Rhodopseudomonas viridis Bacteriochlorophylls

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. 

Figure 23-31 (A) Stereoscopic ribbon drawing of the photosynthetic reaction center proteins of Rhodopseudomonas viridis. Bound chromophores are drawn as wire models. The H subunit is at the bottom the L and M subunits are in the center. The upper globule is the cytochrome c. The view is toward the flat side of the L, M module with the L subunit toward the observer. (B) Stereo view of only the bound chromophores. The four heme groups Hel-He4, the bacteriochlorophylls (Bchl) and bacteriopheophytins (BPh), the quinones QA and QB/ and iron (Fe) are shown. The four hemes of the cytochrome are not shown in...

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. 

Fig. 1 Schematic representation of the spatial arrangement of the chromophores involved in the first steps of photosynthetic charge-separation in rhodopseudomonas viridis. Orientation of phytyl side-chains is indicated by wavy lines, furthermore the position of an additional bacteriochlorophyll unit is sketched.

Fig. 1 Molecular arrangement of pigments in bacterial photo synthetic reaction centre from Rhodopseudomonas viridis (BChl bacteriochlorophyll BPh bacteriopheophytin) (taken from Deisenhofer et al., 1985).

Thomson, M.A. and Schenter G.K., Excited States of the Bacteriochlorophyll b Dimer of Rhodopseudomonas viridis A QM/MM Study of the Photosynthetic Reaction Center That Includes MM Polarization. J. Phys. Chem. (1995) 99 6374-386. 

Arlt, T., Dohse, B., Schmidt, S., Wachtveitl, J., Laussermair, E., Zinth, W., and Oesterhelt, D., 1996, Electron transfer dynamics of Rhodopseudomonas viridis reaction centers with a modified binding site for the accessory bacteriochlorophyll. Biochemistry, 35 92359 9244. 

LTakiff and SG Boxer (1987) Phosphorescence from the primary electron donor in Rhodobacter sphaeroldes and Rhodopseudomonas viridis reaction centers. Biochim Biophys Acta 932 325-334 L Takiff and SG Boxer (1987) Phosphorescence spectra of bacteriochlorophylls. J Am Chem Soc 110 425-4426... 

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). 

It is well established that there is a pair of bacteriochlorophyll molecules (designated Pgyg from the fact that light of 870 nm is the maximum excitation wavelength) in the reaction center of Rhodopseudomonas viridis the critical pair of chlorophylls is embedded in a protein complex that is in turn an integral part of the photosynthetic membrane. (We shall refer to the bacteriochloro-phylls simply as chlorophylls in the interest of simplifying the discussion.)... 

In 1984, Deisenhofer and colleagues reported an X-ray structure of the RC of the photosynthetic bacterium Rhodopseudomonas viridisJ- As explained in subsequent review articles, -" the real tour de force of the work was their attempt to crystallize the membrane protein. The magnificent crystallographic work which led to the structure is of course equally important. This structure determination can no doubt be regarded as a major scientific event not only because of its direct link to bacterial photosynthesis but also for the many studies which it inspired in various fields of research, from biology to biophysics and chemistry. Figure 2 shows a schematic view of the photosynthetic RC from Rhodopseudomonas viridis, with its special pair (SP) of bacteriochlorophylls, its two accessory bacteriochlorophylls (BCh), and the two bacteriopheophytins (BPh). Above the special pair, a tetraheme cytochrome also plays an important role. 

Figure 13. Stereoview of the X-ray crystal structure of a naturally occurring dimer, the special pair (16) of the photosynthetic reaction center of bacterium Rhodopseudomonas viridis, made from two bacteriochlorophyll b molecules (15)."...

X-ray diffraction methods have provided the detailed structures of the reaction centers from two carotenoid-containing puiple photosynthetic bacterial species, Rhodopseudomonas viridis [1] and Rhodobacter sphaeroides wild type strain 2.4.1 [2]. The coordinates of these structures indicate that the reaction center-bound carotenoid is located in the M subunit, close ( 4A) to the accessory bacteriochlorophyll monomer on the M subunit side and -lO.SA edge-to-edge distance from the primary donor. These structures suggest an involvement of the M-side monomeric bacteriochlorophyll in triplet-triplet energy transfer, but there has been no direct experimental verification of this hypothesis. 

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... 

M. A. Thompson and G. K. Schenter Excited states of the bacteriochlorophyll b dimer of rhodopseudomonas viridis A QM/MM study of the photosynthetic reaction center that includes MM polarization, J. Phys. Chem. 99, 6374-6386 (1995). 

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