Infrared Detection of a Proton Released from Tyrosine Y_D to the Bulk upon Its Photo-oxidation in Photosystem II

Photosystem II (PSII) has two symmetrically located, redox-active tyrosine residues, Y_Z and Y_D. Whereas Y_Z mediates the electron transfer from the water-oxidizing center to P680 in the main electron transfer pathway, Y_D functions as a peripheral electron donor to P680. To understand the mechanism of this functional difference between Y_Z and Y_D, it is essential to know where the proton is transferred upon its oxidation in the proton-coupled electron transfer process. In this study, we used Fourier transform infrared (FTIR) spectroscopy to examine whether the proton from Y_D is released from the protein into the bulk. The proton detection method previously used for water oxidation in PSII [Suzuki et al. (2009) J. Am. Chem. Soc. 131, 7849-7857] was applied to Y_D; a proton released into the bulk upon Y_D oxidation was trapped by a high-concentration Mes buffer, and the protonation reaction of Mes was monitored by FTIR difference spectroscopy. It was shown that 0.84 ± 0.10 protons are released into the bulk by oxidation of Y_D in one PSII center. This result indicates that the proton of Y_D is not transferred to the neighboring D2-His198 but is released from the protein; this is in sharp contrast to the Y_Z reaction, in which a proton is transferred to D1-His190 through a strong hydrogen bond. This functional difference is caused by differences in the hydrogen-bonded structures of Y_D and Y_Z, which are determined by the hydrogen bond partners at the N_π sites of these His residues, i.e., D2-Arg294 and D1-Asn298, which function as a hydrogen bond donor and acceptor, respectively. This FTIR spectroscopy result supports the recent theoretical prediction [Saito et al. (2013) Proc. Natl. Acad. Sci. U.S.A. 110, 7690-7695] based on the X-ray crystallographic structure of PSII and explains the different rates of the redox reactions of Y_D and Y_Z.