Oxygen-evolving complex

The oxygen-evolving complex (OEC), also known as the water-splitting complex, is a water-oxidizing enzyme involved in the <a href="/facts/Oxygen_evolution/vPCs8y9G">photo-oxidation of water</a> during the <a href="/facts/Light_reactions/kBnR8Lcp">light reactions</a> of <a href="/facts/Photosynthesis/TMV7w693">photosynthesis</a>. 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 <a href="/facts/Oxidation/Pyd5RVcj">oxidation</a>. 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 <a href="/facts/Photosystem_II/9tmDPU50">photosystem II</a> moves the system from state S0 to S1 to S2 to S3 and finally to S4. S4 reacts with water producing free <a href="/facts/Oxygen/acyn6o8r">oxygen</a>:

2 H2O → O2 + 4 H+ + 4 e−
This conversion resets the catalyst to the S0 state.
The active site of the OEC consists of a cluster of <a href="/facts/Manganese/j9ELd1TL">manganese</a> and <a href="/facts/Calcium/hf252CC0">calcium</a> with the formula Mn4Ca1OxCl1–2(HCO3)y. This cluster is bound to D1 and CP43 subunits and stabilized by <a href="/facts/Peripheral_membrane_protein/EGsWYg2e">peripheral membrane proteins</a>. Many characteristics of it have been examined by <a href="/facts/Flash_photolysis/g9gSdAHr">flash photolysis</a> experiments, <a href="/facts/Electron_paramagnetic_resonance/TDHV8c9e">electron paramagnetic resonance</a> (EPR), and <a href="/facts/X-ray_spectroscopy/spWfPwYK">X-ray spectroscopy</a>.
The mechanism of the complex is proposed to involve an Mn-oxide which couples by O-O bond formation to a calcium oxide/hydroxide.

Oxygen-evolving complex open-in-new

Oxygen-evolving complex