Photosynthetic water oxidation is catalyzed by the Mn 4 CaO 5 cluster of photosystem II (PSII) with linear progression through five S-state intermediates (S 0 to S 4 ). To reveal the mechanism of water oxidation, we analyzed structures of PSII in the S 1 , S 2 , and S 3 states by x-ray free-electron laser serial crystallography. No insertion of water was found in S 2 , but flipping of D1 Glu 189 upon transition to S 3 leads to the opening of a water channel and provides a space for incorporation of an additional oxygen ligand, resulting in an open cubane Mn 4 CaO 6 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.
Organizational Affiliation: 
Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan. michisuga@okayama-u.ac.jp ago@spring8.or.jp shen@cc.okayama-u.ac.jp.
Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
Research Institute for Interdisciplinary Science and Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan.
RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan.
Department of Picobiology, Graduate School of Life Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan.
Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan.
Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan.
The Institute for Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.
Institute for Nanoscience Design, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.