Reductive Carboxylation of Propionate to Butyrate in Methanogenic Ecosystems

Abstract

During the batch degradation of sodium propionate by the anaerobic sludge from an industrial digestor, we observed a significant amount of butyrate formation. Varying the initial propionate concentrations did not alter the ratio of maximal butyrate accumulation to initial propionate concentration within a large range. By measuring the decrease in the radioactivity of [1- 14 C]butyrate during propionate degradation, we estimated that about 20% of the propionate was converted to butyrate. Labeled butyrate was formed from [1- 14 C]propionate with the same specific radioactivity, suggesting a possible direct pathway from propionate to butyrate. We confirmed this hypothesis by nuclear magnetic resonance studies with [ 13 C]propionate. The results showed that [1- 13 C]-, [2- 13 C]-, and [3- 13 C]propionate were converted to [2- 13 C]-, [3- 13 C]-, and [4- 13 C]butyrate, respectively, demonstrating the direct carboxylation on the carboxyl group of propionate without randomization of the other two carbons. In addition, we observed an exchange reaction between C-2 and C-3 of the propionate, indicating that acetogensis may proceed through a randomizing pathway. The physiological significance and importance of various metabolic pathways involved in propionate degradation are discussed, and an unusual pathway of butyrate synthesis is proposed.