Photosystem II PsbO, manganese-stabilising (IPR002628)

Short name: PSII_MSP

Overlapping homologous superfamilies

Family relationships



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 [PMID: 12518057, PMID: 15100025]. 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 10 kDa), and are involved in PSII assembly, stabilisation, dimerisation, and photo-protection [PMID: 14871485].

In PSII, the oxygen-evolving complex (OEC) is responsible for catalysing the splitting of water to O(2) and 4H+. The OEC is composed of a cluster of manganese, calcium and chloride ions bound to extrinsic proteins. In cyanobacteria there are five extrinsic proteins in OEC (PsbO, PsbP-like, PsbQ-like, PsbU and PsbV), while in plants there are only three (PsbO, PsbP and PsbQ), PsbU and PsbV having been lost during the evolution of green plants [PMID: 15258264].

This family represents the PSII OEC protein PsbO, which appears to be the most important extrinsic protein for oxygen evolution. PsbO lies closest to the Mn cluster where water oxidation occurs, and has a stabilising effect on the Mn cluster. As a result, PsbO is often referred to as the Mn-stabilising protein (MSP), although none of its amino acids are likely ligands for Mn. Calcium ions were found to modify the conformation of PsbO in solution [PMID: 14529295].

GO terms

Biological Process

GO:0010207 photosystem II assembly
GO:0042549 photosystem II stabilization

Molecular Function

GO:0010242 oxygen evolving activity

Cellular Component

GO:0009654 photosystem II oxygen evolving complex

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.