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]
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.
^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.
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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. ISBN978-3-319-12414-8. PMC4688042. PMID25707465.
^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. PMID26083545.