Abstract
The pathway of autotrophic CO2 fixation in Methanobacterium thermoautotrophicum has been investigated by long term labelling of the organism with isotopic acetate and pyruvate while exponentially growing on H2 plus CO2. Maximally 2% of the cell carbon were derived from exogeneous tracer, 98% were synthesized from CO2. Since growth was obviously autotrophic the labelled compounds functioned as tracers of the cellular acetyl CoA and pyruvate pool during cell carbon synthesis from CO2.
M. thermoautotrophicum growing in presence of U-14C acetate incorporated 14C into cell compounds derived from acetyl CoA (N-acetyl groups) as well as into compounds derived from pyruvate (alanine), oxaloacetate (aspartate), α-ketoglutarate (glutamate), hexosephosphates (galactosamine), and pentosephosphates (ribose). The specific radioactities of N-acetylgroups and of the three amino acids were identical. The hexosamine exhibited a two times higher specific radioactivity, and the pentose a 1.6 times higher specific radioactivity than e.g. alanine.
M. thermoautotrophicum growing in presence of 3-14C pyruvate, however, did not incorporate 14C into cell compounds directly derived from acetyl CoA. Those compounds derived from pyruvate, dicarboxylic acids and hexosephosphates became labelled. The specific radioactivities of alanine, aspartate and glutamate were identical; the hexosamine had a specific radioactivity twice as high as e.g. alanine.
The finding that pyruvate was not incorporated into compounds derived from acetyl CoA, whereas acetate was incorporated into derivatives of acetyl CoA and pyruvate in a 1:1 ratio demonstrates that pyruvate is synthesized by reductive carboxylation of acetyl CoA. The data further provide evidence that in this autotrophic CO2 fixation pathway hexosephosphates and pentosephosphates are synthesized from CO2 via acetyl CoA and pyruvate.
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Fuchs, G., Stupperich, E. Acetyl CoA, a central intermediate of autotrophic CO2 fixation in Methanobacterium thermoautotrophicum . Arch. Microbiol. 127, 267–272 (1980). https://doi.org/10.1007/BF00427203
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DOI: https://doi.org/10.1007/BF00427203