Oxygen-evolving complex

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The Kok cycle. The oxidation state of the manganese centres is subject to debate. Kok-cycle.svg
The Kok cycle. The oxidation state of the manganese centres is subject to debate.
X-ray crystal structure of the Mn4O5Ca core of the oxygen evolving complex of Photosystem II at a resolution of 1.9 A. Oxygen Evolving Complex Crystal structure to 1.9 Angstrom Resolution.png
X-ray crystal structure of the Mn4O5Ca core of the oxygen evolving complex of Photosystem II at a resolution of 1.9 Å.

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

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 Mn4Ca1OxCl1–2(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]

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<span class="mw-page-title-main">Metalloprotein</span> Protein that contains a metal ion cofactor

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<span class="mw-page-title-main">Photosystem</span> Structural units of protein involved in photosynthesis

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Oxygenevolution is the process of generating molecular oxygen (O2) by a chemical reaction, usually from water. Oxygen evolution from water is effected by oxygenic photosynthesis, electrolysis of water, and thermal decomposition of various oxides. The biological process supports aerobic life. When relatively pure oxygen is required industrially, it is isolated by distilling liquefied air.

Dioxygen plays an important role in the energy metabolism of living organisms. Free oxygen is produced in the biosphere through photolysis of water during photosynthesis in cyanobacteria, green algae, and plants. During oxidative phosphorylation in cellular respiration, oxygen is reduced to water, thus closing the biological water-oxygen redox cycle.

<span class="mw-page-title-main">Manganese in biology</span> Use of manganese by organisms

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<span class="mw-page-title-main">Water oxidation catalysis</span>

Water oxidation catalysis (WOC) is the acceleration (catalysis) of the conversion of water into oxygen and protons:

R. David Britt is the Winston Ko Chair and Distinguished Professor of Chemistry at the University of California, Davis. Britt uses electron paramagnetic resonance (EPR) spectroscopy to study metalloenzymes and enzymes containing organic radicals in their active sites. Britt is the recipient of multiple awards for his research, including the Bioinorganic Chemistry Award in 2019 and the Bruker Prize in 2015 from the Royal Society of Chemistry. He has received a Gold Medal from the International EPR Society (2014), and the Zavoisky Award from the Kazan Scientific Center of the Russian Academy of Sciences (2018). He is a Fellow of the American Association for the Advancement of Science and of the Royal Society of Chemistry.

References

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  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.