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Activity based metabolomic profiling (ABMP) allows unbiased discovery of enzymatic activities encoded by genes of unknown function, and applies liquid-chromatography mass spectrometry (LC-MS) to analyze the impact of a recombinant enzyme on the homologous cellular extract as a physiologic library of potential substrates and products. The Mycobacterium tuberculosis protein Rv1248c was incompletely characterized as a thiamine diphosphate-dependent α-ketoglutarate decarboxylase. Here, recombinant Rv1248c catalyzed consumption of α-ketoglutarate in a mycobacterial small molecule extract with matched production of 5-hydroxylevulinate (HLA) in a reaction predicted to require glyoxylate. As confirmed using pure substrates by LC-MS, ¹H-NMR, chemical trapping, and intracellular metabolite profiling, Rv1248c catalyzes C-C bond formation between the activated aldehyde of α–ketoglutarate and the …