Energetics of Ionized Water Molecules in the H-Bond Network near the Ca2+ and Cl- Binding Sites in Photosystem II

In photosystem II, water oxidation occurs at the oxygen-evolving complex (OEC). The presence of a hydronium ion (H3O+) was proposed at the Cl- binding site and Ca2+-depleted OEC. Using a quantum mechanical/molecular mechanical approach, we report the stability of H3O+ in the PSII protein environment. Neither release of the proton from ligand water molecule W2 at the OEC nor formation of H3O+ at Cl- is energetically favorable. In contrast, H3O+ can exist at the Ca2+-depleted OEC. Even when H3O+ exists in Ca2+-depleted PSII, the H-bond network of the redox-active tyrosine (TyrZ) remains unaltered, retaining the unusually short low-barrier H-bond with D1-His190, and the redox potential of TyrZ, E-m(TyrZ), remains unaltered. These observations explain why the oxidation of the Ca2+-depleted Mn4O5 cluster by TyrZ (i.e., the S-2 to S-3 transition) is not inhibited at low pH. It seems likely that Ca2+ plays a role in not only (i) maintaining the H-bond network and facilitating TyrZ oxidation [tuning E-m(TyrZ)] but also (ii) providing the valence of +2, decreasing the pK(a) of the ligand molecule (W1), and facilitating the release of the proton from W1 in the S-2 to S-3 transition together with Cl-.