Photosystem II (PS II), D2 subunit. PS II is a multi-subunit protein found in the photosynthetic membranes of plants, algae, and cyanobacteria. It utilizes light-induced electron transfer and water-splitting reactions to produce protons, electrons, and molecular oxygen. The protons generated are instrumental in ATP formation. Molecular dioxygen is released as a by-product. PS II can be described as containing two parts: the photochemical part and the catalytic part. The photochemical portion promotes the fast, efficient light-induced charge separation and stabilization that occur when light is absorbed by chlorophyll. The catalytic portion, where water is oxidized, involves a cluster of Mn ions close to a redox-active tyrosine residue. The Mn cluster and its ligands form a functional unit called the oxygen-evolving complex (OEC) or the water-oxidizing complex (WOC). The D1 and D2 subunits are a pair of intertwined polypeptides. They contain all the cofactors involved directly in water oxidation and plastoquinone reduction. D1 and D2 are highly homologous and are also similar to the L and M proteins in bacterial photosynthetic reaction centers.
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