Evidence against a Step-wise Mechanism for the Fumarase-Catalysed Dehydration of (2S)-Malate

Abstract
Fumarase [from pig heart] does not catalyze 18O exchange between (2S, 3R)-tartrate and solvent water, nor does it catalyze 2H or 3H exchange into (2S)-fluorosuccinate from isotopically labeled water. Both of these substrate analogs are good competitive inhibitors of fumarase. This lack of isotopic exchange provides prima facie evidence against the stepwise carbenium ion and carbanion mechanisms. The fumarase-catalysed dehydration of (2S)-malate apparently occurs by a concerted mechanism in which breaking of the C.sbd.OH bond is much further advanced than that of the C.sbd.H bond in the transition state.