Bacteriochlorophyll molecules

This pair of chlorophyll molecules, which as we shall see accepts photons and thereby excites electrons, is close to the membrane surface on the periplasmic side. At the other side of the membrane the symmetry axis passes through the Fe atom. The remaining pigments are symmetrically arranged on each side of the symmetry axis (Figure 12.15). Two bacteriochlorophyll molecules, the accessory chlorophylls, make hydrophobic contacts with the special pair of chlorophylls on one side and with the pheophytin molecules on the other side. Both the accessory chlorophyll molecules and the pheophytin molecules are bound between transmembrane helices from both subunits in pockets lined by hydrophobic residues from the transmembrane helices (Figure 12.16). 

Figure 12.17 Computer-generated diagram of the stmcture of light-harvesting complex LH2 from Rhodopseudomonas acidophila. Nine a chains (gray) and nine p chains Bight blue) form two rings of transmembrane helices between which are bound nine carotenoids (yellow) and 27 bacteriochlorophyll molecules (red, green and dark blue). (Courtesy of M.Z. Papiz.)...

Figure 12.18 Ribbon diagram showing the a (red) and the P (blue) chains of the light-harvesting complex LH2. Each chain forms one transmembrane a helix, which contains a histidine residue that binds to the Mg atom of one bacteriochlorophyll molecule. (Adapted from G. McDermott et al.. Nature 374 517-521, 1995.)...

Figure 12.21 Schematic diagram of the relative positions of bacteriochlorophylls (green) in the photosynthetic membrane complexes LHl, LH2, and the reaction center. The special pair of bacteriochlorophyll molecules in the reaction center is located at the same level within the membrane as the periplasmic bacteriochlorophyll molecules Chi 875 in LHl and the Chi 850 in LH2. (Adapted from W. Kiihlbrandt, Structure 3 521-525, 1995.)...

Once the special pair has absorbed a photon of solar energy, the excited electron is rapidly removed from the vicinity of the reaction centre to prevent any back reactions. The path it takes is as follows within 3 ps (3 X 10 12 s) it has passed to the bacteriopheophytin (a chlorophyll molecule that has two protons instead of Mg2+ at its centre), without apparently becoming closely associated with the nearby accessory bacteriochlorophyll molecule. Some 200 ps later it is transferred to the quinone. Within the next 100 ps the special pair has been reduced (by electrons coming from an electron transport chain that terminates with the cytochrome situated just above it), eliminating the positive charge, while the excited electron migrates to a second quinone molecule. 

Rapid multistep Coulombic energy transfer takes place as the excitation energy is transferred between the antenna chromophores and the special pair of bacteriochlorophyll molecules (P) in the reaction centre. 

The reaction centres from the photosynthetic purple bacteria are amongst the best characterized. Each reaction centre contains four bacteriochlorophyll molecules, two bacteriophenophytin... 

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. 

In addition to interspecific variations, the number of chlorophylls per reaction center can depend on the PPF present during leaf development. Some algae and leaves of land plants developing under low illumination can have over 700 chlorophyll molecules per reaction center, whereas certain leaves developing under full sunlight can have as few as 100, an example of phenotypic plasticity. The ratio is fairly low in bacteria, where there are 40 to 100 bacteriochlorophyll molecules per reaction center. 

Figure 1. Schematic representation of the LH2 antenna complex from Rps. acidophila [7]. Only bacteriochlorophyll molecules are considered, for clarity. The squares represent bacteriochlorophyll molecules which lie in the plane of the page. Their assembly is called the B800 system (see text). The rectangles represent the BChl molecules which are perpendicular to the plane of the page. These latter are in almost a face-to-face arrangement and constitute the B850 system.

Figure 20.34 shows the reaction center of the related bacterium R. sphaeroides its structure is much more evident when the surrounding protein is stripped away. This complex consists of four bacteriochlorophyll molecules (the special pair and two others), two bacteriopheophytin molecules (which are bacteriochlorophyll molecules in which the central Mg ion is replaced by two hydrogen ions), two ubiquinone molecules (Fig. 20.35), and an iron(II) ion. Interestingly, these... 

Here (BChl)2 stands for the special pair of bacteriochlorophyll molecules, UQ for ubiquinone, and Cyt for the cytochrome protein. Steps 1 and 3 involve excitation of bacteriochlorophyll and transfer of a pair of electrons to a ubiquinone molecule. Steps 2 and 4 restore the special pair to its initial state. Steps 5 and 6 transfer hydrogen ions outside the membrane wall and restore the cytochrome to its reduced form. The net reaction is the light-driven movement of hydrogen ions from inside the cell to outside the cell. 

The change from the initial excited state to the final charge-separated state takes about 3 ps, an extremely short time considering that the primary donor pair and BOa are 17 A apart. In fact, Shuvalov and coworkerssuggested some twenty years ago that a monomeric bacteriochlorophyll molecule (Ba), believed to lie between P and BOa as shown in Fig. 1 (A), may serve as abridging element in electron transfer ... 

The photochemical charge separation in Rp. viridis was studied by Dressier, Umlauf, Schmidt, Hamm, Zinth, Buchanan and Michel " by directly exciting the primary donor with femtosecond pulses at 955 nm. In addition to the previously reported time constants of 3.5 and 200a subpicosecond time constant of 0.65 0.2 ps was also revealed in the spectral region belonging to the and Qy bands of the monomeric bacteriochlorophyll molecule. Except for the slightly smaller value of the decay time (0.65 ps vs. 0.9ps), the charge-separation and electron-flow sequences were interpreted the same way as for Rb. sphaeroides. 

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

The photosynthetic reaction center stores light energy by effecting electron transfer to reduce an electron transfer cofactor and form a proton gradient across the membrane. The arrangement of electron transfer cofactors is indicated in Figure 2 and includes a special pair of bacteriochlorophyll molecules, two accessory bacteriochloroophylls, two bacteriopheophytins, two quinone electron acceptors, and a non-henae iron. The reaction center functions... 

Figure 2. Arrangement of the electron transfer cofactors in the photosynthetic reaction center protein from the bacterium Rhodobacter sphaeroides. The figure shows the special pair of bacteriochlorophylls (top, in green and light blue), two accessory bacteriochlorophyll molecules (dark blue), two bacteriopheophytins (red), the primary quinone (Qa), the secondary quinone (Qb), and the non-heme iron.

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