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.[2] OEC is surrounded by 4 core proteins of photosystem II at the membrane-lumen interface. [3] Electrons extracted from 2 water molecules along with 4 photons produce a singular O₂ molecule.[3] The molecular machinery for dissecting water requires the ability to store energy from the first three photons before the fourth provides sufficient energy for water oxidation.[4] Then, the OEC transfers 4 electrons, 1 at a time, to photosystem II via a tyrosine residue in the reaction center. [4]Based on a widely accepted theory from 1970 by Kok, the complex can exist in 5 states: S₀ to S₄. S₄ being the most oxidized and S₀ the most reduced. Photons trapped by photosystem II move the system from state S₀ to S₄. S₄ is unstable and reacts with water producing free oxygen. For the complex to reset to the lowest state, S₀, it uses 2 water molecules to pull out 4 electrons. [3]

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 OEC appears to have a metalloenzyme core containing both manganese and calcium, with the empirical formula for the inorganic core of Mn₄Ca₁O_xCl_1–2(HCO₃)_y. This cluster is coordinated by D₁ and CP₄₃ subunits and stabilized by peripheral membrane proteins. [3] Other characteristics of it have been reviewed; see [5]

Currently, the mechanism of the complex is not completely understood.[6] The exact order in which manganese oxidation occurs through the photo-induced Kok cycle is unknown.[3] Along with the role of Ca⁺², Cl^-1, and the membrane proteins surrounding the metal cluster not being well understood. Much of what is known has been collected from flash experiments, EPR, and x-ray spectroscopy.[7]

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.[8]