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- W2071329528 abstract "Biogenesis and repair of the inorganic core (Mn4CaOxCly), in the water-oxidizing complex of photosystem II (WOC−PSII), occurs through the light-induced (re)assembly of its free elementary ions and the apo-WOC−PSII protein, a reaction known as photoactivation. Herein, we use electron paramagnetic resonance (EPR) spectroscopy to characterize changes in the ligand coordination environment of the first photoactivation intermediate, the photo-oxidized Mn3+ bound to apo-WOC−PSII. On the basis of the observed changes in electron Zeeman (geff), 55Mn hyperfine (AZ) interaction, and the EPR transition probabilities, the photogenerated Mn3+ is shown to exist in two pH-dependent forms, differing in terms of strength and symmetry of their ligand fields. The transition from an EPR-invisible low-pH form to an EPR-active high-pH form occurs by deprotonation of an ionizable ligand bound to Mn3+, implicated to be a water molecule: [Mn3+(OH2)] ↔ [Mn3+(OH-)]. In the absence of Ca2+, the EPR-active Mn3+ exhibits a strong pH dependence (pH ∼6.5−9) of its ligand-field symmetry (rhombicity Δδ = 10%, derived from geff) and AZ (ΔAZ = 22%), attributable to a protein conformational change. Binding of Ca2+ to its effector site eliminates this pH dependence and locks both geff and AZ at values observed in the absence of Ca2+ at alkaline pH. Thus, Ca2+ directly controls the coordination environment and binds close to the high-affinity Mn3+, probably sharing a bridging ligand. This Ca2+ effect and the pH-induced changes are consistent with the ionization of the bridging water molecule, predicting that [Mn3+−(μ-O-2)−Ca2+] or [Mn3+−(μ-OH-)2−Ca2+] is the first light intermediate in the presence of Ca2+. The formation of this intermediate templates the apo-WOC−PSII for the subsequent rapid cooperative binding and photo-oxidation of three additional Mn2+ ions, forming the active water oxidase." @default.
- W2071329528 created "2016-06-24" @default.
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- W2071329528 date "2006-09-30" @default.
- W2071329528 modified "2023-10-03" @default.
- W2071329528 title "Spectroscopic Evidence for Ca<sup>2+</sup>Involvement in the Assembly of the Mn<sub>4</sub>Ca Cluster in the Photosynthetic Water-Oxidizing Complex" @default.
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- W2071329528 doi "https://doi.org/10.1021/bi061495t" @default.
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