Life on Earth depends on photosynthesis, the conversion of light energy from the Sun to chemical energy. In plants, green algae and cyanobacteria, this process is driven by the cooperation of two large protein-cofactor complexes, photosystems I and II, which are located in the thylakoid photosynthetic membranes. The crystal structure of photosystem I from the thermophilic cyanobacterium Synechococcus elongatus described here provides a picture at atomic detail of 12 protein subunits and 127 cofactors comprising 96 chlorophylls, 2 phylloquinones, 3 Fe4S4 clusters, 22 carotenoids, 4 lipids, a putative Ca2+ ion and 201 water molecules. The structural information on the proteins and cofactors and their interactions provides a basis for understanding how the high efficiency of photosystem I in light capturing and electron transfer is achieved.

In plants and cyanobacteria, the primary step in oxygenic photosynthesis, the light induced charge separation, is driven by two large membrane intrinsic protein complexes, the photosystems I and II. Photosystem I catalyses the light driven electron transfer from plastocyanin/cytochrome c(6) on the lumenal side of the membrane to ferredoxin/flavodoxin at the stromal side by a chain of electron carriers. Photosystem I of Synechococcus elongatus consists of 12 protein subunits, 96 chlorophyll a molecules, 22 carotenoids, three [4Fe4S] clusters and two phylloquinones. Furthermore, it has been discovered that four lipids are intrinsic components of photosystem I. Photosystem I exists as a trimer in the native membrane with a molecular mass of 1068 kDa for the whole complex. The X-ray structure of photosystem I at a resolution of 2.5 A shows the location of the individual subunits and cofactors and provides new information on the protein-cofactor interactions. [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Nature 411 (2001) 909-917]. In this review, biochemical data and results of biophysical investigations are discussed with respect to the X-ray crystallographic structure in order to give an overview of the structure and function of this large membrane protein.

The structure of photosystem I from the thermophilic cyanobacterium Synechococcus elongatus has been recently resolved by x-ray crystallography to 2.5-A resolution. Besides the reaction center, photosystem I consists also of a core antenna containing 90 chlorophyll and 22 carotenoid molecules. It is their function to harvest solar energy and to transfer this energy to the reaction center (RC) where the excitation energy is converted into a charge separated state. Methods of steady-state optical spectroscopy such as absorption, linear, and circular dichroism have been applied to obtain information on the spectral properties of the complex, whereas transient absorption and fluorescence studies reported in the literature provide information on the dynamics of the excitation energy transfer. On the basis of the structure, the spectral properties and the energy transfer kinetics are simultaneously modeled by application of excitonic coupling theory to reveal relationships between structure and function. A spectral assignment of the 96 chlorophylls is suggested that allows us to reproduce both optical spectra and transfer and emission spectra and lifetimes of the photosystem I complex from S. elongatus. The model calculation allowed to study the influence of the following parameters on the excited state dynamics: the orientation factor, the heterogeneous site energies, the modifications arising from excitonic coupling (redistribution of oscillator strength, energetic splitting, reorientation of transition dipoles), and presence or absence of the linker cluster chlorophylls between antenna and reaction center. For the Förster radius and the intrinsic primary charge separation rate, the following values have been obtained: R(0) = 7.8 nm and k(CS) = 0.9 ps(-1). Variations of these parameters indicate that the excited state dynamics is neither pure trap limited, nor pure transfer (to-the-trap) limited but seems to be rather balanced.

Photosystem I (PS I) is a large membrane protein complex that catalyzes the ¢rst step of solar conversion, the light-induced transmembrane electron transfer, and generates reductants for CO 2 assimilation. It consists of 12 di¡erent proteins and 127 cofactors that perform light capturing and electron transfer. The function of PS I includes inter-protein electron transfer between PS I and smaller soluble electron transfer proteins. The structure of PS I is discussed with respect to the potential docking sites for the soluble electron acceptors, ferredoxin/£avodoxin, at the stromal side and the soluble electron donors, cytochrome c 6 /plastocyanin, at the luminal side of the PS I complex. Furthermore, the potential interaction sites with the peripheral antenna proteins are discussed.

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