Acetate assimilation and the synthesis of alanine, aspartate and glutamate inMethanobacterium thermoautotrophicum

Abstract

Cultures of the autotrophic bacterium Methanobacterium thermoautotrophicum were shown to assimilate acetate when grown on CO2 and H2 in the presence of acetate. At 1 mM acetate 10% of the cell carbon came from acetate, the rest from CO2. At higher concentrations the percentage increased to reach a maximum of 65% at acetate concentrations higher than 20 mM. The data suggest that acetate may be an important carbon source under physiological conditions. The incorporation of acetate into alanine, aspartate and glutamate was studied in more detail. The cells were grown on CO2 and H2 in the presence of 1 mM U-14C-acetate. The three amino acids were isolated from the labelled cells by a simplified procedure. Alanine, aspartate and glutamate were found to have the same specific radioactivity. Degradation studies showed that C1 of alanine, C1 and C4 of aspartate, and C1 and C5 of glutamate were exclusively derived from CO2, whereas C2 and C3 of alanine and aspartate, and C3 and C4 of glutamate were partially derived from acetate. These findings and the presence of pyruvate synthase, phosphoenolpyruvate carboxylase and alpha-ketoglutarate synthase in M. thermoautotrophicum indicate that CO2 is assimilated into the three amino acids via acetyl CoA carboxylation to pyruvate, phosphoenolpyruvate carboxylation to oxaloacetate, and succinyl CoA carboxylation to alpha-ketoglutarate.

AMP-forming acetyl-CoA synthetases in Archaea show unexpected diversity in substrate utilization

“…Acetyl-CoA synthetase activity was first detected in archaea in Methanothermobacter marburgensis (formerly Methanobacterium thermoautotrophicum Marburg) (Oberlies et al 1980), a thermophilic chemolithoautotrophic methanoarchaeon that can utilize H 2 /CO 2 as the sole carbon and energy source (Zeikus and Wolfe 1972). When M. marburgensis (closely related to M. thermautotrophicus) was grown on H 2 /CO 2 in the presence of acetate, 10% of cellular carbon was derived from acetate with the remainder derived from CO 2 (Fuchs et al 1978). Oberlies et al (1980) subsequently demonstrated ACS activity in M. marburgensis cells grown with limiting H 2 /CO 2 and proposed that ACS allows assimilation of acetate as a cellular carbon source in order to spare limited supplies of CO 2 .…”

Autotrophic Carbon Dioxide Assimilation in Thermoproteales Revisited

“…The pH was adjusted to pH 5.3 with 1 M NaOH. Precipitated brown material was removed by centrifugation (21,22).…”

The Phosphoenolpyruvate Carboxylase from Methanothermobacter thermautotrophicus Has a Novel Structure

“…PPCs probably evolved from PpcA through a process that added allosteric sites to the enzyme. The reverse is also equally possible.The synthesis of oxaloacetate (OAA) is a major and essential CO 2 -fixation reaction in the methanarchaea (10,11,15,16,50,52,58,60). These organisms possess an incomplete tricarboxylic acid (TCA) cycle which is used to generate intermediates (OAA and ␣-ketoglutarate [␣-KG]) and a carrier (succinate) for the biosynthesis of amino acids and tetrapyrroles (10,11,15,16,50,52,58,60).…”

“…These organisms possess an incomplete tricarboxylic acid (TCA) cycle which is used to generate intermediates (OAA and ␣-ketoglutarate [␣-KG]) and a carrier (succinate) for the biosynthesis of amino acids and tetrapyrroles (10,11,15,16,50,52,58,60). The organisms belonging to the orders of Methanococcales, Methanobacteriales, and Methanomicrobiales, which primarily use hydrogen as an energy source (2), employ a reductive sequence starting at OAA and terminating at ␣-KG (10,11,15,16,50,52,60).Methanosarcina species, which predominantly depend on acetotrophic or methylotrophic methanogenesis for energy generation (2), use an oxidative branch of the TCA cycle that initiates with OAA and acetate and terminates at ␣-KG (52, 58). Hence, OAA synthesis is a central anabolic process in methanarchaea.…”