Light‐harvesting Complex

Abstract

Photosynthesis in purple photosynthetic bacteria takes place in a photosynthetic unit located in the cell membrane. Part of this unit is the light‐harvesting complex, an assembly of protein and pigment molecules.

Keywords: light capture; bacterial photosynthesis; integral membrane protein; bacteriochlorophyll; carotenoid

Figure 1.

The upper half shows the molecular structure of Bchla, giving the positions of the Qx and Qy dipoles and the numbering scheme used in this article. The lower half shows the absorption spectrum of an LH2 complex. The three bands around 500 nm are due to the carotenoid, the band at 590 nm comes from Bchla absorbing through the Qx dipole and the bands at 800 and 850 nm are from Bchla absorption through the Qy dipoles. The spectrum of an LH3 complex is similar except the band at 850 nm is shifted to 820 nm. The spectrum of the LH1 complex has a single band at about 870 nm instead of the two bands labelled B800 and B850.

Figure 2.

Two views of the light‐harvesting complex from Rps. acidophila. (a) The nonameric complex viewed from the surface of the membrane. The traces of the protein chains are coloured white, B800 Bchla green, B850 Bchla red and carotenoid yellow. (b) The broad ribbon trace for the helices indicate approximately the hydrophobic region of the membrane. Reprinted by permission from McDermott et al.Nature374: 517–521 © 1995 Macmillan Magazines Ltd.

Figure 3.

The arrangement of the Bchla molecules viewed from the surfaces of the cell membrane. (a) The bacteriochlorin rings of the eighteen B850 molecules are perpendicular to the membrane surface and form a complete ring enclosed by the helices of the α‐apoproteins (yellow) and β‐apoproteins (green). (b) Viewed from the opposite direction, the bacteriochlorin rings of the B800 molecules are parallel to the membrane surface and lie between the β‐apoproteins. They are coordinated to the N‐terminal residues of the α‐apoproteins. Figure 3(a) reprinted by permission from McDermott et al.Nature374: 517–521 © 1995 Macmillan Magazines Ltd.

Figure 4.

The arrangements of the pigment molecules showing the packing of the bacteriochlorin rings and intertwining of the phytyl chains and carotenoids. The B800 Bchla are coloured green; B850 coordinated to the α‐apoprotein are red; B850 coordinated to the β‐apoprotein are orange; carotenoids are yellow.

Figure 5.

The proposed arrangement of light‐harvesting complexes and reaction centre in the membrane. The view is a slice through the membrane across the diameters of adjacent LH2 and LH1 complexes. The shading within the proteins shows the positions of the Bchla pigments. The directions and times for energy transfer are shown. RC = reaction centre.

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References

Freer AA, Prince SM, Sauer K et al. (1996) Pigment–pigment interactions and energy transfer in the antenna complex of the photosynthetic bacterium Rhodopseudomonas acidophila. Structure 4: 449–462.

Gudowska‐Nowak E, Newton MD and Fajer J (1990) Conformation and environmental effects on bacteriochlorophyll optical spectra: correlations of calculated spectra with structural results. Journal of Physical Chemistry 94: 5795–5801.

Koepke J, Hu X, Muenke C, Schulten K and Michel H (1996) The crystal structure of the light‐harvesting complex II (B800‐850) from Rhodospirillum molischianum. Structure 4: 581–597.

McDermott G, Prince SM, Freer AA et al. (1995) Crystal structure of an integral membrane light‐harvesting complex from photosynthetic bacteria. Nature 374: 517–521.

McLuskey K (1999) Crystallographic Studies on Integral Membrane Light Harvesting Complexes from Photosynthetic Bacteria. PhD thesis, University of Glasgow.

Prince SM, Papiz MZ, Freer A et al. (1997) Apoprotein structure in the LH2 complex from Rhodopseudomonas acidophila strain 10050: modular assembly and protein pigment interactions. Journal of Molecular Biology 268: 412–423.

Savage H, Cyrklaff M, Montoya G, Kuhlbrandt W and Sinning I (1996) Two‐dimensional structure of light harvesting complex II (LHII) from the purple bacterium Rhodovulum sulfidophilum and comparison with LHII from Rhodopseudomonas acidophila. Structure 4: 243–252.

Sundstrom V, Pullerits T and van Grondelle R (1999) Photosynthetic light‐harvesting: Reconciling dynamics and structure of purple bacterial LH2 reveals function of photosynthetic unit. Journal of Physical Chemistry B 103: 2327–2346.

Walz T, Jamieson SJ, Bowers CM, Bullough PA and Hunter CN (1998) Projection structures of three photosynthetic complexes from Rhodobacter sphaeroides: LH2 at 6 Å, LH1 and RC‐LH1 at 25 Å. Journal of Molecular Biology 282: 833–845.

Further Reading

Cogdell RJ, Isaacs NW, Freer AA et al. (1997) Structure and function of the LH2 (B800‐850) complex from the purple photosynthetic bacterium Rhodopseudomonas acidophila strain 10050. Progress in Biophysics and Molecular Biology 68: 1–27.

Hu X, Damjanovic A, Ritz T and Schulten K (1998) Architecture and mechanism of the light‐harvesting apparatus of purple bacteria. Proceedings of the National Academy of Sciences of the USA 95: 5935–5941.

Hu X, Ritz T, Damjanovic A and Schulten K (1997) Pigment organisation and transfer of electronic excitation in the purple bacteria. Journal of Physical Chemistry B 101: 3854–3871.

Cogdell RJ, Fyfe PK, Howard TD et al. (1999) The structure and function of the LH2 complex from Rhodopseudomonas acidophila strain 10050, with special reference to the bound carotenoid. In: Frank HA, Young AJ, Britton G and Cogdell RJ (eds) The Photochemistry of Carotenoids, pp. 71–80. Dordrecht, The Netherlands: Kluwer Academic Publishers

Cogdell RJ, Isaacs NW, Howard TD, McLuskey K, Fraser NJ and Prince SM (1999) How photosynthetic bacteria harvest solar energy. Journal of Bacteriology 181: 3869–3879.

Papiz MZ, Prince SM, Hawthornthwaite‐Lawless AM et al. (1996) A model for the photosynthetic apparatus of purple bacteria. Trends in Plant Science 1: 198–206.

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How to Cite close
Isaacs, Neil W, and Cogdell, Richard J(Apr 2001) Light‐harvesting Complex. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0000663]