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(center dot) together with the interaction of the coupled His residue, D1-His190, using light-induced Fourier transform infrared (FTIR) difference spectroscopy. The Y-Z(center dot)-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 N-15 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(center dot). 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 similar to 2800 cm(-1) band reflects a large proton polarizability in the hydrogen bond between Y-Z(center dot) 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(center dot)-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 S-2 -> S-3 transition, which requires proton release before electron transfer because of an excess positive charge on the Mn4Ca cluster.