Detection of acetyl coenzyme A as an early CO2 assimilation intermediate in Methanobacterium

The pivotal role of acetyl coenzyme A in CO2 assimilation by autotrophic methanogenic bacteria has been demonstrated by pulse-labelling of growing Methanobacterium thermoautotrophicum with 14CO2. After very short incubation with 14CO2 (∼1.5 s) approximately 1% of label incorporated into the soluble cell fraction was contained in acetyl coenzyme A. The percentage distribution of 14C within acetyl CoA markedly decreased with time, which is indicative for acetyl CoA being an immediate 14CO2 fixation product. Label in the acetate molecule first appeared in the carboxyl carbon, but the methyl carbon became equally labelled within only ∼10 s. The acetyl CoA was compared with authentic material by various criterions and its cellular concentration was determined to be 52 μM. This small cellular pool size of acetyl CoA as compared to e.g. alanine (6.4 mM) provides an explanation for the observed labelling kinetics. The data are fully consistent with autotrophic carbon assimilation via a total synthesis of acetyl coenzyme A from 2 CO2.