Photosystem I

Abstract

In oxygenic photosynthesis, the two protein–pigment complexes photosystem I and photosystem II are involved in the light‐driven charge separation across the thylakoid membrane, which results in the oxidation of water to O2 and in the reduction of NADP+ to NADPH. In addition to a photosynthetic reaction centre, each of these photosystems contains a core antenna system. The crystal structure of photosystem I reveals the spatial organization of protein subunits and cofactors.

Keywords: photosystem I; oxygenic photosynthesis; electron transfer; core antenna; crystal structure

Figure 1.

Overall structure of PS I. (a) Structural model of the PS I trimer, including the identified α helices (represented by cylinders) except those of PsaC and PsaD, the Chl a of the core antenna system and the Chl a and the phylloquinone cofactors of the ET chain. The Fe4S4 clusters are shown as red cubes. View from the stromal side of the thylakoid membrane. (b) Arrangement of the α helices of one PS I monomer, shown with parts of the adjoining monomers (separated by bold, broken lines), and tentative assignment of the membrane intrinsic subunits. View as in (a), s marks the C3 symmetry axis of the PS I trimer. The two sets of α helices related by the pseudo‐C2 symmetry axis of the PsaA/PsaB heterodimer are labelled a to o (green) and a′ to o′ (blue), respectively. The region occupied by the Chl a of the PS I core antenna is indicated by light grey shading (dots). α Helices of the stromal subunits PsaC and PsaD have been omitted for clarity. (c) Side view, parallel to the membrane, of the arrangement shown in (b), but including the α helices of the stromal subunits PsaC and PsaD. The pseudo‐C2 axis of PsaA/PsaB is labelled C2(AB); the crystallographic C3 symmetry axis of the PS I trimer is denoted C3 (modified with permission from Schubert et al., Copyright © 1997 Academic Press Ltd.).

Figure 2.

Arrangement of the PS I cofactors. (a) Cofactors of the ET chain with centre‐to‐centre distances (Å) and assignments to the spectroscopically identified cofactors are indicated; acc. Chl a are the accessory Chl a. View parallel to the membrane plane. (b) Complete set of 89 chlorophyll cofactors of the core antenna and the ET chain identified in the structure of PS I, are shown together with the three iron–sulfur clusters. Chlorophyll molecules of the ET chain (marked by broken line) are coloured red and ‘connecting’ Chl a cC and cC′ are yellow, iron sulfur clusters are shown as grey cubes. View perpendicular to the membrane plane from the stromal side, s marks the C3 axis of the PS I trimer.

Figure 3.

Proposed structural models for the core structures of heliobacterial PSH, green sulfur bacterial PSC and cyanobacterial or higher plant PS II on the basis of the structures of PS I and PbRC and known sequence similarities (Reproduced with permission from Schubert et al., Copyright © 1998 Academic Press Ltd.).

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References

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Klukas O, Schubert WD, Jordan P et al. (1999b) Localization of two phylloquinones, QK and QK′, in an improved electron density map of photosystem I at 4 Å resolution. Journal of Biological Chemistry 274: 7361–7367.

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Further Reading

Brettel K (1997) Electron transfer and arrangement of the redox cofactors in photosystem I. Biochimica et Biophysica Acta 1318: 322–373.

Chitnis PR, Xu Q, Chitnis VP and Nechushtai R (1995) Function and organization of photosystem I polypeptides. Photosynthesis Research 44: 23–40.

Golbeck JH (1994) Photosystem I in cyanobacteria. In: Bryant DA (ed.) The Molecular Biology of Cyanobacteria, pp. 319–360. Dordrecht: Kluwer Academic.

Golbeck JH (1999) A comparative analysis of the spin state distribution of in vitro and in vivo mutants of PsaC. Photosynthesis Research 61: 107–144.

Lancaster CRD, Ermler U and Michel H (1995) The structures of photosynthetic reaction centers from purple bacteria as revealed by X‐ray crystallography. In: Blankenship RE et al. (eds) Anoxygenic Photosynthetic Bacteria, pp. 503–526. Dordrecht: Kluwer Academic.

Marcus RA and Sutin N (1985) Electron transfers in chemistry and biology. Biochimica et Biophysica Acta 811: 265–322.

Moser CC, Keske JM, Warncke K, Farid RS and Dutton PL (1992) Nature of biological electron transfer. Nature 355: 796–802.

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Krauß, Norbert, and Saenger, Wolfram(Dec 2001) Photosystem I. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0000665]