Rhodobacter sphaeroides bacteria reaction centers

Experiments for testing intracellular redox reactions were carried out [108] with Rhodobacter sphaeroides bacteria. In these works different hydrophobic and hydrophilic redox mediators were used for carrying the redox state between SECM microelectrode in solution and intracellular redox sites. The bacteria have two membranes, the outer cell membrane and the cytoplasmic membrane. Hydrophobic mediators are capable of permeating both. The effective rate constants of redox reactions of such mediators with intracellular redox moieties showed correlation with the formal potential of the mediator. Hydrophilic iraiic species on the other hand can only cross the outer membrane of the bacteria and can react with redox centers in its periplasm. The relationship between reaction rate coefficient and formal potential of the hydrophilic mediator however is not linear. 

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. 

Fritzsch G, Ermler U, Merckel M and Michel H (1996) Crystallization and structure of the photo synthetic reaction centres from Rhodobacter sphaeroides—wild type and mutants. In Michel-Beyerle M-E (ed) Reaction Centers of Photosynthetic Bacteria. Structure and Dynamics, pp 3-13. Springer-Verlag, Berlin... 

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... 

Triplet energy transfer between the primary donorand carotenoids in Rhodobacter sphaeroides R-26.1 reaction centers incorporated with spheroidene analogs havi ng different extents of ff-electron conjugation. Photochem Photobiol 66 97-104 Feher G and Okamura Y (1978) Chemical composition and properties of reaction centers. In Clayton RK and Sistrom WR (eds) The Photosynthetic Bacteria, pp 349-386. Plenum Press, New Y ork... 

Vos M H, Jones M R, Hunter C N, Breton J, Lambry J C and Martin J L 1996 Femtosecond spectroscopy and vibrational ooherenoe of membrane-bound RCs of Rhodobacter sphaeroides genetically modified at positions M210 and LI 81 The Reaction Center of Photosynthetic Bacteria—Structure and Dynamics ed M E Michel-Beyerle (Berlin Springer) pp 271-80... 

Zysmilich MG, McDermott A (1994) Photochemically induced dynamic nuclear polarization in the solid-state 15N spectra of reaction centers from photosynthetic bacteria Rhodobacter sphaeroides R-26. J Am Chem Soc 116 8362-8363... 

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 photoreceptor complexes of Rhodobacter sphaeroides and Rhodobacter caosulatus have many features in common. Their reaction centers have about 80 % homology between the amino acid sequences of their respective L, M, and H polypeptides (1). The electrochemical and spectroscopic details for the primary photochemical events are nearly identical (2). Both have a core light-harvesting (LH) complex with a Qy band near 875 nm and an accessory LH complex, B800-850, whose amount varies inversely with the light intensity used for growing the bacteria. 

In Rhodobacter sphaeroides and Rhodospirillum rubrum, a new type of antenna Bchl-form whose energy level is lower than that of the special pair Bchl of the reaction center (RC) has been reported (4-6). Such components should be surveyed in other photosynthetic bacteria to account for the functional role and structural basis for the specific energy level. 

Fig. 3. The Q-cycle operating with the reaction center of the pigment system of the purple sulfur-, purple non-sulfur and green non-sulfur bacteria, B870 = special pair of bacteriochlorophyll (BChl) a or b B870 = B870 in the S, excited state BChl = bacteriochlorophyll a or b molecules directly reduced by B870 BPheo = bacteriopheophytin a or b Q = bound quinone [ubiquinone (UQ) in Rhodobacter sphaeroides menaquinone (MQ) in Rhodobacter viridis and Chloroflexus spp.l Qb = mobile quinone [UQ in Rb. sphaeroides and Rb. virldis MQ in Chloroflexus spp.l which exchanges with QbHj at the sites marked with an X QbHj = fully reduced (quinol) form of Qg QbH- = semiquinone form of Qb Fe-S = Rieske iron-sufur center b(Fe ) or c(Fe +) = reduced forms of cytochromes b or o b(Fe ) or c(Fe ) = oxidized forms of cytochromes b or c X = Qb/QbH2 exchange site.

The rates of the electron transfer processes in reaction centers (RC s) of photosynthetic bacteria are controlled both by the spatial and the electronic structure of the involved donor and acceptor molecules. The spatial structure of bacterial RC s has been determined by X-ray diffraction for Rhodopseudomonas (Rp.) viridis and for Rhodobacter (Rb.) sphaeroides,- The electronic structure of the transient radical species formed in the charge separation process can be elucidated by EPR and ENDOR techniques. The information is contained in the electron-nuclear hyperfine couplings (hfc s) which, after assignment to specific nuclei, yield a detailed picture of the valence electron spin density distribution in the respective molecules. 

F. Lendzian, B. Endeward, M. Plato, D. Bumann, W. Lubitz, a K. Mobius, ENDOR and TRIPLE resonance investigation of the primary donor cation radical P in single crystals of Rhodobacter sphaeroides R-26 reaction centers, in "Reaction Centers of Riotosynthetic Bacteria," M.-E. Michel-Beyerle, ed.. Springer, Berlin (1990). 

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