The Propionic Acid Fermentation

The most important property of the genus Propionibacterium is the production of propionic acid as a result of the propionic acid fermentation dependent on coenzyme B12. If the dependence on coenzyme Bn is disregarded, some clostridial strains that do not form spores may be erroneously attributed as propionibacteria. For example, Cl botulinum, Cl propionicum and some other species can produce propionic acid, but propionibacteria have the GC-type DNA (65-67 mol% G+C in classical and 53-62 mol% in cutaneous bacteria), while clostridial DNA is of the AT-type (25-30 mol% G+C). 

Propionic acid fermentation is not limited to propionibacteria it functions in vertebrates, in many species of arthropods, in some invertebrates imder anaerobic conditions (Halanker and Blomquist, 1989). In eukaryotes the propionic acid fermentation operates in reverse, providing a pathway for the catabolism of propionate formed via p-oxidation of odd-numbered fatty acids, by degradation of branched-chain amino acids (valine, isoleucine) and also produced from the carbon backbones of methionine, threonine, thymine and cholesterol (Rosenberg, 1983). The key reaction of propionic acid fermentation is the transformation of L-methylmalonyl-CoA(b) to succinyl-CoA, which requires coenzyme B12 (AdoCbl). In humans vitamin B deficit provokes a disease called pernicious anemia. 

The addition of fumarate to the suspension of P. freudenreichii oxidizing lactate was shown to result in a partial oxidation of cytochromes b and 02 (de Vries et al., 1977). In the presence of 2H-heptyl-4-hydroxyquinoline-N-oxide, an inhibitor of cytochrome b, lactate dehydrogenase activity was reduced by 85% if fumarate served as an electron acceptor, but only by 25% in the presence of methylene blue (in the latter case H2 is transferred to the dye without participation of cytochrome b). Keeping in mind the evidence presented above, the propionic acid fermentation can be viewed as a harmonious combination of the soluble and membrane-bound redox enzymes. The involvement of cytochrome b in anaerobic electron transport to fiimarate confirms the suggestion (Bauchop and Elsden, 1960) that there is an oxidative phosphorylation site between fumarate and succinate in propionibacteria. 

The uniqueness of propionic acid fermentation is due to the participation of PEP carboxytransphosphorylase, the enzyme not found in the other organisms that synthesize propionate. Due to the presence of this enzyme the propionic acid fermentation functions as a cyclic process (for the significance of cycling, see above). Another peculiarity of this fermentation is related to the way propionate is formed, which is coupled with the... 

Typical for the propionic acid fermentation is the formation of identical fermentation products from Ce-, C5-, C4- and Cs-compounds. The ratio of products may differ, depending to a large extent on the degree of oxidation of the utilized carbon source. The ratio of propionic to acetic acid in glycerol medium was found to be 2 1, in lactate medium it was 1 1.5, and in pyruvate medium it was 1 2 (Vorobjeva, 1958a). Utilization of pyruvate resulted in the production of acetic acid with a constant rate (Vorobjeva, 1958b). The yield of propionic acid was twofold lower, and after four days it was almost unchanged (Fig. 3.6). No equimolarity between the acetate and CO2... 

For the biosynthesis of cell components a microorganism must be supplied with appropriate low molecular weight compounds such as sugars, organic acids, amino acids etc. Many of 2-, 3-, 4- and 5-carbon compounds are formed in catabolic reactions. In propionic acid bacteria these reactions comprise the propionic acid fermentation, TCA cycle and hexose monophosphate shunt. The latter supplies the cell with erythrose-phosphate, ribose-5-phosphate and reducing equivalents (NADPH) needed for many syntheses. Erythrose-4-phosphate is used in the formation of aromatic amino acids phenylalanine, tryptophane, tyrosine. Ribose-5-phosphate is incorporated into nucleic acids. The pentose cycle and propionic acid fermentation, as mentioned before, have a number of common precursors and enzymes. The inclusion of common precursors into one or another pathway is regulated by the level of ATP (Labory, 1970), and this regulation in fact determines the ratio of catabolic and anabolic processes in the cell. 

Allen SHG, Kellermeyer RW, Stjemholm R and Wood HG (1964) Purification and properties of enzymes involved in the propionic acid fermentation. J Bacteriol 87 171-187... 

Stjernholm, R. and Wood, H. G. (1960) Glycerol dissimilation and the occurrence of a symmetrical three-carbon intermediate in the propionic acid fermentation. J. Biol. Chem. 235,2757-2761. 

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