Pathway of propionate oxidation by a syntrophic culture of Smithella propionica and Methanospirillum hungatei

The pathway of propionate conversion in a syntrophic coculture of Smithella prapionica and Methanospirillum hungatei JF1 was investigated by C-13-NMR spectroscopy. Cocultures produced acetate and butyrate from propionate. [3-C-13]propionate was converted to [2-C-13]acetate, with no [1-C-13]acetate formed, Butyrate from [3-C-13]propionate was labeled at the C2 and C4 positions in a ratio of about 1:1.5. Double-labeled propionate (2,3-C-13) yielded not only double-labeled acetate. but also single-labeled acetate at the CI or C2 position. Most butyrate formed from [2,3-C-13] propionate was also double labeled in either the C1 and C2 atoms or the C3 and C4 atoms in a ratio of about 1:1.5. Smaller amounts of single-labeled butyrate and other combinations were also produced. 1-C-13-labeled propionate yielded both [1-C-13]acetate and [2-C-13]acetate. When C-13-labeled bicarbonate was present, label was not incorporated into acetate, propionate, or butyrate, In each of the incubations described above, C-13 was never recovered in bicarbonate or methane. These results indicate that S. propionica does not degrade propionate via the methyl-malonyl-coenzyme A (CoA) pathway or any other of the known pathways, such as the acryloyl-CoA pathway or the reductive carboxylation pathway. Our results strongly suggest that propionate is dismutated to acetate and butyrate via a six-carbon intermediate.