Lipids in the Structure of Photosystem I, Photosystem II and the Cytochrome b 6 f Complex

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Summary

This chapter describes the data accumulated in the last decade regarding the specific function of lipids in oxygenic photosynthesis, based on crystal structures of at least 3.0 Å resolution of the main photosynthetic membrane protein—pigment complexes, photosystem I, photosystem II and cytochrome b 6 f. Comparisons with other structures of membrane protein complexes like the bacterial reaction center and the external antenna system from the plant light harvesting complexes II reveal the functional versatility of integral lipids. A detailed structural description of the membrane protein complexes pinpoints the various interactions of integral lipids between protein and pigments (e.g., chlorophylls, carotenoids, quinones) and gives a deep insight into their functional roles. A particular focus in this chapter is on the lipid-filled plastoquinone exchange cavities in photosystem II and cytochrome b 6 f. The differences in extent and lipophilic character of these cavities will be discussed in the light of the resulting plastoqui-none/plastoquinol exchange mechanism. An exceptional feature of PS II is the water splitting reaction enabled by the Mn4Ca cluster. This results in the release of protons to the lumenal aqueous phase, release of electrons to a chain of acceptors, which provides metabolically available reduction equivalents, and release of dioxygen to the atmosphere. The high content of lipids in the interior of photosystem II will be correlated with possible diffusion pathways of the dioxygen and the turnover of the D1 protein, necessary to counteract the photodamage occurring within photosystem II. More structural details of integral lipids derived from higher resolution data from these remarkable membrane protein complexes in combination with data from mutant and/or spectroscopic studies will lead to extended functional insights in the future.