Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 116, Issue 34, Pages 16841-16846Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1817526116
Keywords
Photosystem II; WOC/OEC; EPR; EDNMR; methanol
Categories
Funding
- Max Planck Society
- MANGAN project - Bundesministeriums fur Bildung und Forschung [03EK3545]
- Australian Research Council [FT140100834]
- Vetenskapradet [2016-05183]
- Cluster of Excellence RESOLV - German Research Council (DFG) [EXC 1069]
- DFG research unit FOR2092 [NO 836/3-2]
- Deutsch-Israelische Projektkooperation Grant [LU 315/17-1]
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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.
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