Special pairs Rhodopseudomonas viridis

Crystallization in a form suitable for X-ray structure analysis of the protein complex containing this part of the photosynthetic unit has resisted all efforts until Deisenhofer et al. solved such a structure for a complex isolated from rhodopseudomonas viridis (Deisenhofer et al., 1984). In this complex the special pair ((BC)2) and the pheophytin (BP) were found to be spatially separated by a centre-to-centre distance of no less than 17 A, while the centre-to-centre distance between the pheophytin and the menaquinone (MQ)... 

Returning to the photosynthetic system it now seems plausible that the explanation for the short charge-separation times in the primary steps must be found in the nature of the medium between the redox-sites involved. In this connection it is interesting to note that saturated hydrocarbon chains (i.e. phytyl sidechains) extend from the special pair and from the menaquinone towards the intermediate bacteriopheophytin (see Fig.l). At this moment it is not clear, however, whether in rhodopseudomonas viridis any of these phytyl sidechains plays the role of a molecular wire (see also Kuhn, 1986) that we attribute to the hydrocarbon bridges in l(n). For rhodopseudomonas sphaeroides a fivefold decrease in the rate of the reverse electron transfer from the quinone (ubiquinone) to the bacteriopheophytin was recently reported to result upon removal of the isoprenoid sidechain from the quinone (Gunner et al., 1986). 

Dohse, B., Mathis, P., Wachtveitl, J., Laussermair, E., Iwata, S., Michel, H., and Oesterhelt, D., 1995, Electron-transfer from the tetraheme cytochrome to the special pair In the Rhodopseudomonas viridis reaction-centeroeffect of mutations of tyrosine L162 Biochemistry 34 11335nll343. 

Fig, 1. Arrangement of the pigments in the reaction center of Rhodopseudomonas viridis, based on the crystallographic data in Ref. 16. Upon excitation an electron is transferred from the special pair P to the menaquinone Q. (From Ref. 18). 

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 all of the hgures mentioned, down corresponds to inside the membrane, and up is outside. In other words, in Rhodopseudomonas viridis the special pair of chlorophylls is near the cytoplasmic side of the membrane (inside the cell), whereas in both photosystems I and II, the special pairs are near the stroma and away from the thy-lakoid lumen, that is, facing outward. This is interesting because other structures in the thylakoid membrane are also upside down compared to bacterial and mitochondrial systems. 

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)."...

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). 

THEORETICAL STUDIES ON THE ELECTRONICAL STRUCTURE OF THE SPECIAL PAIR DIMER AND THE CHARGE SEPARATION PROCESS FOR THE REACnON CENTER RHODOPSEUDOMONAS VIRIDIS... 

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