Photosynthetic bacteria Rhodopseudomonas viridis

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). 

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). 

Den Blanken HJ and Hoff AJ (1982) High-resolution optical absorption-difference spectra of the triplet state of the primary donor in isolated reaction centers of the photosynthetic bacteria Rhodopseudomonas sphaeroi-des R-26 and Rhodopseudomonas viridis measured with optically detected magnetic resonance at 1.2 K, Biochim. Biophys. Acta 681, 365-374. 

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 larger part of research on light-driven eleetron transfer in purple photosynthetic bacteria has involved three speeies, Rhodopseudomonas (Rps.) viridis, Rhodobacter (Rb.) sphaeroides and Rb. capsulatus. The bulk of this article is written in reference to the Rb. sphaeroides reaetion eentre, the subject of the majority of spectroscopic and mutagenesis work earried out to date. However, much of the research described below has involved the reaction centre from Rb. capsulatus or Rps. viridis, or reaetion eentres from other species of purple bacteria. 

Photosynthetic prokaryotes such as cyanobacteria and photosynthetic bacteria lack chloroplasts and in these organisms the light reactions that drive photosynthesis take place in the cell s inner plasma membrane. The photosynthetic apparatus of purple bacteria, for example, is contained in a system of rntra-cytoplasmic membranes. Fig. 1 depicts the morphologies of two such purple bacteria - Rhodobacter (Rb.) sphaeroides [Fig. 1 (A)], formerly called Rhodopseudomonas sphaeroides, and Rhodopseudomonas (Rp.) viridis [Fig. 1 (B)] - species that are commonly used for photosynthesis studies. The former contains bacteriochlorophyll a (BChl a), which absorbs in the 800-880 run region in vivo, while the latter contains BChl b, which absorbs in the 960-1020 run region. 

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... 

Two types of the photosynthetic reaction center (RC) complexes are known in pxirple bacteria, the distribution of which depends on bacterial species (1). In one type, the RC complexes have a cytochrome subunit with four c-type hemes. The other type of RC does not have the cytochrome subunit (Fig. 1). Three demensional structures of both types of RCs have been revealed in Rhodopseudomonas viridis (2) and Rhodobacter sphaeroides (3) the former has the bound cytochrome subunit. The major difference between the two types of RC is only in the presence or absence of the cytochrome subunit and the structure of the other three peptides with pigments and quinones is similar to each other. Evolutionary relationships between the two types of RC and the role of the bound cytochrome subunit are interesting subjects in the photosynthetic electron transfer system in purple bacteria. 

The acceptor side of the PS II reaction center is structurally and functionally homologous to the reducing side of reaction centers from a number of photosynthetic bacteria, including Rhodopseudomonas viridis. Rhodobacter sphaeroides and capsulatus. and Chloroflexus aurantiacus. The reaction center complexes of viridis and sphaeroides have been crystallized, and the three-dimensional structure of these has been determined at high resolution [3-7]. With the exception of (a) the His residues in the bacterial reaction center that serve as ligands to the Mg of the accessory bacteriochlorophylls, and (b) the Glu residue that serves as a ligand to the non-heme iron between and Q0, all of the amino acid residues that function as important... 

The photochemically active pigments of photosystem II (PSII) are housed in an apoprotein environment provided by the D1 and D2 polypeptides which also contain binding sites for the acceptor quinones (Q and Q ). The organisation of the polypeptides and chromophores has been inferred (1-3) from various similarities and homologies between PSII reaction centres and the photosynthetic reaction centres of purple bacteria, such as Rhodopseudomonas viridis, which have been structurally resolved in considerable detail (e.g. 4). By analogy with the L and M polypeptides of bacterial reaction centres, D1 and D2 each contain 5 transmembrane helical spans. There is strong sequence homology... 

Despite the fact that photosynthetic PPi formation only has been revealed in R. rubrum it is plausible to suggest that other phototro-phic bacteria would be capable of catalyzing PPi synthesis. Thus a membrane bound PPase has recently been found in Rhodopseudomonas pa-lustris (2,3). We have also examined the possibility of PPi synthesis in chromatophores from Rhodopseudomonas viridis. Rhodopseudomonas blastica and Rhodobacter capsulatus. 

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