Rhodopseudomonas viridis, photosynthesis

Fig. 4. Absorbance-change kinetics of photooxidation due to the primary eiectron donor and its decay (re-reduction) [upper paneis] and the oxidation of a c-type cytochrome [iower panels] in C. vinosum (left) and Rp. viridis [right panels]. The C. vinosum sample was poised at a redox potential so that Cyt c555 ("Cyt c422 ) is reduced before flash excitation the ambient redox potential in Rp. viridis was -250 mV, so that only Cyt c5S8 is present in the reduced state before excitation. Figure source left panels (C. vinosum) from Parson (1968) The role of P870 in bacterial photosynthesis. Biochim Biophys Acta 153 254 right panels (Rp. viridis) from Shopes, Levine, Molten and Wraight (1987) Kinetics of oxidation of the bound cytochromes in reaction centers from Rhodopseudomonas viridis. Photosynthesis Res 12 167.

RJ Shopes, LMA Levine, D Molten and CA Wraight (1987) Kinetics of oxidation of the bound cytochromes in reaction centers from Rhodopseudomonas viridis. Photosynthesis Res 12 165-180... 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

It is interesting to compare the thermal-treatment effect on the secondary structure of two proteins, namely, bacteriorhodopsin (BR) and photosynthetic reaction centers from Rhodopseudomonas viridis (RC). The investigation was done for three types of samples for each object-solution, LB film, and self-assembled film. Both proteins are membrane ones and are objects of numerous studies, for they play a key role in photosynthesis, providing a light-induced charge transfer through membranes—electrons in the case of RC and protons in the case of BR. 

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. 

While the preexponential factor in these equations is close to that in eq.(l), a much slower exponential decrease of k with increasing distance is found. Thus for the edge-to-edge distances occurring in the primary steps of photosynthesis (8-10 A) eq.(4) predicts charge separation times of 6.2-32 ps, i.e. quite similar to those actually observed (see Fig. 1) in rhodopseudomonas viridis. 

It is especially timely to review the subject of exciton effects because, with the advent of the X-ray structural model of the Rhodopseudomonas viridis RC [9-12], it is becoming apparent that analyses of exciton effects exhibit a dichotomy. On the one hand there are analyses based on incomplete structural information, on the other there are those based on X-ray structural models. The former generally seem theoretically straightforward and consistent with all experimental data, while the latter tend to be theoretically involuted and inconsistent with at least some of the data. Because the underlying interactions are quite important in photosynthesis, it is worthwhile exploring this situation and trying to understand what underlies it. In Section 2 basic theoretical concepts are briefly summarized. Exciton analyses based on partial structural information are discussed in Sections 3 and 4, and those based on X-ray models are considered in Sections 5-7. 

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. 

Deisenhofer J and Michel H (1989) The photosynthesis reaction center from the bacterium Rhodopseudomonas viridis. EMBO 18 2149-2170... 

Lancaster CRD and Michel H (1999a) The structure of the Rhodopseudomonas viridis reaction centre—an overview and recent advances. In Garab G (ed) Photosynthesis Mechanisms and Effects, Vol II, pp 673-678. Kluwer Academic Publishers, Dordrecht... 

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). 

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. 

The primary electron transfer event in photosynthesis is the transfer of an electron from the excited state reaction centre chlorophyll [P]to pheophytin [I]. This charge separation is then stabilised by transfer of the electron to a chain of acceptors and the rereduction of the reaction centre chlorophyll by an electron donor. In the purple photosynthetic bacterium Rhodopseudomonas viridis the electron acceptors are quinones and the electron donors are cytochrome haems. The acceptor complex is thought to consist of a primary quinone [Qa], which is a menaquinone, and a secondary quinone [Qb], which is ubiquinone. Qa is tightly bound to the reaction centre and undergoes... 

T p y+l)/k0j, where k j is the RC trapping rate and V is the ratio of the probabilities of finding the exciton in the antenna system and RC. It can be shown that from nine experimental observables (fluorescence and phosphorescence intensities and quantum yields, 7) qj) only one is independent. In all the transfer regimes, the observables depend only on V which is in general a function of time, intramolecular rate constants, size of the photosynthetic unit and initial conditions. Therefore, V (t) is the maximum information obtainable from the observables. These and further results representing general theoretical answers to problems l)-5) were illustrated on the case of the bacterial photosynthesis (Rhodopseudomonas viridis) where they are valid for the whole range of the physically acceptable values of the Forster radius. 

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. 

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. 

The photosynthetic reaction centres (RCs) are transmembrane protein-pigment complexes that perform light-induced charge separation during the primary steps of photosynthesis. RCs from purple bacteria consist of three protein subunits, L, M and H, and bind four bacteriochlorophylls, two bacteriopheophytins, two quinones, one non-haem iron and one carotenoid. The elucidation at atomic resolution of the three-dimensional structures of the bacterial RCs from Rhodopseudomonas (Rps.) viridis (1) and Rhodobacter (Rb,) sphaeroides (2-4) has provided impetus for theoretical and experimental work on the mechanism of primary charge separation in the RCs. The structures revealed that the cofactors are bound at the interface between the L and M subunits and are organised around a pseudo C2 symmetry axis. However, the structural symmetry does not result in functional symmetry as the electron transfer proceeds only along the L branch (5). 

The proteins most relevant to photosynthesis are membrane-bound and are therefore difficult to crystallize for structural analysis. Nevertheless, the RC from the purple bacterium Rhodopseudomonas (Rp.) viridis was the first membrane-bound protein from which well-ordered three-dimensional crystals were grown (Michel, 1982). The structure ofthis RC... 

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). 

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