American Association for the Advancement of Science Acta Medica Okayama 0036-8075 366 6463 2019 An oxyl/oxo mechanism for dioxygen bond formation in PSII revealed by X-ray free electron lasers 334 338 EN Michihiro Suga Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Fusamichi Akita Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Keitaro Yamashita RIKEN SPring-8 Center Yoshiki Nakajima Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Go Ueno RIKEN SPring-8 Center Hongjie Li Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Takahiro Yamane Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Kunio Hirata RIKEN SPring-8 Center Yasufumi Umena Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Shinichiro Yonekura Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Long-Jiang Yu Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Hironori Murakami Japan Synchrotron Radiation Research Institute Takashi Nomura RIKEN SPring-8 Center Tetsunari Kimura Department of Chemistry, Graduate School of Science, Kobe University Minoru Kubo RIKEN SPring-8 Center Seiki Baba Japan Synchrotron Radiation Research Institute Takashi Kumasaka Japan Synchrotron Radiation Research Institute Kensuke Tono RIKEN SPring-8 Center Makina Yabashi RIKEN SPring-8 Center Hiroshi Isobe Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Kizashi Yamaguchi The Institute for Scientific and Industrial Research, Osaka University Masaki Yamamoto RIKEN SPring-8 Center Hideo Ago RIKEN SPring-8 Center Jian-Ren Shen Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University Photosynthetic water oxidation is catalyzed by the Mn4CaO5 cluster of photosystem II (PSII) with linear progression through five S-state intermediates (S0 to S4). To reveal the mechanism of water oxidation, we analyzed structures of PSII in the S1, S2, and S3 states by x-ray free-electron laser serial crystallography. No insertion of water was found in S2, but flipping of D1 Glu189 upon transition to S3 leads to the opening of a water channel and provides a space for incorporation of an additional oxygen ligand, resulting in an open cubane Mn4CaO6 cluster with an oxyl/oxo bridge. Structural changes of PSII between the different S states reveal cooperative action of substrate water access, proton release, and dioxygen formation in photosynthetic water oxidation. No potential conflict of interest relevant to this article was reported.