Salmonella typhimurium LT2 catabolizes propionate via the 2-methylcitric acid cycle

We previously identified the prpBCDE operon, which encodes catabolic functi ons required for propionate catabolism in Salmonella typhimurium, Results f rom C-13-labeling experiments have identified the route of propionate break down and determined the biochemical role of each Prp enzyme in this pathway . The identification of catabolites accumulating in wild-type and mutant st rains was consistent with propionate breakdown through the 2-methylcitric a cid cycle. Our experiments demonstrate that the alpha-carbon of propionate is oxidized to yield pyruvate. The reactions are catalyzed by propionyl coe nzyme A (propionyl-CoA) synthetase (PrpE), 2-methylcitrate synthase (PrpC), 2-methylcitrate dehydratase (probably PrpD), 2-methylisocitrate hydratase (probably PrpD), and 2-methylisocitrate lyase (PrpB), In support of this co nclusion, the PrpC enzyme was purified to homogeneity and shown to have 2-m ethylcitrate synthase activity in vitro. H-1 nuclear magnetic resonance spe ctroscopy and negative-ion electrospray ionization mass spectrometry identi fied 2-methylcitrate as the product of the PrpC reaction. Although PrpC cou ld use acetyl-CoA as a substrate to synthesize citrate, kinetic analysis de monstrated that propionyl-CoA is the preferred substrate.