![]() Since the advent of X-ray free electron lasers (XFELs) there have been efforts to study intermediate states of PS II ( 15– 21). The Glu189 residue of the D1 protein as a ligand of Ca moves away during the transition with the insertion of oxygen (O X = O 2− or OH −) as a new bridging ligand between Mn1 and Ca (adapted from ref. ( C) The structures of the Mn 4Ca cluster, showing the major changes between the S 2 and S 3 states (Mn 3+ yellow, Mn 4+ purple, Ca 2+ green, O red W1, 2, 3, 4 are water ligands to Mn4 and Ca). The S 2 → S 3 transition, marked by the red box, is discussed in detail in the main text. The white and gray areas show the redox changes of the OEC (S 1 to S 0) and Y Z together with the charge separation reactions, while the outer blue circle shows the concomitant acceptor side chemistry of the quinones. ( B) Kok cycle of the water oxidation reaction that is triggered by the absorption of photons (nanosecond light flashes, 1F to 4F) in the antenna and the reaction center P 680. ( A) The structure of PS II with the membrane-embedded helices and the large membrane extrinsic regions on the luminal side of PS II are shown as cartoon in gray and in color the main electron transfer components involved in charge transfer (P 680, Pheo) and stabilization with the acceptor quinones, Q A and Q B, toward the cytoplasm, and Y Z and OEC with catalytic Mn 4Ca cluster on the donor side at the interface between the membrane and the lumen-bound part of PS II. After two light-induced charge separations, the dihydroplastoquinol, Q BH 2, formed at the Q B site is replaced by a new plastoquinone molecule ( Fig. At the electron acceptor side of PS II, the electrons created in the charge separation process are transferred to plastoquinone Q A and subsequently to plastoquinone Q B. Once four oxidizing equivalents are accumulated (S 4-state), the formation of dioxygen occurs spontaneously and leads to the release of O 2 and the reformation of the S 0-state. The one-electron photochemistry at the reaction center is coupled to the four-electron redox chemistry at the OEC via a redox active tyrosine residue D1-Tyr161, also known as Y Z. These oxidation reactions are driven by the light-induced charge separations in the reaction center of PS II that comprises the chlorophyll a moiety P 680 (most likely consisting of the four excitonically coupled chlorophylls P D1, P D2, Chl D1, and Chl D2) and the neighboring pheophytin molecules Pheo D1/D2 as well as the two plastoquinones Q A and Q B. To allow this reaction to take place, the OEC accumulates four oxidizing equivalents, stepping through intermediates that are referred to as the S-states (S i, i = 0 through 4) ( Fig. This indicates that the oxidation state change and the insertion of water as a bridging atom between Mn1 and Ca are highly correlated. 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 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. This is followed by Mn1 and Mn4 moving apart, and the insertion of O X(H) at the open coordination site of Mn1. 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. At the electron acceptor site, changes due to the two-electron redox chemistry at the quinones, Q A and Q B, are observed. 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. ![]() 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. 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). Sauter, Uwe Bergmann, Athina Zouni, Johannes Messinger, Jan Kern, Vittal K. Orville, Nicholas Saichek, Trent Northen, Alexander Batyuk, Sergio Carbajo, Roberto Alonso-Mori, Kensuke Tono, Shigeki Owada, Asmit Bhowmick, Robert Bolotovsky, Derek Mendez, Nigel W. Simon, Casper de Lichtenberg, Petko Chernev, Isabel Bogacz, Cindy C. Fuller, Sheraz Gul, In-Sik Kim, Philipp S. Mohamed Ibrahim, Thomas Fransson, Ruchira Chatterjee, +35, Mun Hon Cheah, Rana Hussein, Louise Lassalle, Kyle D.
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