Oxygen-evolving complex

The oxygen-evolving complex (OEC), also known as the water-splitting complex, is a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis.^[3] OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. The mechanism for splitting water involves absorption of three photons before the fourth provides sufficient energy for water oxidation.^[4] Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states, denoted S₀ to S₄, with S₀ the most reduced and S₄ the most oxidized. Photons trapped by photosystem II move the system from state S₀ to S₁ to S₂ to S₃ and finally to S₄. S₄ reacts with water producing free oxygen:

2 H₂O → O₂ + 4 H⁺ + 4 e⁻

The Kok cycle. The oxidation state of the manganese centres is subject to debate.^[1]

X-ray crystal structure of the Mn₄O₅Ca core of the oxygen evolving complex of Photosystem II at a resolution of 1.9 Å.^[2]

This conversion resets the catalyst to the S₀ state.

The active site of the OEC consists of a cluster of manganese and calcium with the formula Mn₄Ca₁O_xCl_1–2(HCO₃)_y. This cluster is bound to D₁ and CP₄₃ subunits and stabilized by peripheral membrane proteins. Many characteristics of it have been examined by flash photolysis experiments, electron paramagnetic resonance (EPR), and X-ray spectroscopy.^[5]

The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide.^{[6][7] [8]}

References

1. Kok, B.; Forbush, B.; McGloin, M. (1970). "Cooperation of charges in photosynthetic O2 evolution. I. A linear four-step mechanism". Photochem. Photobiol. 11 (6): 467–475. doi:10.1111/j.1751-1097.1970.tb06017.x. PMID 5456273. S2CID 31914925.
2. Umena, Yasufumi; Kawakami, Keisuke; Shen, Jian-Ren; Kamiya, Nobuo (May 2011). "Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å" (PDF). Nature. 473 (7345): 55–60. Bibcode:2011Natur.473...55U. doi:10.1038/nature09913. PMID 21499260. S2CID 205224374.
3. Raymond, J.; Blankenship, R. (2008). "The origin of the oxygen-evolving complex". Coordination Chemistry Reviews. 252 (3–4): 377–383. doi:10.1016/j.ccr.2007.08.026.
4. Johnson, James. "The Origin of Life - The Rise of the Oxygen Evolving Complex". www.chm.bris.ac.uk. Florida University. Retrieved 2020-04-30.
5. Abstract : Manganese: The Oxygen-Evolving Complex & Models1 : Encyclopedia of Inorganic Chemistry : Wiley InterScience
6. Askerka, Mikhail; Brudvig, Gary W.; Batista, Victor S. (2017). "The O2-Evolving Complex of Photosystem II: Recent Insights from Quantum Mechanics/Molecular Mechanics (QM/MM), Extended X-ray Absorption Fine Structure (EXAFS), and Femtosecond X-ray Crystallography Data". Accounts of Chemical Research. 50 (1): 41–48. doi:10.1021/acs.accounts.6b00405. PMID 28001034.
7. Yano, Junko; Kern, Jan; Yachandra, Vittal K.; Nilsson, Håkan; Koroidov, Sergey; Messinger, Johannes (2015). "Light-Dependent Production of Dioxygen in Photosynthesis". In Peter M.H. Kroneck and Martha E. Sosa Torres (ed.). Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. Vol. 15. Springer. pp. 13–43. doi:10.1007/978-3-319-12415-5_2. ISBN 978-3-319-12414-8. PMC 4688042. PMID 25707465.
8. Oyala, Paul H.; Stich, Troy A.; Debus, Richard J.; Britt, R. David (2015). "Ammonia Binds to the Dangler Manganese of the Photosystem II Oxygen-Evolving Complex". Journal of the American Chemical Society. 137 (27): 8829–8837. doi:10.1021/jacs.5b04768. PMID 26083545.