Crystallographic Data Support the Carousel Mechanism of Water Supply to the Oxygen-Evolving Complex of Photosystem II

[Image: see text] Photosystem II (PSII) oxidizes water to produce oxygen through a four-step photocatalytic cycle. Understanding PSII structure–function relations is important for the development of biomimetic photocatalytic systems. The quantum mechanics/molecular mechanics (QM/MM) analysis of substrate water binding to the oxygen-evolving complex (OEC) has suggested a rearrangement of water ligands in a carousel mechanism around a key Mn center. Here, we find that the most recently reported X-ray free-electron laser (XFEL) crystallographic data obtained for the dark-stable S(1) state and the doubly flashed S(3) state at 2.25 Å resolution support the carousel mechanism. The features in the XFEL data and QM/MM model-simulated difference Fourier maps suggest that water displacement may occur from the so-called “narrow” channel, resulting in binding of a new water molecule to the OEC, and thus provide new insights into the nature of rearrangements of water ligands along the catalytic cycle before O=O bond formation.