The structure of Photosystem II (see Chapter 4) has only been determined from two different but closely related thermophilic cyanobacteria: TS elongatus [1, 13, 14] and TS vulcanus . Photosystem II is a dimer in cyanobacteria, green algae and plants. One momomer of Photosystem II represents one functional unit. In cyanobacteria, it consists of 20 proteins, which bind 35 chlorophylls, 11 carotenoids, 14 lipids, 2 hemes, 1 non-heme iron, and the Mn4Ca cluster that catalyzes water oxidation. The majority of the protein subunits and cofactors are membrane integral. The core of the complex, which binds all cofactors of the electron transport chain, is formed by the subunits D1 and D2, which show strong similarities
to the L and M subunits of the purple bacterial reaction center (see also Figure 1.3 and Chapter 12). The D1/D2 core is flanked by the antenna proteins CP43 and CP47, which bind the antenna chlorophylls. CP47 is located close to the dimeriza-tion domain while CP43 is located at the periphery of the PSII dimer. All small membrane intrinsic subunits are located peripheral to the PSII core subunits. Photosystem II is relatively flat on the stromal side but contains a hump that extends about 100 A into the lumen. This luminal domain stabilizes the oxygen-evolving complex and consists of three membrane extrinsic subunits and large loops of the major core proteins of PSII: D1, D2, CP47, and CP43.
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