Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation

D1-Tyr161 (TyrZ) forms a low-barrier H-bond with D1-His190 and functions as a redox-active group in photosystem II. When oxidized to the radical form (TyrZ-O^•), it accepts an electron from the oxygen-evolving Mn₄CaO₅ cluster, facilitating an increase in the oxidation state (S; n = 0-3). In this study, we investigated the mechanism of how TyrZ-O^• drives proton-coupled electron transfer during the S₂ to S₃ transition using a quantum mechanical/molecular mechanical approach. In response to TyrZ-O^• formation and subsequent loss of the low-barrier H-bond, the ligand water molecule at the Ca²⁺ site (W4) reorients away from TyrZ and donates an H-bond to D1-Glu189 at Mn4 of Mn₄CaO₅ together with an adjacent water molecule. The H-bond donation to the Mn₄CaO₅ cluster triggers the release of the proton from the lowest pK_a site (W1 at Mn4) along the W1…D1-Asp61 low-barrier H-bond, leading to protonation of D1-Asp61. The interplay of the two low-barrier H-bonds, involving the Ca²⁺ interface and forming the extended Grotthuss-like network [TyrZ…D1-His190]-[Mn₄CaO₅]-[W1…D1-Asp61], rather than the direct electrostatic interaction, is likely a basis of the apparent long-distance interaction (11.4 Å) between TyrZ-O^• formation and D1-Asp61 protonation.