| Description | Oxygenic photosynthesis uses two multi-subunit photosystems (I and II) located in the cell membranes of cyanobacteria and in the thylakoid membranes of chloroplasts in plants and algae. Photosystem II (PSII) has a P680 reaction centre containing chlorophyll 'a' that uses light energy to carry out the oxidation (splitting) of water molecules, and to produce ATP via a proton pump. Photosystem I (PSI) has a P700 reaction centre containing chlorophyll that takes the electron and associated hydrogen donated from PSII to reduce NADP+ to NADPH. Both ATP and NADPH are subsequently used in the light-independent reactions to convert carbon dioxide to glucose using the hydrogen atom extracted from water by PSII, releasing oxygen as a by-product.PSII is a multisubunit protein-pigment complex containing polypeptides both intrinsic and extrinsic to the photosynthetic membrane [ , , ]. Within the core of the complex, the chlorophyll and β-carotene pigments are mainly bound to the antenna proteins CP43 (PsbC) and CP47 (PsbB), which pass the excitation energy on to the reaction centre proteins D1 (Qb, PsbA) and D2 (Qa, PsbD) that bind all the redox-active cofactors involved in the energy conversion process. The PSII oxygen-evolving complex (OEC) oxidises water to provide protons for use by PSI, and consists of OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ). The remaining subunits in PSII are of low molecular weight (less than 10kDa), and are involved in PSII assembly, stabilisation, dimerisation, and photo-protection [].This family represents the D2 protein (PsbD), which forms the reaction core of PSII as a heterodimer with the D1 protein. The accumulation of D2 protein appears to be a key step in the assembly of the PSII reaction centre complex [ ]. In higher plants, the N-terminal residues of both proteins, which are exposed to the stromal surface, can be reversibly phosphorylated. The D1/D2 core binds to a number of cofactors, including: a 4-atom manganese cluster, which is located on the lumenal surface of the D1 and D2 proteins []; two pheophytin molecules, only one of which is phytochemically active; non-haem iron; and two quinones, Qa (bound to D2) and Qb (bound to D1). Upon light excitation, an electron is transferred from the primary donor (chlorophyll a) via intermediate acceptor pheophytin to the primary quinone Qa, then to the secondary quinone Qb. At the oxidising side of PSII, a redox-active residue in the D1 protein reduces P680, the oxidised tyrosine then withdrawing electrons from a manganese cluster, which in turn withdraw electrons from water, leading to the splitting of water and the formation of molecular oxygen. PSII thus provides a source of electrons that can be used by photosystem I to produce the reducing power (NADPH) required to convert CO2 to glucose. | Name | Photosystem II D2 protein |
| Short Name | PSII_PsbD/D2 | Type | Family |
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