Oxygen-evolving complex

The Kok cycle. Note that 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 Å.[1]

The oxygen-evolving complex, (OEC) also known as the water-splitting complex, is a water-oxidizing enzyme involved in the photooxidation 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.[2] Much of the known data have been collected from flash experiments, EPR, and X-ray spectroscopy.[3]

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 [4]

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.

References

  1. ↑ Umena, Yasufumi; Kawakami, Keisuke; Shen, Jian-Ren; Kamiya, Nobuo. "Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å". Nature 473 (7345): 55–60. doi:10.1038/nature09913.
  2. ↑ 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.
  3. ↑ 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.
  4. ↑ Abstract : Manganese: The Oxygen-Evolving Complex & Models1 : Encyclopedia of Inorganic Chemistry : Wiley InterScience

[1] Also see 'Life Ascending' by Nick Lane, Profile Books 2009; pp 83–87, which outlines a possible route for the evolution of the OEC.


  1. ↑ "The origin of the oxygen-evolving complex". Coordination Chemistry Reviews 252: 377–383. doi:10.1016/j.ccr.2007.08.026.
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