Elucidation of the Molecular Mechanisms of Photosystem II Complex Based on Its Crystal Structure Analysis

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Photosystem II (PSII) is a supra-molecular membrane-protein complex consisting of 14-17 membrane-spanning subunits and 3 membrane-peripheral (extrinsic) subunits with a total molecular mass of 350 kDa. This research aimed to analyze the crystal structure of PSII and, on the basis of this, to elucidate the molecular mechanisms of electron transfer, water-splitting and oxygen-evolving reactions taken place in PSII. For this purpose, we crystallized the PSII complex from a thermophilic cyanobacterium Thermosynechococcus vulcanus and analyzed its crystal structure at 3.7Å resolution in which, we assigned 70-80% residues of PSII large subunits CP47,CP43,D1,D2 based on the electron density maps of some residue's large side chains which were visible at the current resolution. We built the structures of all the 3 extrinsic proteins involved in oxygen evolution, of which, the structure of 12 kDa protein was reported for the first time. The whole structure contained in addition 14 trans-membrane helices, some of which were assigned to some low-molecular mass subunits including the α and β-subunits of cytochrome b559,and other helices were not identified. In the reaction center 4 chlorophylls, we identified that the "special dimer" PD1-PD2 has a shorter distance than those between them and the two "accessory chlorophylls", suggesting that the PSII reaction center is not a homogenous "tetramer". We assigned two β-carotenes between the region of D2 and cytochrome b559, thus implied that the secondary electron transfer pathway in PSII is from cytochrome b559 or ChlZD_2 via the two β-carotenes in series and then to ChlD_2,PD_2,PD_1. We also obtained the electron density for the Mn-cluster which is a Y-shaped or "3+1" model as reported previously. We further improved the PSII crystal resolution to 3.5Å, modified our original PSII structure model, and analyzed the PSII functions in more details based on the modified structure.