Oxygen-evolving complex

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The Kok cycle. The oxidation state of the manganese centres is subject to debate.

File: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. 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. 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. Energy from the photons captured by photosystem II moves the system from state S₀ to S₁ to S₂ to S₃ and finally to S₄. S₄ reacts with water producing free oxygen:

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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 Mn₄Ca₁O_xCl_1–2(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.

The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide.

References

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