Structural basis for impaired oxygen evolution in extrinsic-protein-reconstituted photosystem II

Photosystem II (PSII) is a membrane-bound pigment-protein complex in oxygenic photosynthesis that catalyzes water splitting and oxygen evolution. Here, we present the X-ray crystallographic structure of a PsbO/V/U-reconstituted PSII from Thermosynechococcus vulcanus at 2.0 [A] resolution, revealing proper rebinding of the three extrinsic subunits, PsbO, PsbV, and PsbU. The overall geometry of the Mn4CaO5 cluster is largely preserved, although a subtle shortening of the Mn2-O2 bond suggests a minor local rearrangement. Structural analysis identifies perturbations that may underlie the reduced oxygen-evolving activity, including altered bicarbonate-binding interactions on the electron acceptor side and the loss of water molecules W658 and W660 in the O1 channel, disrupting a hydrogen-bond network critical for water delivery. In contrast, the Cl-1 and O4 channels remain intact. These findings suggest that disrupted water delivery and electron transport, together with minor rearrangements within the Mn4CaO5 cluster, may contribute to the decreased activity of the reconstituted PSII.