The pathway of the oxidation of propionate to pyruvate in Escherichia coliinvolves five enzymes, only two of which, methylcitrate synthase and 2-methylisocitrate lyase, have been thoroughly characterized. Here we report that the isomerization of (2S,3S)-methylcitrate to (2R,3S)-2-methyl-isocitrate requires anovel enzyme,methylcitratedehydratase (PrpD), and the well-known enzyme, aconitase (AcnB), of the tricarboxylic acid cycle.