Photosystem II — Details of Cofactor-Protein Interactions in the Light of the 3 Å Resolution Crystal Structure

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The large membrane intrinsic protein complex Photosytem II (PSII) catalyses light-driven charge separation accompanied by the oxidation of water during oxygenic photosynthesis. In this contribution we will discuss the recent X-ray crystallographic structural model at 3 Å resolution (Loll et al. 2005) in relation to various spectroscopic and biochemical data. Special emphasis will be given on: (A) the quinone binding pockets, the proposed diffusion pathway of quinones into the QB binding site and the possible binding of additional quinones within the complex; (B) the catalytic center for light-induced water oxidation (the Mn4-Ca cluster). The arrangement of metal cations in the cluster, their coordination and protein surroundings are discussed with regard to spectroscopic and mutagenesis studies. Limitations of the presently available structural data are pointed out and the very recent results of X-ray spectroscopy (XANES and EXAFS) on PSII single crystals (Yano et al. 2006) are described in comparison with the X-ray crystallographic model of the water oxidizing complex.