Purple bacteria light-harvesting complexes

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. 

A short, intermediate-level review of the structure and function of the light-harvesting complex of the purple bacteria and exci-ton flow to the reaction center. 

Figure 23-27 Illustration of proposed exciton transfer of the energy of light absorbed by bacteriochlorophyll a of purple bacteria. Energy absorbed by the light harvesting complex LH2 is transferred in steps to another LH2, to LH1 and to the reaction center. The short lines within the circles represent the edges of the BChla chromophores. After Kiihlbrandt300 with permission.

These reactions taking place in the RC are the primary chemical reactions of photosynthesis. (A detailed description ot these primary chemical reactions in RCs of purple bacteria is given in Chapter 3.) The primary physical processes of photosynthesis are light absorption and transfer of excitation energy. These processes take place mainly in the light-harvesting complexes (LHCs) described in Chapter 11. 

Comparison of the major light-harvesting complexes in purple bacteria. Taken from Ref. 12. 

Fig. 5. Specific structural elements (amino acid residues boxed) of antenna polypeptides of B880 (A), B800-850 (B) and B800-820 (C) light-harvesting complexes from purple bacteria.

Single-Molecule Spectroscopy on Light-Harvesting Complexes from Purple Bacteria... 

T.M.H. Creemers, C. de Caro, R.W. Visschers, R. van Grondelle, S. Volker, Spectral hole burning and fluorescence line narrowing in subunits of the light harvesting complex LHl of purple bacteria. J. Phys. Chem. B 103, 9770-9776 (1999)... 

Fig. 12. Simple models of various photosystems (A) purple bacteria, (B) green bacteria, (C) cyanobacteria and red algae, (D) bro n algae and (E) green plants. RC=reaction center FCPA=fucoxanthin-chlorophyll-protein assembly LHC=light-harvesting complex PS ll=photosystem II PS l=photosystem I. See text for discussion.

We now summarize in Fig. 11 the reaction-center structure and the known electron-transport reactions in purple bacteria. A simplified representation of the reaction-center and the light-harvesting complexes contained in the bacterial membrane is shown in Fig. 11 (A), followed by a column model and a cofactor model in Fig. 11 (B). The cofactor model is used to illustrate the various electron-transport steps with the associated rate constants in Fig. 11 (C), where the cofactors in the starting state (oxidized or reduced) are shown in solid black. When a cofactor first becomes reduced or oxidized, it is shown as an open symbol. We will also use this cofactor model and reaction sequence as a framework for introducing the remaining chapters throughout the section on photosynthetic bacteria. 

Crystal Structure of the Light-Harvesting Complexes of Purple Photosynthetic Bacteria.71... 

Fig. 2. Natural selection of carotenoid configuration in the reaction center (RC) and light-harvesting complex (LHC) of purple photosynthetic bacteria. See text for discussion. Figure source Koyama (1991) Structure and function of carotenoids in photosynthetic systems. J Photochem Photobiol, B Biol 9 208.

Mathis P (1994) Electron transfer between cytochromeC2 and the isolated reaction center of purple bacterium Rhodobacter sphaeroides. Biochim Biophys Acta 1187 177-180 McDermott G, Prince SM, Freer AA, Hawthornwaite-Lawless AM, Papiz MZ, Cogdelt RJ and Isaacs NW (1995) Crystal structure of an integral membrane light-harvesting complex from photosynthetic bacteria. Nature 374 517-521 Michel H (1982) Three-dimensional crystals of a membrane protein complex. J Mol Biol 158 567-572 Michel H (1983) Crystallization of membrane proteins. Trends Biochem Sci 8 56-59... 

Chachisvilis M and Sundstrom V 1996 Femtosecond vibrational dynamics and relaxation in the core light-harvesting complex of photosynthetic purple bacteria Chem. Phys. Lett. 261 165-74... 

Carotenoids in aerobically grown cells of Erythrobacter sp. OCh 114 have been identified [2]. Spheroidenone is dominant, and small amounts of 2,2 -diketospirilloxanthin and OH-spheroidenone are also found. All of these carotenoids are bound to a photosynthetic reaction center or light-harvesting complexes, whose properties are similar to those of the purple photosynthetic bacteria [1]. 

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