In salmonella enterica, 2-methylcitrate blocks gluconeogenesis

Strains of Salmonella enterica serovar Typhimurium LT2 lacking a functional 2-methylcitric acid cycle (2-MCC) display increased sensitivity to propionate. Previous work from our group indicated that this sensitivity to propionate is in part due to the production of 2-methylcitrate (2-MC) by the Krebs cycle enzyme citrate synthase (GltA). Here we report in vivo and in vitro data which show that a target of the 2-MC isomer produced by GltA (2-[MC.sup.ltA]) is fructose-l,6-bisphosphatase (FBPase), a key enzyme in gluconeogenesis. Lack of growth due to inhibition of FBPase by 2-[MC.sup.GltA] was overcome by increasing the level of FBPase or by micromolar amounts of glucose in the medium. We isolated an fbp allele encoding a single amino acid substitution in FBPase (S123F), which allowed a strain lacking a functional 2-MCC to grow in the presence of propionate. We show that the 2-[MC.sup.gltA] and the 2-MC isomer synthesized by the 2-MC synthase (PrpC; 2-[MC.sup.PrpC]) are not equally toxic to the cell, with 2-[MC.supGltA] being significantly more toxic than 2-[MC.sup.PrpC]. This difference in 2-MC toxicity is likely due to the fact that as a si-citrate synthase, GltA may produce multiple isomers of 2-MC, which we propose are not substrates for the 2-MC dehydratase (PrpD) enzyme, accumulate inside the cell, and have deleterious effects on FBPase activity. Our findings may help explain human inborn errors in propionate metabolism. doi: 10.1128/JB.01301-09