Organization of transmembrane helices in photosystem II: comparison of plants and cyanobacteria

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Abstract
Electron microscopy and X–ray crystallography are revealing the structure of photosystem II. Electron crystallography has yielded a 3D structure at sufficient resolution to identify subunit positioning and transmembrane organization of the reaction–centre core complex of spinach. Single–particle analyses are providing 3D structures of photosystem II–light–harvesting complex II supercomplexes that can be used to incorporate high–resolution structural data emerging from electron and X–ray crystallography. The positions of the chlorins and metal centres within photosystem II are now available. It can be concluded that photosystem II is a dimeric complex with the transmembrane helices of CP47/D2 proteins related to those of the CP43/D1 proteins by a twofold axis within each monomer. Further, both electron microscopy and X–ray analyses show that P 680 is not a 'special pair‘ and that cytochrome b 559 is located on the D2 side of the reaction centres some distance from P 680 . However, although comparison of the electron microscopy and X–ray models for spinach and Synechococcus elongatus show considerable similarities, there seem to be differences in the number and positioning of some small subunits.

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