Characterization bacteriochlorophyll

Hiraishi A, Y Yonemitsu, M Matsushita, YK Shin, H Kuraishi, K Kawahara (2002) Characterization of Porphybacter sanguineus sp. nov., an aerobic bacteriochlorophyll-containing bacterium capable of degrading biphenyl and dibenzofuran. Arch Microbiol 178 45-52. 

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

The hole in the bacteriochlorophyll special pair is filled by electron transfer from a cytochrome. In Rps. viridis, the cytochrome donor is tightly bound to the PRC at the membrane surface. This subunit contains four hemes in a nearly linear array oriented perpendicular to the membrane. The reduction potentials of the hemes alternate from high (>250 mV Vi. NHE) to low (<50mV) as the distance from (Bchl)2 increases. The heme closest to the special pair, cytochrome C559, has the highest potential and fills the (Bchl)2 + hole in about 200 ns. The next well-characterized process is ET from cytochrome C556 to cytochrome C559 in 2 ts over a distance of 27.9 A. ... 

Thus, most of the data collected on bacterial RC seem to agree on the presence of a bacteriochlorophyll pair which can be oxidized upon illumination. Thermodynamically this donor has been characterized by studying the dependence of photooxidation from the ambient redox potential in Rp. sphaeroides the E -j was found to be 0.44 V and pH independent [14]. 

The biochemical architecture of photosynthetic bacteria is not as complex as that of green plants. For example, photosynthetic bacteria have only one photosystem, while green plants have two. The reaction center protein from several species of photosynthetic bacteria can be isolated from the photosynthetic membrane. Reaction centers from the species Rhodopseudomonas sphaeroides have been extensively studied. Although minor details will change from one species to another, the important features are nearly identical. The reaction center protein has a molecular weight of about 70,000 daltons. Within the reaction center protein extracted from the carotenoidless mutant strain R26 of the species R. sphaeroides, are found four molecules of bacteriochlorophyll a, two molecules of bacteriopheophytin a, one atom of nonheme iron, and, depending on the isolation procedure used, one or two molecules of ubiquinone. The absorption spectrum of the isolated reaction center has been well characterized. It is shown in Fig. 4. Based on in vitro absorption spectra, the bands at 870, 800, and 600 nm have been assigned to the bacteriochlorophyll a molecule. Bands at 760 and 530 nm have been attributed to the bacteriopheophytin a. 

The luminescence originates from a Cp biq LLCT triplet. The complex shares some remarkable features with the photosynthetic reaction center which is characterized by a pair of two bacteriochlorophyll molecules in close proximity but in an oblique orientation ( special pair ). In the primary photochemical step charge separation takes place by excited state electron transfer from this special pair to an acceptor. In the case of [Cp2Zr biq] + the special pair is represented by both Cp ligands. However, in this complex charge separation takes place by a direct optical transition. 

Farchaus JW, Wachtveitl J, Mathis P, and Oesterhelt D. Tyrosine-162 of the Photosynthetic Reaction-Center L-Subunit Plays a Critical Role in the Cytochrome-C(2) Mediated Rereduction of the Photooxidized Bacteriochlorophyll Dimer in Rhodobacter-Sphaeroides. 1. Site-Directed Mutagenesis and Initial Characterization. Biochemistry 1993 32 10885-10893. 

Svec, W.A., The isolation, preparation, characterization, and estimation of the chlorophylls and the bacteriochlorophylls, in The Porphyrins, Vol. 5, Physical Chemistry, Part C, Dolphin, D. (Ed.), Academic Press, New York, 1978. 

Functionally important is the fact, that in all the complexes which accept modified bacteriochlorophylls, the reversible photobleaching is retained. Kinetik and other spectroscopic work is in progress to further characterize RC with modified pigments, in order to understand the structure/func-tion relationship of the tetrapyrroles in bacterial RC. 

Also indicated in Fig. 2 are the time constants associated with the various electron transfer processes. There are two views on the initial electron transfer process. In one view, initial charge separation takes place in about 3 ps to yield an intermediate having the electron on the accessory bacteriochlorophyll (B) on the A branch of the reaction center (Holzapfel et al. (1990)). Then, in an even more rapid step of less than one ps, the electron is transferred to the bacteriopheophydn (H) on the A branch. In the other limit, charge separation is viewed as moving an electron directly to bacteriopheophytin (H) in about 3.5 ps (Kirmaier and Holten (1987)). In any case, tie intermediate H" is relatively well characterized in terms of its energy and lifetime since it can be trapped. With a time constant of200 ps the electron is transferred to the quinone, Qa, located near the membrane water interface tm the cytr lasmic ade. Finally, the electron is transferred laterally to Qb in 200 ps (Feher et al. (1989), Parson (1991)). 

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