Introduction

Photosystem II (PSII) is a multisubunit complex embedded in the thylakoid membranes of higher plants, algae and cyanobacteria. It uses light energy to catalyze a series of electron transfer reactions resulting in the splitting of water into molecular oxygen, protons and electrons. These reactions take place on an enormous scale, being responsible for the production of atmospheric oxygen and indirectly for almost all the biomass on the planet. Despite its importance, the catalytic properties of PSII have never been reproduced in any artificial system. Understanding its unique chemistry is not only important in its own right, but could have implications for the agricultural industry since PSII is a main site of damage during environmental stress. The aim of this article is to review our current knowledge of the three-dimensional structure of PSII in higher plants, an area of research which has developed rapidly over recent years. To aid this process we first outline the photochemical reactions that take place in this photosystem. We then summarize the main structural features of individual subunits, with particular focus on their likely transmembrane helical content, as well as on their cofactor and organizational characteristics. Electron microscopy of PSII will be reviewed in the following sections in order to relate the subunit and cofactor composition of PSII to its three-dimensional structure. Low resolution structural data on PSII, obtained from freeze-etch and freeze-fracture studies of thylakoid membranes will be reviewed initially. Such studies have provided information on the location, heterogeneity, as well as the overall size and shape of PSII and its antenna system in the thylakoid membrane at resolutions of 40-50Å. To obtain higher resolution information (~15-40Å), two other approaches have been used: single particle image averaging of detergent solubilised PSII complexes, and analysis of two-dimensional crystals. The former has yielded considerable information on the oligomeric state and subunit organization of PSII and its antenna systems while the latter offers the potential of an atomic resolution structure. Results obtained from both approaches are discussed in terms of the question of whether PSII exists as a monomer or dimer in vivo. Finally, the conclusions emerging from these studies are compared with biochemical and cross-linking data.

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