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Five-coordinate Mn-IV intermediate in the activation of nature's water splitting cofactor
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2019 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 116, no 34, p. 16841-16846Article in journal (Refereed) Published
Abstract [en]

Nature's water splitting cofactor passes through a series of catalytic intermediates (S-0-S-4) before O-O bond formation and O-2 release. In the second last transition (S-2 to S-3) cofactor oxidation is coupled to water molecule binding to Mn1. It is this activated, water-enriched all Mn-IV form of the cofactor that goes on to form the O-O bond, after the next light-induced oxidation to S-4. How cofactor activation proceeds remains an open question. Here, we report a so far not described intermediate (S-3') in which cofactor oxidation has occurred without water insertion. This intermediate can be trapped in a significant fraction of centers (> 50%) in (i) chemical-modified cofactors in which Ca2+ is exchanged with Sr2+; the Mn4O5Sr cofactor remains active, but the S-2-S-3 and S-3-S-0 transitions are slower than for the Mn4O5Ca cofactor; and (ii) upon addition of 3% vol/vol methanol; methanol is thought to act as a substrate water analog. The S-3' electron paramagnetic resonance (EPR) signal is significantly broader than the untreated S-3 signal (2.5 T vs. 1.5 T), indicating the cofactor still contains a 5-coordinate Mn ion, as seen in the preceding S-2 state. Magnetic double resonance data extend these findings revealing the electronic connectivity of the S-3' cofactor is similar to the high spin form of the preceding S-2 state, which contains a cuboidal Mn3O4Ca unit tethered to an external, 5-coordinate Mn ion (Mn-4). These results demonstrate that cofactor oxidation regulates water molecule insertion via binding to Mn-4. The interaction of ammonia with the cofactor is also discussed.

Place, publisher, year, edition, pages
2019. Vol. 116, no 34, p. 16841-16846
Keywords [en]
Photosystem II, WOC/OEC, EPR, EDNMR, methanol
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:umu:diva-163062DOI: 10.1073/pnas.1817526116ISI: 000481935500034PubMedID: 31391299OAI: oai:DiVA.org:umu-163062DiVA, id: diva2:1361828
Funder
Swedish Research Council, 2016-05183Available from: 2019-10-17 Created: 2019-10-17 Last updated: 2019-10-17Bibliographically approved

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Nilsson, HåkanMessinger, Johannes

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