Reaction centers viridis

Deisenhofer J, Epp O, Miki K, Huber R and Michei H 1984 X-ray structure anaiysis of a membrane-protein compiex eiectron density map at 3 A resoiution and a modei of the chromophores of the photosynthetic reaction center from Rhode pseudomonas viridis J. Mol. Biol. 180 385-98... 

MR Gunner, B Homg. Electrostatic control of midpoint potentials m the cytochrome subunit of the Rhodopseudomonas viridis reaction center. Proc Natl Acad Sci USA 88 9151-9155, 1991. 

MR Gunner, A Nicholls, B Honig. Electrostatic potentials in Rhodopseudomonas viridis reaction centers Implications for the driving force and directionality of electron transfer. J Phys Chem 100 4277-4291, 1996. 

No region of the cytochrome penetrates the membrane nevertheless, the cytochrome subunit is an integral part of this reaction center complex, held through protein-protein interactions similar to those in soluble globular multisubunit proteins. The protein-protein interactions that bind cytochrome in the reaction center of Rhodopseudomonas viridis are strong enough to survive the purification procedure. However, when the reaction center of Rhodohacter sphaeroides is isolated, the cytochrome is lost, even though the structures of the L, M, and H subunits are very similar in the two species. 

Figure 12.15 Schematic arrangement of the photosynthetic pigments in the reaction center of Rhodopseudomonas viridis. The twofold symmetry axis that relates the L and the M subunits is aligned vertically in the plane of the paper. Electron transfer proceeds preferentially along the branch to the right. The periplasmic side of the membrane is near the top, and the cytoplasmic side is near the bottom of the structure. (From B. Furugren, courtesy of the Royal Swedish Academy of Science.)...

Deisenhofer, J., Michael, H. Nobel lecture. The photosynthetic reaction center from the purple bacterium Rhodopseudomonas viridis. EMBO f. 8 2149-2169, 1989. 

Deisenhofer, J., et al. Structure of the protein subunits in the photosynthetic reaction center of Rhodopseudomonas viridis at 3 A resolution. Nature 318 618-624, 1985. 

Michel, H. Three-dimensional crystals of a membrane protein complex. The photosynthetic reaction center from Rhodopseudomonas viridis.. Mol. Biol. 

Michel, H., et al. The "heavy" subunit of the photosynthetic reaction center from Rhodopseudomonas viridis isolation of the gene, nucleotide and amino acid sequence. EMBO J. 4 1667-1672, 1985. 

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. 

Photosynthetic Electron Transfer in the R. viridis Reaction Center... 

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

FIGURE 22.18 Model of the R. viridis reaction center, (a, b) Two views of the ribbon diagram of the reaction center. Mand L subunits appear in purple and blue, respectively. Cytochrome subunit is brown H subunit is green. These proteins provide a scaffold upon which the prosthetic groups of the reaction center are situated for effective photosynthedc electron transfer. Panel (c) shows the spatial relationship between the various prosthetic groups (4 hemes, P870, 2 BChl, 2 BPheo, 2 quinones, and the Fe atom) in the same view as in (b), but with protein chains deleted. 

Deisenhofer, J., and Michel, H., 1989. The photosyndietic reaction center from die purple bacterium Rhodopseudomonas viridis. Science 245 1463-1473. Published version of die Nobel laureate address by the researchers who first elucidated the molecnlar structure of a photosyndietic reacdon center. 

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. 

Thompson MA, Zemer MC (1991) A theoretical examination of the electronic structure and spectroscopy of the photosynthetic reaction center from Rhodopseudomonas viridis. J Am Chem Soc 113 8210-8215... 

The recent X-ray crystallographic determination of the structure of Rhodopseu-domonas viridis [15-17] and Rhodobacter sphaeroides [18-20] reaction centers has been the starting point of many theoretical studies. This structure, which is represented in Fig. 4 has confirmed the arrangement of the co-factors that had... 

Fig. 4. Schematic representation of the bacterial reaction center (R. sphaeroides). The two branches are noted A, B in studies on R. viridis. Center-to-center distances are reported from Refs. [18, 21], The approximate position of the cytochrome is indicated. The simplified notations P, B, H are used in the text...

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

Both the face-to-face dimers and the T- and L-shaped dimers have relevance to biomimetic systems, especially in light of the work on the x-ray structure of the R. Viridis reaction center ill). As will be discussed in the following, the results obtained for the face-to-face dimer with an interplanar spacing of 5.35 A is prototypical of the results obtained for almost all the dimers investigated, and thus it provides a reasonable basis for illustrating a number of important features that arise from the calculations. However, it should be noted that the lack oP covalent links in our model dimers precludes the possibility of "through-bond" effects. 

Figure 6.2. (a) Photos3fnthetic reaction center of Rhodopseudomonas viridis Reprinted from the Protein Data Bank, H. M. Berman, J. Westbrook, Z. Feng, G. Gilhland, T. N. Bhat, H. Weissig, I. N. Shindyalov, P. E. Bourne, Nucleic Acids Res. 2000, 1, 235 (http // www.pdb.org/) PDB ID IDXR, C. R. D. Lancaster, M. Bibikova, P. Sabatino, D. Oesterhelt, H. Michel, J. Biol. Chem. 2000, 275, 39364. b) arrangement of the essential components in the purple bacterium Rh. sphaeroides (see color insert). [Adapted from Ref. 5.]... 

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. 

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. 

The crystal structure of reaction centers from R. viridis was determined by Hartmut Michel, Johann Deisenhofer, Robert Huber, and their colleagues in 1984. This was the first high-resolution crystal structure to be obtained for an integral membrane protein. Reaction centers from another species, Rhodobacter sphaeroides, subsequently proved to have a similar structure. In both species, the bacteriochlorophyll and bacteriopheophytin, the iron atom and the quinones are all on two of the polypeptides, which are folded into a series of a helices that pass back and forth across the cell membrane (fig. 15.1 la). The third polypeptide resides largely on the cytoplasmic side of the membrane, but it also has one transmembrane a helix. The cytochrome subunit of the reaction center in R. viridis sits on the external (periplasmic) surface of the membrane. 

From QA., an electron moves to the second quinone bound to the reaction center (QB in fig. 15.13). In the meantime, a cytochrome replaces the electron that was removed from P870, preparing the reaction center to operate again. In R. viridis, the electron donor is the bound cytochrome with four hemes shown at the top of figure 15.1 la. In other species, it often is a soluble c-type cytochrome with a single heme, resembling mitochondrial cytochrome c. 

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