Norbert Krauß

Structure of photosystem I

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.

Photosystem I, an Improved Model of the Stromal Subunits PsaC, PsaD, and PsaE

An improved electron density map of photosystem I (PSI) calculated at 4-Å resolution yields a more detailed structural model of the stromal subunits PsaC, PsaD, and PsaE than previously reported. The NMR structure of the subunit PsaE of PSI fromSynechococcus sp. PCC7002 (Falzone, C. J., Kao, Y.-H., Zhao, J., Bryant, D. A., and Lecomte, J. T. J. (1994)Biochemistry 33, 6052–6062) has been used as a model to interpret the region of the electron density map corresponding to this subunit. The spatial orientation with respect to other subunits is described as well as the possible interactions between the stromal subunits. A first model of PsaD consisting of a four-stranded β-sheet and an α-helix is suggested, indicating that this subunit partly shields PsaC from the stromal side. In addition to the improvements on the stromal subunits, the structural model of the membrane-integral region of PSI is also extended. The current electron density map allows the identification of the N and C termini of the subunits PsaA and PsaB. The 11-transmembrane α-helices of these subunits can now be assigned uniquely to the hydrophobic segments identified by hydrophobicity analyses.

Structure of Photosystem I: Suggestions on the docking sites for plastocyanin, ferredoxin and the coordination of P700

Photosystem I of Synechococcus elongatus was crystallized showing maximal diffraction to 4 A resolution (Witt et al. (1988) Ber. Bunsenges. Phys. Chem. 92, 1503–1506). The crystal structure has been determined at a resolution of 6 A (Kraus et al. (1993) Nature 361, 326–331). Based on this structure, docking sites for plastocyanin and ferredoxin are proposed. The arrangement of helices is analysed in terms of hydrophobicity plots and homology to the reaction center of purple bacteria. A possible coordination of the primary donor P700 by the helices in subunits A and B is suggested based on the relative three-dimensional arrangement of the helices.

Localization of Two Phylloquinones, QK and QK′, in an Improved Electron Density Map of Photosystem I at 4-Å Resolution

An improved electron density map of photosystem I from Synechococcus elongatus calculated at 4-Å resolution for the first time reveals a second phylloquinone molecule and thereby completes the set of cofactors constituting the electron transfer system of this iron-sulfur type photosynthetic reaction center: six chlorophyll a, two phylloquinones, and three Fe4S4 clusters. The location of the newly identified phylloquinone pair, the individual plane orientations of these molecules, and the resulting distances to other cofactors of the electron transfer system are discussed and compared with those determined by magnetic resonance techniques.

Relevance of the diastereotopic ligation of magnesium atoms of chlorophylls in Photosystem I

The central magnesium (Mg) atoms of natural occurring tetrapyrroles such as chlorophylls (Chls) and bacteriochlorophylls (BChls) are typically five-coordinated, a fact which leads to the formation of diastereoisomers if the Mg-ligand bond is stable on the time scale of the observation method. This possibility has only been briefly addressed before in a CD-study of BChl c aggregates [T.S. Balaban, A.R. Holzwarth, K. Schaffner, J. Mol. Struct. 349 (1995) 183]. On the basis of the chlorophyll-protein complex photosystem I (PSI), which has recently been characterized by single crystal crystallography [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Nature 411 (2001) 909], we find that chlorophyll a molecules are much more frequently bound by the protein matrix from one side (anti) than the other one (syn) in a ratio of 82:14, which corresponds to a significant DeltaDeltaG value of 4.3 kJ/mol. Syn and anti denote the orientation of the Mg-ligand with respect to the 17-propionic acid esterified by phytol. Furthermore, by parallel sequence analysis we find that the binding sites for both syn and anti chlorophylls have been strongly conserved during evolution-a fact which stresses the nonrandom manner in which chlorophylls are bound by the apoprotein in antenna complexes, in order to exert efficiently their light harvesting function and energy funnelling. Most remarkably, all the syn chlorophylls are part of the inner core antenna system. Results from semiempirical quantum mechanical and detailed exciton coupling calculations allow us to speculate on the functional relevance of the diasteretopicity for PSI functioning.

The assembly of protein subunits and cofactors in photosystem I

The recently determined crystal structures of photosystems I and II at 2.5 A and 3.8 A resolution, respectively, have improved the structural basis for understanding the processes of light trapping, exciton transfer and electron transfer occurring in the primary steps of oxygenic photosynthesis. Understanding the assembly of the 12 protein subunits and 128 cofactors in photosystem I allows us to study the possible functions of the individual players in this protein-cofactor complex.

Molecular orbital study of the primary electron donor P700 of photosystem I based on a recent X-ray single crystal structure analysis

Abstract The X-ray structure analysis of photosystem (PS) I single crystals showed that the primary electron donor P700 is a heterodimer formed by one chlorophyll (Chl) a and one Chl a ′ [Nature 411 (2001) 909]. The electronic structure of the cation radical P700 + of the primary donor, which is created in the charge separation process, has been probed by semiempirical molecular orbital calculations including spin polarization effects (RHF-INDO/SP). The calculations, which were based on the X-ray structure, clearly show that P700 is a supermolecule formed by two chlorophyll species. They furthermore predict an asymmetrical charge and spin density distribution in favor of the monomeric Chl a half of this dimer in accordance with results from earlier EPR and ENDOR studies [J. Phys. Chem. B 105 (2000) 1225]. The stepwise inclusion of various electrostatic interactions of the dimer with its nearest surrounding (one threonine forming a hydrogen bond to the keto group of Chl a ′ and two histidines liganding the Mg atoms of the two chlorophylls) leads to a systematic enhancement of this electronic asymmetry yielding a spin density ratio of almost 5:1 as also found experimentally. A large part of this value is caused by spin polarization effects. This result is only weakly affected by the electrostatic field of more remote amino acid residues and other pigment molecules (‘accessory’ Chl a molecules) present in PS I. A separate group of calculations involving local geometry optimizations by energy minimization techniques yields a further enhancement of the spin density asymmetry. A particularly strong effect is obtained by allowing for variations of the geometry of the vinyl groups on both chlorophylls of the P700 dimer. Theoretical results for individual isotropic proton and nitrogen hyperfine coupling constants, showing a satisfactory agreement with experimental findings, are also presented.

The structural organization of the PsaC protein in Photosystem I from single crystal EPR and X-ray crystallographic studies

In Photosystem I (PS I) the terminal electron acceptors, FA and FB, are iron-sulfur (4Fe-4S) centers, which are bound to the stromal subunit PsaC. The orientation of PsaC is determined relative to the whole PS I complex (see Schubert, W.-D. et al. (1995) in From Light to Biosphere (Mathis, P. ed.), Vol. II, pp. 3-10, Kluwer) from which a molecular model for the structure of PsaC within PS I is derived. Two strategies are followed: (i) PS I single crystal EPR data on the orientation of the g tensors of both FA- and FB- relative to each other and relative to the crystal axes (see preceding paper) are used in conjunction with the central structural part of the bacterial 2 [Fe4S4] ferredoxins, the cysteine binding motifs of which are known to be homologous to those of PsaC; (ii) the same core structure is fitted into the intermediate resolution electron density map of PS I. The PsaC orientation obtained both ways agree well. The local twofold symmetry axis inherent to the ferredoxin model leaves a twofold ambiguity in the structural conclusion. Deviations from this C2-symmetry in the amino acid sequence of PsaC are analyzed with respect to observable properties which would resolve the remaining structural ambiguity. Arguments both for and against FA being the distal iron-sulfur center (to FX) are discussed.

New Structural Details of Photosystem I at 4 Å Resolution

In water oxidising photosynthesis, the primary electron transfer processes occur within two multi-subunit protein pigment complexes embedded in the thylakoid membrane, the photosystems I and II (PSI and PSII). PSI belongs to the group of type-I (iron-sulphur type) photosynthetic reaction centres (RC), whereas PSII is a representative of the type-II (quinone type) RCs [1]. The X-ray crystallographic analysis of PSI isolated from the cyanobacterium Synechococcus elongatus at 4 A resolution [2,3] presently provides the most detailed structural information available for a type-I reaction centre.