Reaction-Center and Light-Harvesting Complexes

Incorporation of Carotenoids into Reaction Center and Light-Harvesting Pigment-protein Complexes... 

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]. 

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.

Carotenoids are constituents of the photosynthetic reaction centers and the lightharvesting complexes of the antennae [13], playing their role as redox intermediates in electron transfer processes of photosystem II [14] and as accessory pigments in light harvesting [5, 15]. 

Rhodobacter capsulatus cells lacking the B800/850 antenna complex (strain U43 with episomal expression of the reaction center and B875 antenna complex) and Rhodospirillwn rubrum cells (strain SI) were grown to late log phase anaerobically in the light at 30 C and harvested. Cells were either broken using a French press and chromatophores isolated as described by Woodbury et al. (8), or cells were ruptured by grinding in alumina and chromatophores isolated as describe by Jackson and Crofts (9). 

Table 1 Per Cent of Lipids Referred to the Total Lipid Content of Chloroplasts from Green (Chl ) and Yellow-Green (Chly ) Leaf Areas and of Photosystem II Particles (PS II), Reaction Center Complexes (RCC) and Light-Harvesting Complexes (LHC) from Green Leaf Areas of the Tobacco Mutant Nicotiana tabacum NC 95... 

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. 

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. 

Wavepacket motion is now routinely observed in systems ranging from the very simple to the very complex. In the latter category, we note that coherent vibrational motion on functionally significant time scales has been observed in the photosynthetic reaction center [15], bacteriorhodopsin [16], rhodopsin [17], and light-harvesting antenna of purple bacteria (LH1) [18-20]. Particularly striking are the results of Zadoyan et al. [21] on the... 

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.

---