Photosystem II

The Proteins of Photosystem II

Proteins that form the photosystem II core complex.

Light-harvesting complex components are not shown. The letters E, F, H-N, R, S, W and X indicate photosystem b (Psb) components.

Reprinted from EMBO Reports Sep 4(9), V. Calderone, M. Trabucco, A. Vujicic, R. Battistutta, G.M. Giacometti, F. Andreucci, R. Barbato, G. Zanotti, Crystal structure of the PsbQ protein of photosystem II from higher plants, pp900-905, 2003, PMID: 12949587

The advent of oxygenic photosynthesis brought about an increase in protein complexity from the three to four subunits found in anoxygenic reaction centres to around thirty subunits found in PSII. A few of these proteins show homology to one another, and as such may have arisen by gene duplication - for instance, the D1 (PsbA) and D2 (PsbD) reaction centre core proteins, the CP43 (PsbC) and CP47 (PsbB) core antenna proteins, and the PsbE and PsbF subunits of cytochrome b559. However, most of the proteins making up PSII are unrelated to one another or to other protein families, suggesting a period of rapid protein diversification, perhaps in response to the toxic effects of oxygen from which cells would need protection. The development of oxygenic photosynthesis brought about many changes, requiring alterations to existing pigments, the generation of an oxygen evolution complex, and protection against the toxic effects of oxygen by-products.

PSII is a multi-subunit, pigment-protein complex localised in the chloroplast thylakoid membranes. It consists of around 30 subunits and several cofactors. The major redox components are present in the heterodimer reactive centre core, which is composed of polypeptides D1 (PsbA) and D2 (PsbD) that bind to chlorophyll a, beta-carotene and iron. These chlorophylls participate in energy transfer from the proximal antennae complexes of CP43 (PsbC) and CP47 (PsbB) to the reactive centre core chromophores. The antenna pigment-protein complex CP43-CP47 also binds chlorophyll a and beta-carotene, and acts to transfer excitation energy from the peripheral antenna of PSII toward the photochemical reaction centre. Cytochrome b559 (proteins PsbE and PsbF) is closely associated with the core, and may be involved in a secondary electron transfer pathway that helps to protect PSII from photodamage. Associated with the core is an oxygen-evolving complex (OEC) that acts as the active site of the water oxidation centre. The OEC is composed of the extrinsic polypeptides OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ), as well as a tetranuclear manganese (Mn) cluster, one calcium ion and one chloride ion. OEE1 acts to stabilise the ligation of the Mn cluster in the dark and to promote rapid redox cycling in the light. Finally, there are at least ten small (<10 kDa) hydrophobic peptides, many of which contain transmembrane helices, which are required for the assembly, stability or dimerisation of the PSII complex, as well as for facilitating the fast conformational changes required for photosynthetic activity. Some of the small polypeptides, such as PsbH and PsbT, are involved in photoprotection, which help protect against the damaging effects of the reactive oxygen species generated during photosynthesis.

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