Bacteriochlorophyll a-proteins

Olson, J.M. Bacteriochlorophyll a-proteins from green bacteria. In The Photosynthetic Bacteria (Clayton, R.K., Sistrom, W.R., eds.). New York Plenum Press 1978, pp. 161-178... 

The bacteriochlorophyll a protein from the green phot osynthetic bacterium Prosthecochloris aestuarii has been determined at 2.8 A resolution.389 It is made up of three identical subunits, tightly packed around a three-fold symmetry axis. Each subunit consists of a core of seven bacteriochlorophyll a molecules enclosed within a bag of protein. There are extensive contacts between the phytyl chains of the seven bacteriochlorophylls within each subunit. These tails form an inner hydrophobic core. The seven magnesiums appear to be five-coordinate. In five cases the fifth ligand is a histidine side-chain, in one case a protein backbone carbonyl oxygen, and in the other case a water molecule. It appears that the removal of one or more of the bacteriochlorophyll molecules would destabilize the protein. It is unlikely that the chlorophyll can be reversibly removed from the protein. 

Because the aminolipid is a zwitterion at physiological pH, it may act as a membrane anchor for proteins with negatively and positively charged groups. The water-soluble bacteriochlorophyll a-protein (8) found only in green sulfur bacteria might be bound in this way. 

Niederman, R.A, Mallon, D.E. and Langan, J.J, (1976) Membranes of Rhodopseudomonas sphaeroides. IV. Assembly of chromatophores in low aeration cell suspensions. Biochim. Biophys. Acta 440, 429-447 Niederman, R.A., Mallon, D.E. and Parks, L.C. (1979) Membranes of Rhodopseudomonas sphaeroides. VI. Isolation of a fraction enriched in newly synthesised bacteriochlorophyll a protein con5)lexes. Biochim. Biophys. Acta 555, 210-220... 

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

Sunlight is efficiently absorbed throughout the solar spectrum by an antenna system containing bacteriochlorophyll and carotenoid chromophores in a protein environment. 

The CD features of bacteriochlorophyll a in light-harvesting bacteriochlorophy 11-protein complexes from native bacteria is more interesting and is not compatible with Boxer s experiments. Bacteriochlorophyll(Bchl)-protein complexes exhibit various CD spectral profiles, depending on the species of bacteria and their culture conditions 201 206). Thus, CD of Bchl arises from the asymmetric environment in which the Bchl is situated, and from the specialized arrangement which affords a specific interaction between Bchl molecules, as well as from the asymmetry of Bchl... 

Metal ion chelates of various porphyrins, differing in their substituents at positions 1-8, are intimately involved in a great number of life processes. Iron protoporphyrin (13) is the most common form and serves as the cofactor of a large number of enzymes. Usually (13) is non-covalently bound to its conjugate apoenzymes. Examples of covalently attached (13) are provided by c-type cytochromes, the attachment being between two vinyl side chains of (13) and two cysteine residues of the protein. Other biologically important derivatives of porphyrin include chlorophyll a (14), bacteriochlorophyll a and heme a (B-79MI11002). 

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

The chlorophyll or bacteriochlorophyll that undergoes pho-tooxriiation is bound to a protein in a complex caffed a reaction center. Reaction centers have been purified by disrupt-... 

A view of the core of the reaction center of Rh. viridis69 is shown in Figure 2.36. It consists of three tetrapyrrolic cofactors the so-called special pair (SP), which is a dimer of bacteriochlorophylls, a monomeric bacteriochloro-phyll (BCh), and a bacteriopheophytin (BPh). As noted above, all these chro-mophores are arranged within the protein structure with oblique orientations to one another. In this bacterial triad, SP functions as the electron donor in... 

Another antenna complex where high-resolution structural information is available is the bacteriochlorophyll a binding protein (also known as the Fenna-Matthews-Olson or FMO protein) from green sulfur bacteria. This complex serves as the bridge between the peripheral chlorosome complex and the membrane-bound reaction center complexes. In this... 

JM Olson, B Ke and KH Thompson (1976) Exciton interaction among chlorophyll molecules in bacteriochlo-rophyll-a proteins and bacteriochlorophyll-a reaction center complexes from green bacteria. Biochim Biophys Acta 430 524-537... 

CD measurements at 77 °K of a bacteriochlorophyll protein from Chloropseudomonas ethylica indicated exciton splitting near 800 and 600 nm of bacteriochlorophyll a molecules. Distances of 12—15 A between the interacting molecules in a protein subunit were estimated (269). 

The reaction center of purple bacteria contains three protein subunits (L, M, and H) located in the plasma membrane (Figure 8-35). Bound to these proteins are the prosthetic groups that absorb light and transport electrons during photosynthesis. The prosthetic groups include a special pair of bacteriochlorophyll a molecules equivalent to the reaction-center chlorophyll a molecules in plants, as well as several other pigments and two quinones, termed Qa and Qb, that are structurally similar to mitochondrial ubiquinone. 

M subunit (white) each form five transmembrane a helices and have a very similar structure overall the H subunit (light blue) is anchored to the membrane by a single transmembrane a helix. A fourth subunit (not shown) is a peripheral protein that binds to the exoplasmic segments of the other subunits. (Bottom) Within each reaction center is a special pair of bacteriochlorophyll a molecules (green), capable of initiating photoelectron transport two voyeur chlorophylls (purple) two pheophytins (dark blue), and two quinones, Qa and Qb (orange). Qb is the primary electron acceptor during photosynthesis. [After M. H. Stowell etal., 1997, Science 276 812.]... 

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