Sulfate-reducing bacteria electron transfer

Although electron transfers in biological systems are generally expected to be non-adiabatic, it is possible for some intramolecular transfers to be close to the adiabatic limit, particularly in proteins where several redox centers are held in a very compact arrangement. This situation is found for example in cytochromes C3 of sulfate-reducing bacteria which contain four hemes in a 13 kDa molecule [10, 11], or in Escherichia coli sulfite reductase where the distance between the siroheme iron and the closest iron of a 4Fe-4S cluster is only 4.4 A [12]. It is interesting to note that a very fast intramolecular transfer rate of about 10 s was inferred from resonance Raman experiments performed in Desulfovibrio vulgaris Miyazaki cytochrome Cj [13]. 

Odom, J. M. and Peck, H. D. (1984) Hydrogenase, electron-transfer proteins, and energy coupling in the sulfate reducing bacteria Desulfovibrio. Ann. Rev. Microbiol., 38, 551. 

Le Gall, J., DerVartanian, D.V., Peck, H.D., Jr. In Flavoproteins, Iron-Proteins, and Hemo-proteins as Electron Transfer Components of the Sulfate-Reducing Bacteria (ed. Sanadi, R.), New York, Academic Press 1979, Current Topics in Bioenergetics 9, 237... 

Fig. 4.2. The oxidation mechanisms of lactate by sulfate in the sulfate-reducing bacteria of Desulfovibrio genus. Circled numbers 1, lactate dehydrogenase (cytochrome c-553) 2, pyruvate-ferredoxin 2-oxidoreductase (CoA-acetylating) 3, phosphate acetyltransferase 4, acetate kinase 5, sulfate adenylyltransferase 6, adenylylsulfate reductase 7, sulfite reductase 8, adenylate kinase. ATP adenosine 5 -triphosphate is also biosynthesized by the catalysis of ATP synthase using the energy liberated by the electron transfer around this part...

LeGall J, Peck HD Jr (1987) Amino-terminal amino acid sequences of electron transfer proteins from Gram-negative bacteria as indicators of their cellular localization the sulfate-reducing bacteria. FEMS Microbiol Rev 46 35-40... 

SCHEME I. Anaerobic metabolism in Sulfate Reducing Bacteria (SRB) from Desulfovibrio sp. ETC electron transfer chains Ferredoxins, Flavodoxins, Cytochromes, etc. 

Long-range electron transfer reactions are critical for biological energy conservation. The electron transfer chain of sulfate reducing bacteria has been discussed extensively, but is still not completely established. 

The microenviromnent of AOT/isooctane reverse micelles was considered as a biomimetic system for the solubilization of flavodoxin, aldehyde oxidoreductase, and a membrane-associated hydrogenase, all isolated from the sulfate reducing bacteria Desulfovibrio gigas [153,154], The main perspective of this research group was the encapsulation of aU the three microbial enzymes mentioned above in reverse micelles and the observation of the electron-transfer chain under the water restricted conditions. 

The multiheme protein cytochrome C3, an electron transfer protein from the sulfate-reducing bacteria Desulfovibrio vulgaris (strain Miyazaki), was the first example of a heme protein exhibiting a reversible electrode reaction at a mercury electrode. Due to the sophisticated structure of proteins, there is great difficulty in most cases to achieve electron transfer of enzyme molecules via an electrode. 

Some prokaryotes are anaerobic heterotrophs. These include the denitrifiers, sulfate reducers, and fermenters, as well as the bacteria capable of reducing metals, such as Fe(lll) to Fe(II) and Mn(lV) to Mn(II). Because the oxidized metals are present as solids, e.g., FeOOH(s), Fe203(s), and Mn02(s), these bacteria must be in direct contact with the mineral surface and have a mechanism for transferring electrons across their cell membranes. One bacterium that appears to have such a mechanism is the facultative anaerobe Shewanella oneidensis, which produces a specific protein on its outer membrane only under anaerobic conditions when it is in direct contact with a suitable... 

In bacteria PAPS is a substrate for sulfate reduction. In plants, adenosine-5 -phosphosulfate is the substrate. Thioredoxin, a small thiol-containing protein, reduces the sulfate in PAPS to sulfite (SOS "). Sulfite is reduced by sulfite reductase in a six electron transfer through the intermediates NADPH, FAD, FMN, an iron-sulfur center, and the porphyrin siroheme. The end product is H2S. 

Similar c-type cytochromes are involved in many kinds of energy metabolism in bacteria, such as phototrophes, methylotrophes, sulfate reducers, nitrogen-fixers, and denitrifiers. For example, in the anaerobic electron chain of the denitrification system in Gram-negative bacteria, c-type cytochromes transfer electrons from the cyt bc complex to cytochrome cdy nitrite reductase, N2O... 

As discussed above, sulfate is activated by ATP to form APS. Electrons from donors are transferred to APS. The sulfate ion of APS is reduced to sulfite and then to sulfide, which is excreted from cells. The first step, sulfate to sulfite reduction, involves two-electron transfer followed by six-electron transfer during the sulfite to sulfide reduction. The bacteria are capable of performing catabolic sulfate reduction. 

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