The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the active site of 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 S<sub>0</sub> to S<sub>4</sub>, with S<sub>0</sub> the most reduced and S<sub>4</sub> the most oxidized. Energy from the photons captured by photosystem II moves the system from state S<sub>0</sub> to S<sub>1</sub> to S<sub>2</sub> to S<sub>3</sub> and finally to S<sub>4</sub>. S<sub>4</sub> reacts with water producing free oxygen:
This conversion resets the catalyst to the S<sub>0</sub> state.
The active site of the OEC consists of a cluster of manganese and calcium with the formula Mn<sub>4</sub>Ca<sub>1</sub>O<sub>x</sub>Cl<sub>1âÂÂ2</sub>(HCO<sub>3</sub>)<sub>y</sub>. This cluster is bound to D<sub>1</sub> and CP<sub>43</sub> 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.