Assignment of the slowly exchanging substrate water of nature's water-splitting cofactor

• 成果类型: Article
• 署名作者: de Lichtenberga, Casper; Rapatskiy, Leonid; Reusc, Michael; Heyno, Eiri; Schneggc, Alexander; Nowaczykd, Marc M.; Lubitz, Wolfgang; Messinge, Johannes; Cox, Nicholas
• 署名单位: Uppsala University; Umea University; Max Planck Society; Ruhr University Bochum; Australian National University; University of Rostock
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-10282
• DOI: 10.1073/pnas.2319374121
• 发表日期: 2024-03-12
• 关键词: oxygen-evolving complex; o bond formation; photosystem-ii; s-2 state; electronic-structure; catalytic cycle; proton release; detected nmr; mu-oxo; oxidation

摘要

Identifying the two substrate water sites of nature's water- splitting cofactor (Mn4CaO5 cluster) provides important information toward resolving the mechanism of O - O bond formation in Photosystem II (PSII). To this end, we have performed parallel substrate water exchange experiments in the S-1 state of native Ca-PSII and biosynthetically substituted Sr-PSII employing Time- Resolved Membrane Inlet Mass Spectrometry (TR-MIMS) and a Time- Resolved O-17- Electron- electron Double resonance detected NMR (TR-O-17- EDNMR) approach. TR-MIMS resolves the kinetics for incorporation of the oxygen- isotope label into the substrate sites after addition of (H2O)-O-18 to the medium, while the magnetic resonance technique allows, in principle, the characterization of all exchangeable oxygen ligands of the Mn4CaO5 cofactor after mixing with (H2O)-O-17. This unique combination shows i) that the central oxygen bridge (O5) of Ca-PSII core complexes isolated from Thermosynechococcus vestitus has, within experimental conditions, the same rate of exchange as the slowly exchanging substrate water (WS) in the TR-MIMS experiments and ii) that the exchange rates of O5 and W-S are both enhanced by Ca2+-> Sr2+ substitution in a similar manner. In the context of previous TR-MIMS results, this shows that only O5 fulfills all criteria for being W-S. This strongly restricts options for the mechanism of water oxidation.