Photosystem I

Model of photosystem I

The protein is shown by the position of C_a-atoms only, joined to form a backbone. In the initial view, the protein is oriented with the acceptor side (stroma facing domain) up, and the donor-side (lumen facing domain) down. The chlorophylls of the antenna are shown as green-blue wireframe models; the FeS-centers are shown in CPK coloring (S-yellow, Fe-red), the speculative position of a phylloquinone (A₁) is shown by the magenta sphere. The reaction center core chlorophylls are shown as (Chl)₂ (P700), green; ancillary Chls, blue-green; A₀, pale blue-green.

Note

The six chlorophylls, two acting as a special pair (the primary donor, P700), two ancillary Chl, and two "acceptor" Chl (the one on the phylloquinone side is thought to be A₀), in a symetrical arrangement about the vertical axis.
Ten transmembrane helices in two sets of five about the symetry axis, provide the framework supporting the Chls. It is thought that the two sets are contributed by the C-terminal half of each of the two main subunits of PS I (psaA, psaB). These show a weak sequence homology with PS II D1 and D2 polypeptides.
The phylloquinone is not a well defined feature. It is modelled in on the basis of evidence that PS I contains two phylloquinones/reaction center, one of which (A₁) is thought to act as acceptor of electrons from the Chl monomer.
The two subunits are held together by the 4F.4S center (center X), which is the acceptor from the phylloquinone.
Two 4Fe.4S centers, which are oriented the same way as the similar centers in ferredoxin from Peptococcus aerogenes (1fdx.pdb)
On the donor side, the general layout of the reaction center core is strikingly similar to that in the bacterial reaction center.
The antenna chlorophylls occupy the peripheral structure. This is assumed to be made up mainly of the N-terminal half of the two main subunits. These show weak sequence homology with the CP43, CP47 proteins which form the reaction center antenna of photosystem II.

The chromophores of the photochemical reaction

Chlorophylls (Chl) of the special pair (green) and the ancillary pair (blue-green), the "acceptor" Chl (A₀ and non-active twin), pale blue-green; phylloquinone magenta, FeS (CPK).

The protein

The protein added as a gray wireframe model.

The antenna

The antenna chlorophylls added.

Structure of a photosystem I ancillary protein, PsaE

See ref 2.

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References

N. Krauss, W. Hinrichs, I. Witt, P. Fromme, W.Prizkow, Z. Dauter, C. Betzel, K.S. Wilson, H.T. WITT and W. Saenger (1993) Three-dimensional structure of system I of photosynthesis at 6 Å resolution. Nature 361, 326-
N. Krauss, W.D. Schubert, O. Klukas, P. Fromme, H.T. Witt, and W.Saenger (1996) Photosystem I at 4 Å resolution represents the first structural model of a joint photosynthetic reaction center and core antenna complex. Nature, Struc. Biol. 3, 965
W.D. Schubert, O. Klukas, N. Krauss, W. Saenger, P. Fromme, and H.T. Witt (1997) Photosystem I of Synechococcus elongatus at 4 Å resolution: Comprehensive structure analysis. J. Mol. Biol. 272, 741
C. J. Falzone, Y. H. Kao, J. Zhao, K. L. MacLaughlin, D. A. Bryant & J. T. Lecomte (1994) 1H and 15N NMR assignments of PsaE, a photosystem I subunit from the cyanobacterium Synechococcus sp. strain PCC 7002. Biochemistry 33: 6043-51