Hydrogen-bonded structure of tyrosine Y_D in photosystem II

Abstract

The redox-active tyrosine Y_D (D2-Tyr160) 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^·. The X-ray structures of the PSII core complex showed that D2-His189 and D2-Gln164 are located near Y_D forming a H-bond network, which should play an important role in proton-coupled electron transfer. In this study, we have studied the H-bond structure of Y_D using Fourier transform infrared (FTIR) spectroscopy. Light-induced Y_D^·/Y_D FTIR difference spectra were obtained with PSII core complexes from Thermosynechococcus elongatus. The 1252 cm^-1 band was assigned to the coupled mode of CO stretching and COH bending vibrations by labeling of Tyr residues with [4-¹³C]Tyr and measurement in D₂O. From quantum chemical analysis of this CO/COH band, it is concluded that Y_D is H-bonded with both D2-His189 and D2-Gln164.