Bacteria Rhodopseudomonas viridis

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. 

Two new acyclic end-groups have been found. Phytoene-1,2-oxide (15) was isolated from tomatoes. In the bacteria Rhodopseudomonas viridis it was found that although neurosporene and lycopene (16) were present, most of the carotenoids previously thought to be these two compounds, were in fact the corresponding 1,2-dihydro-derivatives [e.g. (17)]. In addition, l,2-dihydro-3,4-dehydrolycopene (18) was present. 

The problem of bacterial photosynthesis has attracted a lot of recent interest since the structures of the photosynthetic reaction center (RC) in the purple bacteria Rhodopseudomonas viridis and Rhodobacterias sphaeroides have been determined [56]. Much research effort is now focused on understanding the relationship between the function of the RC and its structure. One fundamental theoretical question concerns the actual mechanism of the primary ET process in the RC, and two possible mechanisms have emerged out of the recent work [28, 57-59]. The first is an incoherent two-step mechanism where the charge separation involves a sequential transfer from the excited special pair (P ) via an intermediate bacteriochlorophyll monomer (B) to the bacteriopheophytin (H). The other is a coherent one-step superexchange mechanism, with P B acting only as a virtual intermediate. The interplay of these two mechanisms can be studied in the framework of a general dissipative three-state model (AT = 3). 

Photoreactions that produce chemical energy by excitation of BChl or Chi molecules take place in RCs. The process is referred to as the primary charge separation. Purple bacteria use a type of photosynthesis that, to some extent, resembles green plant photosynthesis in PSll. In the 1980s, two purple bacteria, Rhodopseudomonas viridis and Rhodobacter sphoeroides, reached a prominence that few had expected from species living at the bottom of ponds and similar places. Two German chemists, Johann Deisenhofer and Hartmut Michel, managed to dissolve the protein from the membrane, crystallize it, and determine its structure. 

The crystallization of RCs in view of structure determination by x-ray crj taUography requires special methods that have worked successfully for two kinds of purple bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides) and for Photosystems I and II of thermophilic cyanobacteria. 

Reaction centers of purple bacteria. The exact composition varies, but the properties of reaction centers from several genera of purple bacteria are similar. In Rhodopseudomonas viridis there are three peptide chains designated H, M, and L (for heavy, medium and light) with molecular masses of 33,28, and 24 kDa, respectively. Together with a 38-kDa tetraheme cytochrome (which is absent from isolated reaction centers of other species) they form a 1 1 1 1 complex. This constitutes reaction center P870. The three-dimensional structure of this entire complex has been determined to 0.23-nm resolution288 319 323 (Fig. 23-31). In addition to the 1182 amino acid residues there are four molecules of bacteriochlorophyll (BChl), two of bacteriopheophytin (BPh), a molecule of menaquinone-9, an atom of nonheme iron, and four molecules of heme in the c type cytochrome. In 1984, when the structure was determined by Deisenhofer and Michel, this was the largest and most complex object whose atomic structure had been described. It was also one of the first known structures for a membrane protein. The accomplishment spurred an enormous rush of new photosynthesis research, only a tiny fraction of which can be mentioned here. 

Rhodopseudomonas viridis (now Blasatochloris) Green plants and Cyanobacteria Green sulfur bacteria, heliobacteriaa... 

Reaction centers of purple bacteria typically contain three polypeptides, four molecules of bacteriochlorophyll, two bacteriopheophytins, two quinones, and one nonheme iron atom. In some bacterial species, both quinones are ubiquinone. In others, one of the quinones is menaquinone (vitamin K2), a naphthoquinone that resembles ubiquinone in having a long side chain (fig. 15.10). Reaction centers of some species, such as Rhodopseudomonas viridis, also have a cytochrome subunit with four c-type hemes. 

Everybody knows of the spectacular success of unravelling the structure and kinetics of the photosynthetic bacteria, rhodopseudomonas sphaeroides and viridis the structure by Deisenhoffer, Huber and Michel (Deisenhofer et al., 1984) following the isolation and crystallisation by Michel (Michel, 1982) and the picosecond kinetics (which came first) by Rockley, Windsor, Cogdell and Parson (Rockley et al., 1975) and also by Dutton, Rentzepis, Netzel et al. (Netzel et al., 1977). 

See, e.g., J. Deisenhofer, H. Michel, The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas-Viridis. Science 1989, 245, 1463-1473 M. E. Michel-Beyerle, M. Plato, J. Deisenhofer, H. Michel, M. Bixton, J. Jortner, Unidirectionality of Charge Separation in Reaction Centers of Photosynthetic Bacteria. Biochim. Biophys. Acta 1988, 932, 52-70. 

The membrane-bound ATP synthetase couples phosphorylation to a proton gradient [90] which is generated by the cyclic electron transfer system (Fig. 3). This system includes the RC, a UQ pool [91], a Cyt bic complex [92,93], and a specialized Cyt c (E j = -fO.34 V) for transferring electrons to the oxidized primary donor (P-870 or P-970 ) of the RC. In some bacteria such as Chromatiurn vi-nosum and Rhodopseudomonas viridis this specialized Cyt c is bound to the RC in the membrane [93,94], whereas in other bacteria such as Rb. sphaeroides and Rhodospirillum rubrum this cytochrome is a periplasmic protein (Cyt C2) that binds to the membrane-bound RC [90]. 

Ubiquinone (UQ), also known as coenzyme Q, has a benzoquinone structure with a long side chain. The name ubiquinone is for the ubiquitous nature of the quinone. Some bacteria also contain menaquinone (vitamin K2), either in addition to ubiquinone or in place of it. For instance, the reaction center of Rhodopseudomonas viridis contains one ubiquinone and one menaquinone, while in some other bacterial reaction centers both quinones are ubiquinones. Menaquinone has a naphthoquinone structure with a long isoprenoid side chain. The long hydrocarbon side chains in ubiquinone and menaquinone render a high degree of hydrophobicity to these molecules. 

Fig. 9. Stereo view of the three-dimensional arrangement of the pigment moiecules and cofactors in the Rp. viridis reaction center without the background protein structures. He=heme. Figure constructed as a composite from Deisenhofer, Michel and Huber (1985) The structural basis of photosynthetic light reactions in bacteria. Trends Biochem Sci, 10 245 and Deisenhofer and Michel (1993) Three-dimensional structure of the reaction center of Rhodopseudomonas viridis. In J Deisenhofer and JR Norris (eds) The Photosynthetic Reaction Center, Vol. II, p 348. Acad Press.

Fig. 2. Absorption spectra of BChl a in petroleum ether and the Rb. sphaeroides R-26 reaction-center preparation (A) and of BChl b in ether and in the BChl b-containing Rhodopseudomonas viridis reaction-center preparation (B). Figure source (A) Reed and Peters (1972) Characterization of the pigments in reaction center preparation from Rhodopseudomonas sphaeroides. J Biol Chem 246 7148 (B) Parson, Scherz and Warshel (1985) Calculation of spectroscopic properties of bacterial reaction centers. In ME Michel-Bayerle (ed) Antennas and Reaction Centers of Photosynthetic Bacteria, p 123. Springer Verlag.

WZinth, MC Nuss, MA Franz, W Kaiser and H Michel (1985) Femtosecond studies of the reaction center of Rhodopseudomonas viridis. The very first dynamics of the electron-transfer processes. In ME Michel-Beyerle (ed) Antennas and Reaction Centers of Photosynthetic Bacteria, pp 286-291. Springer... 

I Sinning, J Koepke and H Michel (1990) Recent advances in the structure analysis of Rhodopseudomonas viridis mutants. In M-E Michel-Beyerle (ed) Springer Series in Biophysics, Vol 6, Reaction Centers of Photosynthetic Bacteria, pp 199-208... 

As seen earlier in Chapter 2 on bacterial reaction centers, crystallization of the reaction-center protein of the photosynthetic h iCttn xm Rhodopseudomonas viridis by Michel in 1982 and subsequent determination ofthe three-dimensional structure ofthe reaction center by Deisenhofer, Epp, Miki, Huber and Michel in 1984 led to tremendous advances in the understanding ofthe structure-function relationship in bacterial photosynthesis. Furthermore, because of certain similarities between the photochemical behavior of the components of some photosynthetic bacteria and that of photosystem II, research in photosystem-II was greatly stimulated to its benefit by these advances. In this way, it became obvious that the ability to prepare crystals from the reaction-center complexes of photosystems I and II would be of great importance. However, it was also recognized that, compared with the bacterial reaction center, the PS-I reaction center is more complex, consisting of many more protein subunits and electron carriers, not to mention the greater number of core-antenna chlorophyll molecules. 

Photosynthetic bacteria such as Rhodopseudomonas viridis contain a photo-synthetic reaction center that has been revealed at atomic resolution. The bacterial reaction center consists of four polypeptides L (31 kd), M (36 kd), and H (28 kd) subunits and C, a c-type cytochrome with four c-type hemes (figure 19.9). Sequence comparisons and low-resolution structural studies have rmaled that the bacterial reaction center is homologous to the more complex plant systems. Thus, many of our observations of the bacterial system will apply to plant systems as well. 

Proteobacteria (Imhoff, 1995). The functions of carotenoids in photosynthetic bacteria have been investigated in most detail in the Rhodospirillaceae (other chapters in this book). Their RC resembles that of PS 11 of green plants. Their major BChl is BChl a or b. The RC was firstly crystallized from Bla. (previously, Rhodopseudomonas) viridis, and the localization of one carotenoid, 1,2-dihydroneuro-sporene, four BChl b and two bacteriopheophytin b molecules was determined (Deisenhofer et al., 1995). A similar localization of spheroidene in the RC of Rba. sphaeroides has also been described (Yeates et al., 1988 Ermler et al., 1994). The fine crystal structure of the LH II antenna complex from Rps. acidophila strain 10050 has shown the localization of one rhodopin glucoside and three BChl a molecules per ap monomer (McDermott et al., 1995). A similar localization of lycopene in the LH II complex from Rsp. molischiamm has also described (Koepke et al,... 

Lancaster CRD and Michel H (1996) New insights into the X-ray structure of the reaction center from Rhodopseudomonas viridis. In Michel-Beyerle ME(ed) Reaction Centers of Photosynthetic Bacteria. Structure and Dynamics, pp 23-35. Springer-Verlag, Berlin... 

Lancaster CRD, ErmlerU and Michel H (1995) The structures of photosynthetic reaction centers from purple bacteria as revealed by X-ray crystallography. In Blankenship RE, Madigan MT and Bauer CE (eds) Anoxygenic Photosynthetic Bacteria, pp 503-526. Kluwer Academic Publishers, Dordrecht Lancaster CRD, Michel H, Honig B and Gunner MR (1996) Calculated coupling of electron and proton transfer in the photosyntheticreaction centre of Rhodopseudomonas viridis. Biophys 1 70 2469-2492... 

Michel H and Deisenhofer J (1988) Relevance of the photosynthetic reaction center from purple bacteria to the structure of Photosystem II. Biochemistry 27 1-7 Michel H, Weyer KA, Gruenberg K, Dunger I, Oesterhelt D and Lottspeich F (1986a) The Tight and medium subunits of the photosynthetic reaction centre from Rhodopseudomonas viridis Isolation of the genes, nucleotide and amino acid sequence. EMBOJ5 1149-1158... 

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