This paper, the second of three sections of a review, deals with propionate and acetate formation, carboxylation, decarboxyiation, and transcarboxylation, the roles of biotin and vitamin B12, and the enzymes involved in the propionic-acid fermentation. Pathways for propionate, acetate, and carbon dioxide formation by Propionibacterium have been thoroughly investigated and established. These investigations led to discovery of transcarboxylase, a biotin-containing enzyme, and were the result of studies showing that the formation of propionate does not involve the expected turnover of CO2. This enzyme appears to be unique to propionibacteria; it could be speculated that it is the first discovered of a group of transcarboxylases. One of the breakthroughs relative to the mechanism of biotin fixation reactions was the discovery that biotin can be carboxylated enzymatically.
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Hettinga et al. (1972) studied this question.