Photosystem II, poised for O2 formation

The vast majority of oxygen in Earth's atmosphere is generated by photosynthetic light reactions. This process also plays an integral role in producing energy for Earth's biosphere. Photosystem II (PSII) uses visible light to oxidize water and release O2. Its oxygen-evolving complex (OEC) sequentially advances from its most reduced state (S0), through four photon-driven oxidations, to its most oxidized state (S4), which produces O2. As described on page 334 of this issue, Suga et al. (1) used a powerful x-ray free-electron laser (XFEL) to examine in structural detail the OEC just prior to O=O bond formation, thereby adding to a growing body of work in this area. Such mechanistic details of the OEC's operations could help in the development of synthetic solar fuel reactions aimed at sustainable fuel production from water and sunlight.