Discovering new enzymes and metabolic pathways: Conversion of succinate topropionate by Escherichia coli

The Escherichia coli genome encodes seven paralogues of the crotonase (enoy l CoA hydratase) superfamily. Four of these have unknown or uncertain funct ions; their existence was unknown prior to the completion of the E. coli ge nome sequencing project. The gene encoding one of these, YgfG, is located i n a four-gene operon that encodes homologues of methylmalonyl CoA mutases ( Sbm) and acyl CoA transferases (YgfH) as well as a putative protein kinase (YgfD/ArgK). We have determined that YgfG is methylmalonyl CoA decarboxylas e, YgfH is propionyl CoA:succinate CoA transferase, and Sbm is methylmalony l CoA mutase. These reactions are sufficient to form a metabolic cycle by w hich E. coli can catalyze the decarboxylation of succinate to propionate, a lthough the metabolic context of this cycle is unknown. The identification of YgfG as methylmalonyl CoA decarboxylase expands the range of reactions c atalyzed by members of the crotonase superfamily.