The paper describes an important milestone in research on photosynthesis that relies on two membrane-intrinsic, multi-subunit protein/cofactor complexes, the photosystems I (PSI) and II (PSII). After decades of elusive efforts to crystallize PSII for X-ray diffraction studies to determine its three-dimensional structure, we succeeded in crystallizing PSII isolated from the thermophilic cyanobacterium, Synechococcus elongatus. By interpreting the electron density, the first high-resolution structure of PSII was obtained. It permits us to better interpret a number of biochemical and biophysical data and suggests new experiments to further our understanding of the function of PSII.

Photosystem II (PSII) is embedded in the thylakoid membranes of plants, algae and cyanobacteria. It uses light energy to finally oxidize the extremely stable water to produce oxygen, electrons and protons (H⁺). This photosynthetic process is driven by PSII and has created and maintains oxygen in the atmosphere, and together with photosystem I (PSI), PSII provides most of the global biomass by converting CO₂ to carbohydrates (sugars). PSII is the only protein complex known to oxidize water and to release molecular oxygen and is consequently of prime importance for the presently existing life on earth.

Since PSII isolated from higher plants could not be crystallized in a form suitable for high resolution X-ray diffraction studies, we isolated PSII from the thermophilic cyanobacterium Synechococcus elongatus as it is well known that proteins from thermophilic organisms are better suited for crystallographic studies than those from mesophilic organisms. This PSII consists of at least 17 different proteins and contains at least 13 redox-active cofactors, not to mention at least 26 more chlorophylls (green pigments) that are important for the collection of sunlight.

The crystals obtained from Synechococcus elongatus PSII are fully active when illuminated by light, and the electron density derived at 3.8 Å resolution (Zouni et al.,

Nature 409, p.739, 2001) shows the essential features of this protein-cofactor complex. These are a field of 36 transmembrane a -helices, of which 6 each were assigned to the light-collecting antenna proteins CP43 and CP47, and 5 each were assigned to the central D1 and D2 proteins that form the reaction center at the heart of PSII where all the photochemical reactions take place (Diner & Babcock, 1996 in: Oxygen Photosynthesis: The Light Reactions – Ort.D.R. and Yocum, C.F., Eds., pp. 213-247, Kluwer, Dordrecht). This involves the ultra-fast and ultra-efficient light-induced charge separation and stabilization steps that occur vectorially across the membrane when light is absorbed by the antenna proteins CP43 and CP47 that contain 12 and 14 chlorophylls, respectively.

Besides the antenna and the reaction center proteins, it was possible to assign two transmembrane a -helices to cytochrome b559 (cyt b559), and the 12 remaining transmembrane a -helices belong to smaller subunits of which several but not all could be assigned.

The catalytic part of PSII contains in addition three hydrophilic proteins that are not located in but on the lumenal side of the thylakoid membrane and are exposed to solvent. These are a 12 kDa protein, a cytochrome c550 and a 33 kDa protein that is engaged in stabilizing a cluster of four manganese ions. Since this is where water oxidation takes place, it is the main and unique feature of PSII as it has not been found so far in any other protein. The 3.8 Å resolution electron density provides information on the location, size and shape of this cluster.

At the present resolution, it is not yet possible to assign individual amino acids to the electron density and to provide structural knowledge at atomic detail. This requires a resolution of at least 2.8 Å. At this level, information about the ligands that stabilize the manganese cluster will be obtained that are of importance to finally understand the functioning of PSII. We are confident that this will be achieved in the near future, similar to the 2.5 Å resolution structure of the other photosystem, PSI, that has recently been published by our group (Jordan et al., Three-dimensional structure of cynobacterial photosystem I at 2.5 Å resolution, Nature 411, pp 909 – 917, 2001).