The oxygen-evolving complex (OEC), also known as the water-splitting complex, is the portion of photosystem II where photo-oxidation of water occurs during the light reactions of photosynthesis. The OEC is surrounded by four core protein subunits of photosystem II at the membrane-lumen interface. In the overall water oxidation reaction, four electrons are extracted from two water molecules so that four protons and one O2 molecule are produced. The molecular machinery for oxidizing water requires the ability to store the oxidizing potential resulting from three light-induced charge separations before the fourth provides sufficient oxidizing power for water oxidation. Species distribution and subcellular location The OEC and Photosystem II are only found in chloroplasts and cyanobacteria. Within the chloroplasts, Photosystem II spans the thylakoid membrane, composed of both granal thylakoids and stromal thylakoids. Structure The OEC has a metalloenzyme core containing both manganese and calcium, with the empirical formula in the S1 state for the inorganic core of Mn4O5Ca. This cluster is coordinated by amino acid side chains of the D1 and CP43 subunits and is stabilized by peripheral membrane proteins. These peripheral membrane proteins include OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ); a fourth PsbR peptide is associated nearby. For other characteristics, see. The Mn4O5Ca cluster is assembled within the Photosystem II protein in a light-driven process called photoactivation. Mn2+ ions diffuse into the active site and are oxidized there by the same components responsible for water oxidation: the redox-active tyrosine of D1, which is oxidized by light-generated P680+. The oxo bridges of the cluster derive from water molecules bound to the metal ions. The quantum efficiency of photoactivation is low. The process is not well understood. Mechanism The OEC transfers four electrons, one at a time, to P680+. A tyrosine residue of the D1 polypeptide, YZ, is the immediate electron donor to P680+. According to the model proposed in 1970 by Bessel Kok, the complex can exist in five states: S0 to S4, S0 being the least oxidized and S4 the most oxidized. Photons absorbed by photosystem II move the system in four steps from state S0 to S4. S4 is unstable and it releases molecular oxygen. For the complex to reset to the lowest state, S0, it binds two new water molecules to enable the cycle to repeat. Currently, although the mechanism of water oxidation by the complex is not completely understood, the main features of the mechanism have