Biochemistry - The Chemical Reactions of Living Cells Volume 2 - D. Metzler 1980

Organization of Metabolism: Catabolic Pathways
Fermentation: "Life Without Oxygen"
Propionic Acid Fermentation

Propionic acid Bacteria are particularly abundant in the digestive tract of ruminants. The rumen contains bacteria capable of hydrolyzing Cellulose to yield glucose, which is then converted into lactate and other products. Propionic acid bacteria can convert both glucose and lactate into propionic and acetic acids, which are subsequently absorbed into the host's bloodstream. Small amounts of succinic acid are formed simultaneously.

The foundation of Propionic acid Fermentation is The conversion of Pyruvate to oxaloacetate via carboxylation, followed by transformation through succinate and succinyl-CoA into methylmalonyl-CoA and propionyl-CoA. These reactions constitute a pathway that is almost exactly the reverse of the one considered in Section D, 2, when discussing The oxidation of propionate in animal Tissues. However, while the carboxylation of pyruvate to oxaloacetate in the animal body requires ATP, propionic acid bacteria manage to save one equivalent of ATP by utilizing carboxyltransferase (Chapter 8, Section C, 1). This enzyme transfers a carboxyl group from protein-bound carboxybiotin, which is formed during the decarboxylation of methylmalonyl-CoA at the penultimate stage of the complete reaction chain (Fig. 9-10). A second ATP molecule is conserved through the direct coupling, mediated by CoA-transferase, of the conversion of succinate to succinyl-CoA with the Cleavage of propionyl-CoA to yield propionate (Chapter 7, Section E, 4). The redox balance is maintained when two-thirds of the glucose molecules are converted into propionate and one-third into acetate:

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FIG. 9-10. Propionic acid fermentation in Propionibacterium and Veillonella bacteria. Oxidative steps are designated by "O", and reductive steps by "R". Asterisks denote the two coupled Reactions Catalyzed by carboxyltransferase.

It should be noted that this fermentation process yields a greater number of carboxyl groups and CO₂ molecules (2²/₃ mol per 1 mol of glucose) than standard Lactic acid fermentation. Consequently, ΔG' becomes more negative, and the ATP yield is also higher (likewise 2²/₃ mol per 1 mol of fermented glucose).

Utilizing the same mechanism (Fig. 9-10), propionic acid bacteria are able to take up lactate produced by the fermentation of other bacteria and convert it further into propionate and acetate:

The net gain is one molecule of ATP. This reaction apparently accounts for the existence of a specific ecological niche in the rumen of ruminants colonized by propionic acid bacteria.