Photosystem II protein D1/D2 superfamily (Photo_II_D1/D2_sf)
Type: Homologous_superfamily

This entry describes the photosynthetic reaction centre L and M subunits, and the homologous D1 (PsbA) and D2 (PsbD) photosystem II (PSII) reaction centre proteins from cyanobacteria, algae and plants. The D1 and D2 proteins only show approximately 15% sequence homology with the L and M subunits, however the conserved amino acids correspond to the binding sites of the phytochemically active cofactors. As a result, the reaction centres (RCs) of purple photosynthetic bacteria and PSII display considerable structural similarity in terms of cofactor organisation.

The D1 and D2 proteins occur as a heterodimer that form the reaction core of PSII, a multisubunit protein-pigment complex containing over forty different cofactors, which are anchored in the cell membrane in cyanobacteria, and in the thylakoid membrane in algae and plants. Upon absorption of light energy, the D1/D2 heterodimer undergoes charge separation, and the electrons are transferred from the primary donor (chlorophyll a) via pheophytin to the primary acceptor quinone Qa, then to the secondary acceptor Qb, which like the bacterial system, culminates in the production of ATP. However, PSII has an additional function over the bacterial system. 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 .

Also in this entry is the light-dependent chlorophyll f synthase (ChlF) from cyanobacteria such as Chlorogloeopsis fritschii. ChlF synthesizes chlorophyll f or chlorophyllide f, which is able to absorb far red light, probably by oxidation of chlorophyll a or chlorophyllide a and reduction of plastoquinone .

The photosynthetic apparatus in non-oxygenic bacteria consists of light-harvesting (LH) protein-pigment complexes LH1 and LH2, which use carotenoid and bacteriochlorophyll as primary donors . LH1 acts as the energy collection hub, temporarily storing it before its transfer to the photosynthetic reaction centre (RC) . Electrons are transferred from the primary donor via an intermediate acceptor (bacteriopheophytin) to the primary acceptor (quinine Qa), and finally to the secondary acceptor (quinone Qb), resulting in the formation of ubiquinol QbH2. RC uses the excitation energy to shuffle electrons across the membrane, transferring them via ubiquinol to the cytochrome bc1 complex in order to establish a proton gradient across the membrane, which is used by ATP synthetase to form ATP .

The core complex is anchored in the cell membrane, consisting of one unit of RC surrounded by LH1; in some species there may be additional subunits . RC consists of three subunits: L (light), M (medium), and H (heavy). Subunits L and M provide the scaffolding for the chromophore, while subunit H contains a cytoplasmic domain . In Rhodopseudomonas viridis, there is also a non-membranous tetrahaem cytochrome (4Hcyt) subunit on the periplasmic surface.