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

Abstract

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.