Figure 9. Proposed model for succinate production and NADH reoxidation in Dubos medium under anaerobic condition in M. tuberculosis.

Under aerobic conditions, glucose is metabolized to acetyl-CoA by glycolysis and pyruvate dehydrogenase with further oxidation by the TCA cycle. Labeling of TCA cycle intermediates by feeding of cells with [1,4-¹³C₂]-aspartate results in loss of both ¹³C atoms (green highlighting) through oxidative decarboxylation in the TCA cycle. Reduced cofactors (red arrows) are re-oxidized by respiratory complexes. Under anaerobic conditions, the direct incorporation of C3 metabolites from glycolysis into the reductive branch of the TCA cycle is favored resulting in re-oxidation of reduced cofactors. Succinate is secreted into extracellular milieu to maintain a proton motive force. Under these conditions, labeling of the C4 branch of the TCA cycle using [1,4-¹³C₂]-aspartate results in secretion of succinate labeled at the C1 and C4 positions with ¹³C.