{"doi":"10.1073/pnas.2000529117","title":"Untangling the sequence of events during the S <sub>2</sub> → S <sub>3</sub> transition in photosystem II and implications for the water oxidation mechanism","abstract":"In oxygenic photosynthesis, light-driven oxidation of water to molecular oxygen is carried out by the oxygen-evolving complex (OEC) in photosystem II (PS II). Recently, we reported the room-temperature structures of PS II in the four (semi)stable S-states, S 1 , S 2 , S 3 , and S 0 , showing that a water molecule is inserted during the S 2 → S 3 transition, as a new bridging O(H)-ligand between Mn1 and Ca. To understand the sequence of events leading to the formation of this last stable intermediate state before O 2 formation, we recorded diffraction and Mn X-ray emission spectroscopy (XES) data at several time points during the S 2 → S 3 transition. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, Q A and Q B , are observed. At the donor site, tyrosine Y Z and His190 H-bonded to it move by 50 µs after the second flash, and Glu189 moves away from Ca. This is followed by Mn1 and Mn4 moving apart, and the insertion of O X (H) at the open coordination site of Mn1. This water, possibly a ligand of Ca, could be supplied via a “water wheel”-like arrangement of five waters next to the OEC that is connected by a large channel to the bulk solvent. XES spectra show that Mn oxidation (τ of ∼350 µs) during the S 2 → S 3 transition mirrors the appearance of O X electron density. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated.","journal":"Proceedings of the National Academy of Sciences","year":2020,"id":49681,"datarank":4.757635990535866,"base_score":5.480638923341991,"endowment":5.480638923341991,"self_citation_contribution":0.8220958385012987,"citation_network_contribution":3.9355401520345676,"self_endowment_contribution":0.8220958385012987,"citer_contribution":3.9355401520345676,"corpus_percentile":null,"corpus_rank":null,"citation_count":239,"citer_count":100,"citers_with_citation_signal":100,"citers_with_endowment":100,"datacite_reuse_total":0,"is_dataset":false,"is_dataset_confidence":0.9526,"is_data_producer":true,"deposit_databanks":{"PDB":["6W1O","6W1P","6W1Q","6W1R","6W1T","6W1U","6W1V"]},"is_oa":true,"file_count":0,"downloads":0,"has_version_chain":false,"published_date":"2020-01-01","fair_score":null,"fair_percentile":null,"algorithm_id":"datarank_citation_only_1hop_v6","ranking_scope":"data_only","authors":[{"id":240862,"name":"Thomas Fransson","orcid":"0000-0002-3770-9780","position":1,"is_corresponding":false},{"id":240863,"name":"Ruchira Chatterjee","orcid":"0000-0002-0865-061X","position":2,"is_corresponding":false},{"id":240864,"name":"Mun Hon Cheah","orcid":"0000-0001-5732-1524","position":3,"is_corresponding":false},{"id":240865,"name":"Rana Hussein","orcid":"0000-0001-6292-2375","position":4,"is_corresponding":false},{"id":243676,"name":"Louise Lassalle","orcid":null,"position":5,"is_corresponding":false},{"id":240866,"name":"Kyle D. 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