Figure 2. Labelling experiments can probe pathway(s) and the metabolic context of newly identified enzymatic activities.

(A) An enzymatic reaction is identified by activity-based metabolic profiling or another method. The structure is of Mycobacterium tuberculosis CitE ⁹⁹. (B) Possible outcome of a labelling experiment designed to probe the origin of the carbon backbone of succinate. In this case, as glutamate labelling does not generate labelled succinate, a classic GABA shunt starting from glutamate is ruled out. Labelling with dextrose would therefore indicate a mitochondrial role for this newly identified enzyme. (C) Labelling experiment designed to confirm that the enzymatic activity identified belongs to the pathway described. The specific inhibition of synthesis of compound B leads to no change in the pool size of metabolite C. However, it drastically diminishes its labelling, supporting the relationship between A and C.