In vivo nuclear magnetic resonance studies of the metabolism of methanol and pyruvate byMethanosarcina barkeri

Growth of Methanosarcina barkeri (Fusaro) under nonmethanogenic conditions by the fermentation of pyruvate to acetate: ATP synthesis via the mechanism of substrate level phosphorylation

A mutant of Methanosarcina barkeri (Fusaro) is able to grow on pyruvate as the sole carbon and energy source. During growth, pyruvate is converted to CH4 and CO2, and about 1.5 mol of ATP per mol of CH4 is formed (A.-K. Bock, A. Prieger-Kraft, and P. Schönheit, Arch. Microbiol. 161:33-46, 1994). The pyruvate-utilizing mutant of M. barkeri could also grow on pyruvate when methanogenesis was completely inhibited by bromoethanesulfonate (BES). The mutant grew on pyruvate (80 mM) in the presence of 2 mM BES with a doubling time of about 30 h up to cell densities of about 400 mg (dry weight) of cells per liter. During growth on pyruvate, the major fermentation products were acetate and CO2 (about 0.9 mol each per mol of pyruvate). Small amounts of acetoin, acetolactate, alanine, leucine, isoleucine, and valine were also detected. CH4 was not formed. The molar growth yield (Yacetate) was about 9 g of cells (dry weight) per mol of acetate, indicating an ATP yield of about 1 mol/mol of acetate formed. Growth on pyruvate in the presence of BES was limited; after six to eight generations, the doubling times increased and the final cell densities decreased. After 9 to 11 generations, growth stopped completely. In the presence of BES, suspensions of pyruvate-grown cells fermented pyruvate to acetate, CO2, and H2. CH4 was not formed. Conversion of pyruvate to acetate, in the complete absence of methanogenesis, was coupled to ATP synthesis. Dicyclohexylcarbodiimide, an inhibitor of H(+)-translocating ATP synthase, did not inhibit ATP formation. In the presence of dicyclohexylcarbodiimide, stoichiometries of up to 0.9 mol of ATP per mol of acetate were observed. The uncoupler arsenate completely inhibited ATP synthesis, while the rates of acetate, CO2, and H2 formation were stimulated up to fourfold. Cell extracts of M. barkeri grown on pyruvate under nonmethenogenic conditions contained pyruvate: ferredoxin oxidoreductase (0.5 U/mg), phosphate acetyltransferase (12 U/mg), and acetate kinase (12 U/mg). From these data it is concluded that ATP was synthesized by substrate level phosphorylation during growth of the M. barkeri mutant on pyruvate in the absence of methanogenesis. This is the first report of growth of a methanogen under nonmethanogenic conditions at the expense of a fermentative energy metabolism.

13 C Nuclear Magnetic Resonance Studies of Citrate and Glucose Cometabolism by Lactococcus lactis

13 C nuclear magnetic resonance ( 13 C-NMR) was used to investigate the metabolism of citrate plus glucose and pyruvate plus glucose by nongrowing cells of Lactococcus lactis subsp. lactis 19B under anaerobic conditions. The metabolism of citrate plus glucose during growth was also monitored directly by in vivo NMR. Although pyruvate is a common intermediate metabolite in the metabolic pathways of both citrate and glucose, the origin of the carbon atoms in the fermentation products was determined by using selectively labeled substrates, e.g., [2,4- 13 C]citrate, [3- 13 C]pyruvate, and [2- 13 C]glucose. The presence of an additional substrate caused a considerable stimulation in the rates of substrate utilization, and the pattern of end products was changed. Acetate plus acetoin and butanediol represented more than 80% (molar basis) of the end products of the metabolism of citrate (or pyruvate) alone, but when glucose was also added, 80% of the citrate (or pyruvate) was converted to lactate. This result can be explained by the activation of lactate dehydrogenase by fructose 1,6-bisphosphate, an intermediate in glucose metabolism. The effect of different concentrations of glucose on the metabolism of citrate by dilute cell suspensions was also probed by using analytical methods other than NMR. Pyruvate dehydrogenase (but not pyruvate formate-lyase) was active in the conversion of pyruvate to acetyl coenzyme A. α-Acetolactate was detected as an intermediate metabolite of citrate or pyruvate metabolism, and the labeling pattern of the end products agrees with the α-acetolactate pathway. It was demonstrated that the contribution of the acetyl coenzyme A pathway for the synthesis of diacetyl, should it exist, is lower than 10%. Evidence for the presence of internal carbon reserves in L. lactis is presented.