Structural Basis for Impaired Oxygen Evolution in Extrinsic-Protein-Reconstituted Photosystem II

Photosystem II (PSII) is a membrane-embedded pigment-protein complex in oxygenic photosynthesis that catalyzes water splitting and oxygen evolution. Here, we present the X-ray crystallographic structure of PsbO/V/U-reconstituted PSII from Thermosynechococcus vulcanus at 2.0 Å resolution, revealing proper rebinding of the three extrinsic subunits, PsbO, PsbV, and PsbU. The overall geometry of the Mn 4 CaO 5 cluster was largely preserved, with a subtle local change around the Mn2−O2 bond. Structural analysis revealed several perturbations that may contribute to the reduced oxygen-evolving activity, including altered bicarbonate-binding interactions on the electron acceptor side and reduced ordering of the W658 and W660 hydration sites in the O1 channel, together with changes in the local hydrogen-bond network. These findings provide a structural basis for understanding the incomplete recovery of oxygen-evolving activity in the reconstituted PSII, highlighting local donor- and acceptor-side perturbations that remain despite proper rebinding of the extrinsic proteins.