Olaf Klukas

Three-dimensional Structure of Cyanobacterial Photosystem I at 2.5 A Resolution

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.

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 at 4.5 A resolution: a short review including evolutionary aspects

In photosynthesis of higher plants and cyanobacteria two photosystems are responsible for light induced charge separation. Photosystem II catalyses the electron transfer from water at the lumenal side to quinone at the stromal side, during this process oxygen is evolved. Photosystem I catalyses the electron transport from the soluble electron carrier plastocyanin or cytochrome c 6 at the lumenal side to ferredoxin at the stromal side of the membrane. Photosystem II shows homology to the reaction centre of purple bacteria in sequence as well as in content of electron carriers, yet purple bacteria are not able to use water as electron donor. Photosystem I has nearly no sequence homology to the reaction centre of purple bacteria, there are furthermore differences in the content of electron carriers and in the fact that Photosystem I carries its own antenna system of 90 chlorophyll a molecules. Green sulfur bacteria and heliobacteria are related to Photosystem I (for review on evolutionary relationship see Ref. [1 ]). The major part of Photosystem I is constituted by the two large subunits PsaA and PsaB ( = 83 kDa, each), carrying most of the electron transport chain: P700 (a Chl a dimer), A0 (a monomer of Chl a ), AI (phylloquinone/Vit K l ) and the first of the three [4Fe-4S] clusters F x. In addition, the 90 Chl a molecules of the antenna are bound by these subunits. Three subunits are

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.

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.