Structure of photosystem II and substrate binding at room temperature

Light-induced oxidation of water by photosystem II (PS II) in plants, algae and cyanobacteria has generated most of the dioxygen in the atmosphere. PS II, a membrane-bound multi-subunit pigment protein complex, couples the one-electron photochemistry at the reaction centre with the four-electron redox chemistry of water oxidation at the Mn4CaO5 cluster in the oxygen-evolving complex (OEC). Under illumination, the OEC cycles through five intermediate S-states (S-0 to S-4)(1), in which S-1 is the dark-stable state and S-3 is the last semi-stable state before O-O bond formation and O-2 evolution(2,3). A detailed understanding of the O-O bond formation mechanism remains a challenge, and will require elucidation of both the structures of the OEC in the different S-states and the binding of the two substrate waters to the catalytic site(4-6). Here we report the use of femtosecond pulses from an X-ray free electron laser (XFEL) to obtain damage-free, room temperature structures of dark-adapted (S-1), two-flash illuminated (2F; S-3-enriched), and ammonia-bound two-flash illuminated (2F-NH3; S-3-enriched) PS II. Although the recent 1.95 angstrom resolution structure of PS II at cryogenic temperature using an XFEL7 provided a damage-free view of the S-1 state, measurements at room temperature are required to study the structural landscape of proteins under functional conditions(8,9), and also for in situ advancement of the S-states. To investigate the water-binding site(s), ammonia, a water analogue, has been used as a marker, as it binds to the Mn4CaO5 cluster in the S-2 and S-3 states(10). Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site(10-13). This approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O-O bond formation mechanisms.