Rhodopseudomonas sphaeroides bacterial reaction center

Figure 3. The bacterial reaction-center protein model from Rhodopseudomonas sphaeroides the structure and positioning of components are highly speculative.

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.

By contrast, the carotenoids in the bacterial reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis do not span the membrane. They are in a kinked conformation because of a cis double bond in position 15, and oriented roughly perpendicular to e trans-membrane helices (Ermles et al., 1994 Lancaster and Michel, 1996) The carotenoids interact mostly with hydrophobic amino... 

Frank HA and Violette C A (1989) Monomeric bacteriochlorophyll is required for triplet energy transfer between the primary donor and the carotenoid in photosynthetic bacterial reaction centers Biochim Biophys Acta 976 222-232 Frank HA, Machnicki J and Friesner R (1983) Energy transfer between the primary donor bacteriochlorophyll and carotenoids in Rhodopseudomonas sphaeroides. Photochem Photobiol 38 451 56... 

The bacterial reaction center (RC) has been the subject of intense research efforts during the past 25 years. The reasons are that it is an evolutional precursor to the much more elaborate systems of green algae and plants, and that the initial charge separation is located on one protein which, for some species of purple bacteria, can easily be purified in significant amounts from fresh cells. In fact, for two species, Rhodopseudomonas viridis Rp. viridis) and Rhodohacter sphaeroides (Rb. sphaeroides)p-" this protein has been crystallized, and the three-dimensional structure has been solved. 

M. Losche, G. Feher, and M.Y. Okamura, The Stark Effect in Reaction Centers from Rhodobacter Sphaeroides R-26, Rhodopseudomonas Viridis and the D1D2 Complex of Photosystem II from Spinach, in "The Photosynthetic Bacterial Reaction Center", J. Breton and A. Vermeglio, eds.. Plenum Publishing Corp., San Diego (1988). 

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... 

Recently, the structures of the reaction centers from Rhodopseudomonas viridis and Rhodobacter sphaeroides have been solved at the atomic level (1,2). In both organisms the reaction center "core" polypeptides consist of the L and M subunits (responsible for charge separation), a cytochrome, and the H subunit (possibly required for stable assembly). It has become apparent that the bacterial reaction center, particularly the L and M subunits, can be directly compared to the reaction center polypeptides, D1 and D2, of Photosystem II (3). 

F. Lendzian, W. Lubitz, H. Scheer, C. Bubenzer, and K. Mobius, In vivo liquid solution ENDOR and TRIPLE resonance of bacterial reaction centers of Rhodopseudomonas sphaeroides R-26, J. Am. Chem. Soc. 103 4635 (1981). 

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. 

Figure 4-7. Electronic factors in the rate constant calculated for the electron transfers in the bacterial photosynthetic reaction centers of (a) Rhodopseudomonas viridis, and (b) Rhodobactor sphaeroides...

Bacterial photosynthetic reaction centers (PRC) have been among the most actively studied ET proteins since DeVault and Chance first measured C. vinosum tunneling rates in the early 1960s. In many cases, measurements of ET kinetics preceded determination of the three-dimensional structure of the membrane-bound protein assembly. It was not until the X-ray crystal-stracture determinations of the Rhodopseudomonas (Rps.) viridus and Rhodobacter (Rb.) sphaeroides PRCs that distances could be assigned to specific rate constants. The recent crystal structures of photosystems l and from cyanobacteria promise to clarify critical aspects of the ET mechanisms in oxygenic PRC. ... 

C-FI Chang, M Schiffer, D Tiede, U Smith and J Norris (1985) Characterization of bacterial photosynthetic reaction center crystals from Rhodopseudomonas sphaeroides R-26 by X-ray diffraction. J Mol Biol 186 201 -203... 

F/g. 5. Relationship between photochemical activity (measured by AA due to P870 photooxidation and by production of EPR signai at g=2.0026) and the number of ubiquinone molecuies per reaction center in Rb. sphaeroides R-26. See text for other details. Figure modified from Okamura, Isaacson and Feher (1975) Primary acceptor in bacterial photosynthesis Obligatory role of ubiquinone in photoactive reaction centers of Rhodopseudomonas sphaeroides. Proo Nat Acad Sci, USA 72 3494,... 

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). 

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. 

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. 

Natui al photosynthesis undoubtedly represents an exemplary system for supramolecular photochemistry. In a series of irreversible electron transport processes in bacterial photosynthesis, an electron was ejected from bacteriochloro-phyll dimer (specif pair) [43S-438] and transferred to quinone [439-441] via bacteriopheophytin [442-444]. Ferrocytochrome c supplies an electron to the hole of a special pair [445]. The charge separation and each electron transfer have been supposed to proceed at almost 100% efficiency. Those postulates were actually verified in a series of elegant works on structural analyses of reaction center from Rhodopseudomonas (Rps.) viridis and Rb. sphaeroides by Deisen-hofer et al. [428-430]. In 1984 they fotmd that the special pair and bacteriopheophytin were beautifully aligned and oriented with each other in the system [428]. The intermolecular center-to-center distance within the special pair was revealed to be 7.0 A and the distances between the two molecular planes were 3.0 A for Rps. and 3.5 A for Rb., respectively [428-434,446-450] (Fig. 39). 

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. 

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