Fourier Transform Infrared Detection of a Polarizable Proton Trapped between Photooxidized Tyrosine Y_Z and a Coupled Histidine in Photosystem II: Relevance to the Proton Transfer Mechanism of Water Oxidation

The redox-active tyrosine Y_Z (D1-Tyr161) in photosystem II (PSII) functions as an immediate electron acceptor of the Mn₄Ca cluster, which is the catalytic center of photosynthetic water oxidation. Y_Z is also located in the hydrogen bond network that connects the Mn₄Ca cluster to the lumen and hence is possibly related to the proton transfer process during water oxidation. To understand the role of Y_Z in the water oxidation mechanism, we have studied the hydrogen bonding interactions of Y_Z and its photooxidized neutral radical Y_Z^• together with the interaction of the coupled His residue, D1-His190, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. The Y_Z^•-minus-Y_Z FTIR difference spectrum of Mn-depleted PSII core complexes exhibited a broad positive feature around 2800 cm^–1, which was absent in the corresponding spectrum of another redox-active tyrosine Y_D (D2-Tyr160). Analyses by ¹⁵N and H/D substitutions, examination of the pH dependence, and density functional theory and quantum mechanics/molecular mechanics (QM/MM) calculations showed that this band arises from the N–H stretching vibration of the protonated cation of D1-His190 forming a charge-assisted strong hydrogen bond with Y_Z^•. This result provides strong evidence that the proton released from Y_Z upon its oxidation is trapped in D1-His190 and a positive charge remains on this His. The broad feature of the ∼2800 cm^–1 band reflects a large proton polarizability in the hydrogen bond between Y_Z^• and HisH⁺. QM/MM calculations further showed that upon Y_Z oxidation the hydrogen bond network is rearranged and one water molecule moves toward D1-His190. From these data, a novel proton transfer mechanism via Y_Z^•-HisH⁺ is proposed, in which hopping of the polarizable proton of HisH⁺ to this water triggers the transfer of the proton from substrate water to the luminal side. This proton transfer mechanism could be functional in the S₂ → S₃ transition, which requires proton release before electron transfer because of an excess positive charge on the Mn₄Ca cluster