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. It is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states: S0 to S4. Photons trapped by photosystem II move the system from state S0 to S4. S4 is unstable and reacts with water producing free oxygen. Currently, the mechanism of the complex is not completely understood. Much of what is known has been collected from flash experiments, EPR, and X-ray spectroscopy.
The OEC appears to have a metalloenzyme core containing both manganese and calcium, with the empirical formula for the inorganic core of Mn4Ca1OxCl1–2(HCO3)y. Other characteristics of it have been reviewed; see 
It is suggested that the evolution of the OEC was triggered by the presence of the manganese-containing minerals rancieite and hollandite in the early oceans. The minerals were, it is presumed, assimilated by the early Cyanobacteria, which have incorporated them in the active center of the complex.
In order to determine more about the structure, most research focuses on creating manganese-core dimers at various protonation states and comparing the resultant data with observed behavior of the complex.
- 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". Nature. 473 (7345): 55–60. doi:10.1038/nature09913. PMID 21499260.
- "The origin of the oxygen-evolving complex". Coordination Chemistry Reviews. 252: 377–383. doi:10.1016/j.ccr.2007.08.026.
- Yano, Junko; Kern, Jan; Yachandra, Vittal K.; Nilsson, Håkan; Koroidov, Sergey; Messinger, Johannes (2015). "Chapter 2, Section 2 Geometric and Electronic Structure of the Mn4CaO5 Cluster". In Peter M.H. Kroneck and Martha E. Sosa Torres. Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. 15. Springer. pp. 13–43. doi:10.1007/978-3-319-12415-5_2.
- Kok, B.; Forbush, B.; McGloin, M. (1970). "Cooperation of charges in photosynthetic O2 evolution. I. A linear four-step mechanism". Photochem. Photobiol. 11: 467–475. doi:10.1111/j.1751-1097.1970.tb06017.x. PMID 5456273.
- Abstract : Manganese: The Oxygen-Evolving Complex & Models1 : Encyclopedia of Inorganic Chemistry : Wiley InterScience
- 'Life Ascending' by Nick Lane, Profile Books 2009; pp 83–87 - outlines a possible route for the evolution of the OEC.
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