Total synthesis of acetyl coenzyme a involved in autotrophic CO2 fixation inAcetobacterium woodii

Data showed that acetyl CoA is the central intermediate for biosynthesis and excluded the operation of the Calvin cycle in A. woodii.

Abstract

The Gram positive anaerobeAcetobacterium woodii is able to grow autotrophically with a mixture of H2 and CO2 as the energy and carbon source. The question, by which pathway CO2 is assimilated, was studied using long term isotope labeling. Autotrophically growing cultures produced acetate parallel to cell proliferation, and, when U-[14C]acetate was present as tracer, incorporated radioactivity into all cell fractions. The specific radioactivity and the label positions were determined for those representative cell compounds which biosynthetically originated directly from acetyl CoA (N-acetyl groups), pyruvate (alanine), oxaloacetate (aspartate), α-ketoglutarate (glutamate), and hexosephosphates (glucosamine). Per mol compound the same amount of labeled acetate was incorporated into N-acetyl groups, alanine (C-2, C-3), aspartate (C-2, C-3), and twice the amount into glutamate (C-2, C-3, C-4, C-5) and into glucosamine. Consequently, the unlabeled carbon atoms of the C3−C6 compounds must have been derived from CO2 by carboxylation subsequent to acetyl CoA synthesis. When 0.2 mM 2-[14C]pyruvate was added to autotrophically growing cultures, also a substantial amount of radioactivity was incorporated. Two important differences in comparison to the acetate experiment were observed: The N-acetyl groups were almost unlabeled and glutamate contained the same specific radioactivity as alanine or aspartate. These data showed that acetyl CoA is the central intermediate for biosynthesis and excluded the operation of the Calvin cycle inA. woodii. The results were consistent with the operation of a different autotrophic CO2 fixation pathway in which CO2 is converted into acetyl CoA by total synthesis via methyltetrahydrofolate; acetyl CoA is then further reductively carboxylated to pyruvate.