them possibly interacts directly with Y_D,and thus,they may play crucial roles in the Y_D reaction by forming a proton-transfer pathway and tuning the redox potential of Y_D.