Light harvesting complex

Dunn R C, Holtom G R, Mets L and Xie X S 1994 Near-field fluorescence imaging and fluorescence lifetime measurement of light harvesting complexes in intact photosynthetic membranes J. Chem. Phys. 98 3094-8... 

Tietz C, Chekhlov O, Drabenstedt A, Schuster J and Wrachtrup J 1999 Spectroscopy on single light-harvesting complexes at low temperature J. Chem. Phys. B 103 6328-33... 

Bopp M A, Jia Y, Li L, Cogdell R J and Hochstrasser R M 1997 Fluorescence and photobleaching dynamics of single light-harvesting complexes Proc. Natl Acad. Sc/. USA 94 10 630-5... 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

Given the difficulty of obtaining three-dimensional crystals of membrane proteins, it is not surprising that the electron microscope technique is now widely used to study large membrane-bound complexes such as the acetylcholine receptor, rhodopsin, ion pumps, gap junctions, water channels and light-harvesting complexes, which crystallize in two dimensions. 

Figure 12.17 Computer-generated diagram of the stmcture of light-harvesting complex LH2 from Rhodopseudomonas acidophila. Nine a chains (gray) and nine p chains Bight blue) form two rings of transmembrane helices between which are bound nine carotenoids (yellow) and 27 bacteriochlorophyll molecules (red, green and dark blue). (Courtesy of M.Z. Papiz.)...

Chlorophyll molecules form circular rings in the light-harvesting complex LH2... 

Figure 12.18 Ribbon diagram showing the a (red) and the P (blue) chains of the light-harvesting complex LH2. Each chain forms one transmembrane a helix, which contains a histidine residue that binds to the Mg atom of one bacteriochlorophyll molecule. (Adapted from G. McDermott et al.. Nature 374 517-521, 1995.)...

TTie reaction center is surrounded by a ring of 16 antenna proteins of the light-harvesting complex LHl... 

The light-harvesting complex LHl is directly associated with the reaction center in purple bacteria and is therefore referred to as the core or inner antenna, whereas LH2 is known as the peripheral antenna. Both are huilt up from hydrophohic a and p polypeptides of similar size and with low hut significant sequence similarity. The two histidines that hind to chlorophyll with absorption maxima at 850 nm in the periplasmic ring of LH2 are also present in LHl, but the sequence involved in binding the third chlorophyll in LH2 is quite different in LHl. Not surprisingly, the chlorophyll molecules of the periplasmic ring are present in LHl but the chlorophyll molecules with the 800 nm absorption maximum are absent. 

Figure 12.19 Schematic diagrams illustrating the arrangement of hacteriochlorophyll molecules in the light-harvesting complex LH2, viewed from the periplasmic space, (a) Eighteen hacteriochlorophyll molecules (green] are hound between the two rings of a (red) and p (blue) chains. The planes of these molecules are oriented perpendicular to the plane of the membrane and the molecules are bound close to the periplasmic space, (b) Nine hacteriochlorophyll molecules (green) are bound between the p chains (blue) with their planes oriented parallel to the plane of the membrane. These molecules are bound in the middle of the membrane.

Karrasch, S., Bullough, RA., Ghosh, R. 8.5-A projection map of the light-harvesting complex I from Rhodospir-illum rubrum reveals a ring composed of 16 subunits. EMBO J. 14 631-638, 1995. 

Koepke, J., et al. The crystal structure of the light-harvesting complex II (B800-850) from Rhodospirillum molis-chianum. Structure 4 581-597, 1996. 

Kiihlbrandt, W., Wang, D.A., Fujiyoshi, Y. Atomic model of the plant light-harvesting complex. Nature 367 614-621, 1994. 

All carotenoids are bound to the light harvesting complexes or reaction centers in membranal systems of bacterial cells. 

Horton, P. and Ruban, A.V. 1994. The role of light-harvesting complex II in energy quenching. In Photoinhibition of Photosynthesis, eds. N.R. Baker and J.R. Bowyer, pp. 11-128. Oxford BIOS Scientific Publishers Ltd. 

Ruban, A.V. and Horton, P. 1992. Mechanism of ApH-dependent dissipation of absorbed excitation energy by photosynthetic membranes. I Spectroscopic analysis of isolated light harvesting complexes. Biochim. Biophys. Acta 1102 30-38. 

Ruban, A.V., Robert, B., and Horton, P. 1995. Resonance Raman spectroscopy of photosystem B light-harvesting complex of green plants. A comparison of trimeric and aggregated states. Biochemistry 34 2333-2337. 

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