Photosystem II is a multisubunit enzyme complex involved in plant photosynthesis. It uses solar energy to catalyse the breakdown of water to reducing equivalents and molecular oxygen. Native photosystem II comprises more than 25 different subunits, and has a relative molecular mass of more than 600K. Here we report the three-dimensional structure of a photosystem II subcomplex, containing the proteins D1, D2, CP47 and cytochrome b-559, determined by electron crystallography. This CP47 reaction centre, which has a relative molecular mass of 160K, can perform light-mediated energy and electron-transfer reactions but is unable to oxidize water. The complex contains 23 transmembrane alpha-helices, of which 16 have been assigned to the D1, D2 and CP47 proteins. The arrangement of these helices is remarkably similar to that of the helices in the reaction centres of purple bacteria and of plant photosystem I, indicating a common evolutionary origin for these assemblies. The map suggests that redox cofactors in the D1-D2 complex are located in positions analogous to those in the bacterial reaction centre, but the distance between the chlorophylls corresponding to the bacterial 'special pair' is significantly larger.

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H(+)-ATPase is a P-type ATPase that transports protons across membranes using the energy from ATP hydrolysis. This hydrolysis is coupled to a conformational change between states of the protein, in which the proton-binding site is alternately accessible to the two sides of the membrane with an altered affinity. When isolated from Neurospora crassa, H(+)-ATPase is a 600 kDa hexamer of identical 100 kDa polypeptides. We have obtained the three-dimensional structures of both ligand-free and Mg(2+)/ADP-bound states of this complex using single-particle electron cryo- microscopy. Structural comparisons, together with the difference map, indicate that there is a rearrangement of the cytoplasmic domain on Mg(2+)/ADP binding, which consists of a movement of mass towards the 6-fold axis causing the structure to become more compact, accompanied by a modest conformational change in the transmembrane domain.

Understanding the precise role of photosystem II as an element of oxygenic photosynthesis requires knowledge of the molecular structure of this membrane protein complex. The past few years have been particularly exciting because the structural era of the plant photosystem II has begun. Although the atomic structure has yet to be determined, the map obtained at 6 A resolution by electron crystallography allows assignment of the key reaction center subunits with their associated pigment molecules. In the following, we first review the structural details that have recently emerged and then discuss the primary and secondary photochemical reaction pathways. Finally, in an attempt to establish the evolutionary link between the oxygenic and the anoxygenic photosynthesis, a framework structure common to all photosynthetic reaction centers has been defined, and the implications have been described.

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