Total Synthesis of Acetate from CO2

The methyl group of 5-methyltetrahydrofolate or methylcobalamin is converted to acetate in fermentations of pyruvate or α-ketobutyrate and has been studied using a cell free extract of Clostridium thermoaceticum. Evidence supporting the conclusion that the carboxyl of acetate is derived from the carboxyl of α-ketobutyrate (or pyruvate) by a mechanism involving transcarboxylation, and not from free CO2 as previously thought, is as follows: (a) carbon dioxide, a low potential reducing system (hydrogen, hydrogenase, and ferridoxin from Clostridium posteurianum (ATCC 6013)) and an adenosine triphosphate-generating system (acetyl-phosphate and adenosine diphosphate) do not replace the requirement for an α-keto acid in the conversion of 5-methyl-tetrahydrofolate to acetate; (b) attempts to demonstrate a dependence of the carboxylation upon CO2 were unsuccessful; the conversion was independent of CO2 at concentrations as low as 1 x 10-6 m; (c) kinetic tracer experiments conducted in the presence of either 14CO2 and a large pool of unlabeled α-ketobutyrate or α-[1-14C]ketobutyrate and a large pool of unlabeled CO2 demonstrated that the specific radioactivity of the acetate paralleled that of the carboxyl of α-ketobutyrate and not the CO2. The carboxyl of the α-keto acids exchanges rapidly with CO2; thus the acetate formed is labeled in both carbons from CO2 by C. thermoaceticum but not by a direct total synthesis from CO2 per se.