Single-carbon chemistry of acetogenic and methanogenic bacteria

Anaerobic catabolism of formate to acetate and CO2 by Butyribacterium methylotrophicum

The catabolism of sodium formate to acetate and carbon dioxide by the anaerobic acetogen Butyribacterium methylotrophicum was analyzed by fermentation time course and 13C nuclear magnetic resonance studies. Significant hydrogen production and consumption fluxes were observed during formate catabolism but not during the catabolism of formate plus CO. In the latter case, formate and CO were simultaneously consumed and label distribution studies with mixtures of 13C-labeled CO and formate demonstrated their preferential incorporation into the acetate carboxyl and methyl groups, respectively. Hydrogen consumption was inhibited by CO when both were present, whereas hydrogen and formate were simultaneously consumed when CO2 was supplied. Carbon dioxide was required for the conversion of CO to acetate, but a similar need was not observed when methanol plus CO or formate plus CO was present. These analyses indicate a bifurcated single-carbon catabolic pathway in which CO2 is the sole single-carbon compound that directly supplies the carbonyl and methyl group synthesis pathways leading to the formation of acetyl coenzyme A, the primary reduced product. We discuss causes for the reported inability of B. methylotrophicum to use formate as a sole substrate.

Source of carbon and hydrogen in methane produced from formate by Methanococcus thermolithotrophicus

Methanococcus thermolithotrophicus is able to produce methane either from H2-CO2 or from formate. The route of formate entry into the methanogenic pathway was investigated by using 2H2O or [13C]formate and analysis by mass spectrometry. When cells (H2-CO2 or formate grown) were transferred to formate medium in 95% 2H water, the proportion of 2H in methane was 95%. When cells (H2-CO2 or formate grown) were transferred to media containing [13C]formate in the presence of H2-CO2 or He-CO2, the ratio of 13CH4 to 12CH4 increased over time parallel to the ratio of 13CO2 to 12CO2. The cells catalyzed a significant exchange of label between [13C]formate and 13CO2.

Methanosarcina mutant unable to produce methane or assimilate carbon from acetate

Mutants of Methanosarcina barkeri 227 resistant to monofluoroacetate were isolated from monofluoroacetate-treated cultures. Mutant strain FAr9 was 100 times more resistant to monofluoroacetate than the wild-type strain and was deficient in carbon uptake and CH4 and CO2 production from methyl-labeled acetate. Methanol was assimilated at increased levels. Strain FAr9 was unable to shift from using methanol to using acetate for growth and exhibited increased sensitivity to growth inhibition by NaCN in methanol-containing complex medium. Unlike parent strain 227, acetate addition to methanol-containing media did not prevent NaCN inhibition. The specific activities of enzymes of exogenous acetate assimilation, CO dehydrogenase, and enzymes of the tricarboxylic acid cycle were similar for mutant and parent strain cell extracts. Mutation to monofluoroacetate resistance did not confer simultaneous resistance to 2-bromoethanesulfonate or pyruvate or alter propionate uptake. We conclude that strain FAr9 is either an acetate permeability mutant or is defective in an activation step required for the catabolism and anabolism of acetate.

Acetate catabolism by Methanosarcina barkeri: evidence for involvement of carbon monoxide dehydrogenase, methyl coenzyme M, and methylreductase

The pathway of acetate catabolism in Methanosarcina barkeri strain MS was studied by using a recently developed assay for methanogenesis from acetate by soluble enzymes in cell extracts. Extracts incubated with [2-14C]acetate, hydrogen, and ATP formed 14CH4 and [14C]methyl coenzyme M as products. The apparent Km for acetate conversion to methane was 5 mM. In the presence of excess acetate, both the rate and duration of methane production was dependent on ATP. Acetyl phosphate replaced the cell extract methanogenic requirement for both acetate and ATP (the Km for ATP was 2 mM). Low concentrations of bromoethanesulfonic acid and cyanide, inhibitors of methylreductase and carbon monoxide dehydrogenase, respectively, greatly reduced the rate of methanogenesis. Precipitation of CO dehydrogenase in cell extracts by antibodies raised to 95% purified enzyme inhibited both CO dehydrogenase and acetate-to-methane conversion activity. The data are consistent with a model of acetate catabolism in which methylreductase, methyl coenzyme M, CO dehydrogenase, and acetate-activating enzymes are components. These results are discussed in relation to acetate uptake and rate-limiting transformation mechanisms in methane formation.