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PK_a of the ligand water molecules in the oxygen-evolving Mn₄CaO₅ cluster in photosystem II.

ABSTRACT: Release of the protons from the substrate water molecules is prerequisite for O₂ evolution in photosystem II (PSII). Proton-releasing water molecules with low pK_a values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH^-. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH^-. Here we report the pK_a values of the four ligand water molecules, W1 and W2 at Mn4 and W3 and W4 at Ca²⁺, of the Mn₄CaO₅ cluster. pK_a(W1) ≈ pK_a(W2) << pK_a(W3) ≈ pK_a(W4) in the Mn₄CaO₅ cluster in water. However, pK_a(W1) ≈ pK_a(D1-Asp61) << pK_a(W2) in the PSII protein environment. These results suggest that in PSII, deprotonation of W2 is energetically disfavored as far as W1 exists.

SUBMITTER: Saito K

PROVIDER: S-EPMC9814768 | biostudies-literature | 2020 Jul

REPOSITORIES: biostudies-literature

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pKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II.

Saito Keisuke K Nakagawa Minesato M Ishikita Hiroshi H

Communications chemistry 20200716 1

Release of the protons from the substrate water molecules is prerequisite for O2 evolution in photosystem II (PSII). Proton-releasing water molecules with low pKa values at the catalytic moiety can be the substrate water molecules. In some studies, one of the ligand water molecules, W2, is regarded as OH-. However, the PSII crystal structure shows neither proton acceptor nor proton-transfer pathway for W2, which is not consistent with the assumption of W2 = OH<su ...[more]

PMID: 36703312

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Project description:The oxygen-evolving complex of Photosystem II in plants and cyanobacteria catalyzes the oxidation of two water molecules to one molecule of dioxygen. A tetranuclear Mn complex is believed to cycle through five intermediate states (S0-S4) to couple the four-electron oxidation of water with the one-electron photochemistry occurring at the Photosystem II reaction center. We have used X-ray absorption spectroscopy to study the local structure of the Mn complex and have proposed a model for it, based on studies of the Mn K-edges and the extended X-ray absorption fine structure of the S1 and S2 states. The proposed model consists of two di-μ-oxo-bridged binuclear Mn units with Mn-Mn distances of ~2.7 Å that are linked to each other by a mono-μ-oxo bridge with a Mn-Mn separation of ~3.3 Å. The Mn-Mn distances are invariant in the native S1 and S2 states. This report describes the application of X-ray absorption spectroscopy to S3 samples created under physiological conditions with saturating flash illumination. Significant changes are observed in the Mn-Mn distances in the S3 state compared to the S1 and the S2 states. The two 2.7 Å Mn-Mn distances that characterize the di-μ-oxo centers in the S1 and S2 states are lengthened to ~2.8 and 3.0 Å in the S3 state, respectively. The 3.3 Å Mn-Mn and Mn-Ca distances also increase by 0.04-0.2 Å. These changes in Mn-Mn distances are interpreted as consequences of the onset of substrate/water oxidation in the S3 state. Mn-centered oxidation is evident during the S0→S1 and S1→S2 transitions. We propose that the changes in Mn-Mn distances during the S2→S3 transition are the result of ligand or water oxidation, leading to the formation of an oxyl radical intermediate formed at a bridging or terminal position. The reaction of the oxyl radical with OH-, H2O, or an oxo group during the subsequent S state conversion is proposed to lead to the formation of the O-O bond. Models that can account for changes in the Mn-Mn distances in the S3 state and the implications for the mechanism of water oxidation are discussed.

Dataset Information

PK_a of the ligand water molecules in the oxygen-evolving Mn₄CaO₅ cluster in photosystem II.

Publications

pK<sub>a</sub> of the ligand water molecules in the oxygen-evolving Mn<sub>4</sub>CaO<sub>5</sub> cluster in photosystem II.

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Dataset Information

PKa of the ligand water molecules in the oxygen-evolving Mn4CaO5 cluster in photosystem II.

Publications

pK<sub>a</sub> of the ligand water molecules in the oxygen-evolving Mn<sub>4</sub>CaO<sub>5</sub> cluster in photosystem II.

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets

PK_a of the ligand water molecules in the oxygen-evolving Mn₄CaO₅ cluster in photosystem II.