Functional and structural analysis of photosystem II core complexes from spinach with high oxygen evolution capacity

Oxygen-evolving photo system II core complexes were prepared from spinach by solubilizing photosystem II membrane fragments with dodecyl-β-D-maltoside. The core complexes consist of the intrinsic 47-kDa, 43-kDa, D1 and D2 polypeptides, the two subunits of cytochrome b559 and the extrinsic 33-kDa protein. In the presence of 50 mM CaCl2 they exhibit a high oxygen evolution rate of 1.3 ± 0.2 mmol O2 • mg chlorophyll–1 • h–1 with either 2,6-dichloro-benzoquinone or K3[Fe(CN)6] as acceptor. Electron micrographs of these complexes reveal an obtuse triangular structure in when viewed from the top, measuring 15.3 nm on one side and 10.6 nm on the other two sides. An average height of 7.3 nm was determined from the side view position. These data are in good agreement with previously reported dimensions for photosystem II core complexes. In contrast to previous reports the extrinsic 33-kDa subunit could be resolved for the first time. It appears as a small protrusion when the complex is viewed from the side and seems to cover the lumenal side of the core complex appearing as a disk with a thickness of 1.5–3.3 nm.