Pathways & interactions
Photosynthetic reaction centre, M subunit (IPR005781)
Short name: Photo_RC_M
Overlapping homologous superfamilies
- Photosystem II protein D1/D2 superfamily (IPR036854)
- Photosynthetic reaction centre, L/M (IPR000484)
- Photosynthetic reaction centre, M subunit (IPR005781)
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 [PMID: 11005826]. LH1 acts as the energy collection hub, temporarily storing it before its transfer to the photosynthetic reaction centre (RC) [PMID: 15329728]. 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 [PMID: 16931113, PMID: 12872158, PMID: 2676514].
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 [PMID: 11095707]. 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 [PMID: 8027023]. In Rhodopseudomonas viridis, there is also a non-membranous tetrahaem cytochrome (4Hcyt) subunit on the periplasmic surface.
This entry describes the photosynthetic reaction centre M subunit.
The L and M subunits are arranged around an axis of 2-fold rotational symmetry perpendicular to the membrane, forming a scaffold that maintains the cofactors in a precise configuration. The L and M subunits have both sequence and structural similarity, suggesting a common evolutionary origin. The L and M subunits bind noncovalently to the nine cofactors in 2-fold symmetric branches: four bacteriochlorophylls, two bacteriopheophytins, two ubiquinone molecules (QA and QB), and a non-heme iron.