Structure of Sr-substituted photosystem II at 2.1 Å resolution and its implications in the mechanism of water oxidation

Oxygen-evolving complex of photosystem II (PSII) is a tetramanganese calcium penta-oxygenic cluster (Mn₄CaO₅) catalyzing light-induced water oxidation through several intermediate states (S-states) by a mechanism that is not fully understood. To elucidate the roles of Ca ²⁺ in this cluster and the possible location of water substrates in this process, we crystallized Sr²⁺-substituted PSII from Thermosynechococcus vulcanus, analyzed its crystal structure at a resolution of 2.1 Å, and compared it with the 1.9 Å structure of native PSII. Our analysis showed that the position of Srwasmoved toward the outside of the cubane structure of the Mn₄CaO₅-cluster relative to that of Ca²⁺, resulting in a general elongation of the bond distances between Sr and its surrounding atoms compared with the corresponding distances in the Ca-containing cluster. In particular, we identified an apparent elongation in the bond distance between Sr and one of the two terminal water ligands of Ca²⁺,W3, whereas that of the Sr-W4 distance was not much changed. This result may contribute to the decrease of oxygen evolution upon Sr ²⁺-substitution, and suggests a weak binding and rather mobile nature of this particular water molecule (W3), which in turn implies the possible involvement of this water molecule as a substrate in the O-O bond formation. In addition, the PsbY subunit, which was absent in the 1.9 Å structure of native PSII, was found in the Sr-PSII structure.