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 (S0 to S4), in which S1 is the dark-stable state and S3 is the last semi-stable state before O-O bond formation and O2 evolution. 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. 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 (S1), two-flash illuminated (2F; S3-enriched), and ammonia-bound two-flash illuminated (2F-NH3; S3-enriched) PS II. Although the recent 1.95 Å resolution structure of PS II at cryogenic temperature using an XFEL provided a damage-free view of the S1 state, measurements at room temperature are required to study the structural landscape of proteins under functional conditions, 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 S2 and S3 states. Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site. This approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O-O bond formation mechanisms.

Citation

Iris D Young, Mohamed Ibrahim, Ruchira Chatterjee, Sheraz Gul, Franklin Fuller, Sergey Koroidov, Aaron S Brewster, Rosalie Tran, Roberto Alonso-Mori, Thomas Kroll, Tara Michels-Clark, Hartawan Laksmono, Raymond G Sierra, Claudiu A Stan, Rana Hussein, Miao Zhang, Lacey Douthit, Markus Kubin, Casper de Lichtenberg, Pham Long Vo, Håkan Nilsson, Mun Hon Cheah, Dmitriy Shevela, Claudio Saracini, Mackenzie A Bean, Ina Seuffert, Dimosthenis Sokaras, Tsu-Chien Weng, Ernest Pastor, Clemens Weninger, Thomas Fransson, Louise Lassalle, Philipp Bräuer, Pierre Aller, Peter T Docker, Babak Andi, Allen M Orville, James M Glownia, Silke Nelson, Marcin Sikorski, Diling Zhu, Mark S Hunter, Thomas J Lane, Andy Aquila, Jason E Koglin, Joseph Robinson, Mengning Liang, Sébastien Boutet, Artem Y Lyubimov, Monarin Uervirojnangkoorn, Nigel W Moriarty, Dorothee Liebschner, Pavel V Afonine, David G Waterman, Gwyndaf Evans, Philippe Wernet, Holger Dobbek, William I Weis, Axel T Brunger, Petrus H Zwart, Paul D Adams, Athina Zouni, Johannes Messinger, Uwe Bergmann, Nicholas K Sauter, Jan Kern, Vittal K Yachandra, Junko Yano. Structure of photosystem II and substrate binding at room temperature. Nature. 2016 Dec 15;540(7633):453-457

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PMID: 27871088

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