Interactions of water molecules with Y_D in photosystem II

Abstract

The redox-active tyrosine Y_D in photosystem II (PSII) serves as an accessory electron donor to P680. When oxidized, Y_D releases a proton and becomes a neutral radical Y_D^·. Hence, a H-bond network around Y_D plays a crucial role in this reaction. In the last annual meeting, we reported that Y_D has two simultaneous H-bonds. Here, we studied the interactions of water molecules with Y_D using FTIR spectroscopy. Light-induced Y_D/Y_D^· FTIR difference spectra were obtained with the PSII core complexes from Thermosynechococcus elongatus. In the OH stretching region, signals were observed at 3636/3617 and 3594/3584 cm^-1. These peaks downshifted upon H₂¹⁸O substitution, and hence were attributed to two water molecules coupled to Y_D. The recent X-ray structures of PSII revealed that the side chain capable of forming a proper H-bond with Y_D is only D2-His189. Thus, it is likely that one of the two water molecules is directly H-bonded with Y_D.