Higher plant PSII structure and membrane organisation review

- adapted from Hankamer B., Barber J. and Boekema E.J. (1997) Annu. Rev. Plant Phys. Mol. Biol. 48, 641-671

ABSTRACT: Photosystem II (PSII) is a multisubunit complex embedded in the thylakoid membrane of higher plants, algae and cyanobacteria. It contains a large number of cofactors including, chlorophyll, pheophytin, carotenoids, plastoquinone, non-heme iron and manganese, which together trap, transfer and modulate the utilization of solar energy, to drive the water splitting reaction. This article reviews the primary, secondary, tertiary and quaternary structure of PSII as well as the function of its constituent subunits. An outline of the photochemically driven reactions which take place in the complex is followed by a more detailed section covering the primary and secondary structures of the PSII subunits (PsbA-PsbX). The function and cofactor content of these subunits is also described together with a summary of the structures of the Lhcb proteins which form the outer antenna system of PSII. These details are then related to the in vivo organization of PSII, the understanding of which is based in part on freeze-etched and freeze-fracture images of thylakoid membranes. These electron microscopy studies provide structural detail at a resolution of about 40-50Å and so provide information mainly on the localization, heterogeneity, dimensions and shapes of membrane embedded PSII complexes. Higher resolution detail of about 15-40Å resolution has been obtained from single particle images of isolated PSII complexes of defined and differing subunit composition and from electron crystallography of 2-D crystals. The information obtained is discussed in terms of the oligomeric state and subunit organization of PSII and its antenna components.

CONTENTS

ABBREVIATIONS: b-Car, b-carotene; BChl, bacteriochlorophyll; Chl, chlorophyll; CD, circular dichroism; CP, chlorophyll protein; cyt b559, cytochrome b559; ENDOR, electron-nuclear double resonance; EPR, Electron paramagnetic resonance; ESEEM, electron spin echo envelope modulation; XANES, X-ray absorption near edge structure; EXAFS, X-ray absorption fine structure; LHCII, light harvesting complex of PSII; LH2, light harvesting complex of purple photosynthetic bacteria; ORF, open reading frame; PSII, photosystem two; P680, primary electron donor of PSII; Pheo, pheophytin; PQ, plastoquinone; QA and QB, primary and secondary electron acceptors of PSII; RC, reaction center; STEM, scanning transmission electron microscopy