Reaction center protein

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. 

Photosynthetic organisms take advantage of the fact that chlorophyll becomes a strong reductant when it is excited with light. Photooxidation of chlorophyll or bacteriochlorophyll occurs in pigment-protein reaction centers. Chloroplasts have two types of reaction cen-... 

The primary photochemical processes of photosynthesis take place within membrane bound complexes of pigments and protein, reaction centers (Shuvalov and Krasnovsky, 1981 Deisenhofer et al., 1986, Rees et al., 1989 Norris and Shiffer, 1990 Kirmaier and Holten, 1991 Feher et al, 1992 Stowell et al, 1997). One mole of a reaction center from different bacteria contains 4 moles of bacteriochlorophyl (Bchl), 2 moles of bacteriopheophytin (Bph), two moles of ubiquinone (Q) and a non-heme Fe atom. In RC from Rhodobacter speroides, a total of 11 hydrophobic a-helixes create a framework that organizes the cofactor and a hydrophobic band approximately 35 A wide. RC from Rhodopseudomonus viridus has three polypeptides having pronounced hydrophobic properties. The molecular mass of the polypeptides are 37 571 (L), 35902 (M) and 28902 (H). The H subunit does not carry pigments but it is sufficient for the photochemical activity. The protein components of reaction centers from different-bacteria are similar. 

The thylakoid membranes contain the energy-transducing machinery light-harvesting proteins, reaction centers, electron-transport chains, and ATP synthase. They have nearly equal amounts of lipids and proteins. The lipid composition is highly distinctive about 40% of the total lipids are galactolipids and 4% are sulfolipids, whereas only 10% are... 

Staehelin. L. A. Armond, P. A. Miller, K. R. in "Chlorophyll-Proteins, Reaction Centers, and Photosynthetic Membranes" Brookhaven Symposium in Biology No. 28 Brookhaven National Laboratory Upton, Long Island, New York, 1976 ... 

J.M. Olson and G. Hind, Eds., "Chlorophyll-Proteins, Reaction Centers and Photosynthetic Membranes", Brookhaven Symposia Biology, Vol. 28, pp. 105-131 (1977). 

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... 

Both PSI and PSII are necessary for photosynthesis, but the systems do not operate in the implied temporal sequence. There is also considerable pooling of electrons in intermediates between the two photosystems, and the indicated photoacts seldom occur in unison. The terms PSI and PSII have come to represent two distinct, but interacting reaction centers in photosynthetic membranes (36,37) the two centers are considered in combination with the proteins and electron-transfer processes specific to the separate centers. 

Despite considerable efforts very few membrane proteins have yielded crystals that diffract x-rays to high resolution. In fact, only about a dozen such proteins are currently known, among which are porins (which are outer membrane proteins from bacteria), the enzymes cytochrome c oxidase and prostaglandin synthase, and the light-harvesting complexes and photosynthetic reaction centers involved in photosynthesis. In contrast, many other membrane proteins have yielded small crystals that diffract poorly, or not at all, using conventional x-ray sources. However, using the most advanced synchrotron sources (see Chapter 18) it is now possible to determine x-ray structures from protein crystals as small as 20 pm wide which will permit more membrane protein structures to be elucidated. 

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

The reaction center is built up from four polypeptide chains, three of which are called L, M, and H because they were thought to have light, medium, and heavy molecular masses as deduced from their electrophoretic mobility on SDS-PAGE. Subsequent amino acid sequence determinations showed, however, that the H chain is in fact the smallest with 258 amino acids, followed by the L chain with 273 amino acids. The M chain is the largest polypeptide with 323 amino acids. This discrepancy between apparent relative masses and real molecular weights illustrates the uncertainty in deducing molecular masses of membrane-bound proteins from their mobility in electrophoretic gels. 

The L and M subunits show about 25% sequence identity and are therefore homologous and evolutionarily related proteins. The H subunit, on the other hand, has a completely different sequence. The fourth subunit of the reaction center is a cytochrome that has 336 amino acids with a sequence that is not similar to any other known cytochrome sequence. 

Figure 12.12 X-ray diffraction pattern from crystals of a membrane-bound protein, the bacterial photosynthetic reaction center. (Courtesy of H. Michel.)...

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 12.14 The three-dimensional structure of a photosynthetic reaction center of a purple bacterium was the first high-resolution structure to be obtained from a membrane-bound protein. The molecule contains four subunits L, M, H, and a cytochrome. Subunits L and M bind the photosynthetic pigments, and the cytochrome binds four heme groups. The L (yellow) and the M (red) subunits each have five transmembrane a helices A-E. The H subunit (green) has one such transmembrane helix, AH, and the cytochrome (blue) has none. Approximate membrane boundaries are shown. The photosynthetic pigments and the heme groups appear in black. (Adapted from L. Stryer, Biochemistry, 3rd ed. New York ...

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

TTie reaction center is surrounded by a ring of 16 antenna proteins of the light-harvesting complex LHl... 

Naively, one might assume that it should be possible to scan the sequence and pick out regions with about 20 consecutive hydrophobic amino acids. However, no such regions occur in the reaction center proteins. Just as in soluble proteins there are hydrophobic side chains at the... 

Rees, D.C., et al. The bacterial photosynthetic reaction center as a model for membrane proteins. Anna. Rev. Biochem. 58 607-633, 1989. 

Allen, J.R, et al. Structure of the reaction center from Rhodobacter sphaeroides R-26 the protein subunits. 

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

The structure of the UQ-cyt c reductase, also known as the cytochrome bc complex, has been determined by Johann Deisenhofer and his colleagues. (Deisenhofer was a co-recipient of the Nobel Prize in Chemistry for his work on the structure of a photosynthetic reaction center [see Chapter 22]). The complex is a dimer, with each monomer consisting of 11 protein subunits and 2165 amino acid residues (monomer mass, 248 kD). The dimeric structure is pear-shaped and consists of a large domain that extends 75 A into the mito-... 

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. 

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. 

Knaff, D. B., 1991. Regnlatory phosphoryladon of chloroplast antenna proteins. Trends in Biochemical Sciences 16 82-83. Additional discussion of the structure of light-harvesdng antenna complexes associated widi photo-synthedc reaction centers can be found in Trends in Biochemical Sciences 11 414 (1986), 14 72 (1989), and 16 181 (1991). 

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