InterPro domain: IPR037025

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 [ 1 , 2 , 3 ]. Within the core of the complex, the chlorophyll and beta-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 [ 4 ].

This entry represents the alpha subunit (PsbE) superfamily of cytochrome b559, which forms a haem-binding heterodimer with the beta subunit (PsbF) ( IPR006241 ) within the reaction centre core of PSII. Both PsbE and PsbF are essential components for PSII assembly, and are probably involved in secondary electron transport mechanisms that help to protect PSII from photo-damage [ 5 ].

1. Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7-A resolution. Proc. Natl. Acad. Sci. U.S.A. 100, 98-103
2. The evolutionary development of the protein complement of photosystem 2. Biochim. Biophys. Acta 1655, 133-9
3. Thylakoid membrane lipid sulfoquinovosyl-diacylglycerol (SQDG) is required for full functioning of photosystem II in Thermosynechococcus elongatus. J Biol Chem 293, 14786-14797
4. The low molecular mass subunits of the photosynthetic supracomplex, photosystem II. Biochim. Biophys. Acta 1608, 75-96
5. Mossbauer studies of the non-heme iron and cytochrome b559 in a Chlamydomonas reinhardtii PSI- mutant and their interactions with alpha-tocopherol quinone. FEBS Lett. 535, 159-65