Evidence against a step-wise mechanism for the fumarase-catalysed dehydration of (2S)-malate.

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