The conversion of dTDP-glucose into dTDP-4-keto-6-deoxyglucose by Escherichia coli dTDP-glucose 4,6-dehydratase (4,6-dehydratase) takes place in the active site in three steps: dehydrogenation to dTDP-4-ketoglucose, dehydration to dTDP-4-ketoglucose-5,6-ene, and rereduction of C6 to the methyl group. The 4,6-dehydratase makes use of tightly bound NAD(+) as the coenzyme for transiently oxidizing the substrate, activating it for the dehydration step. Dehydration may occur by either of two mechanisms, enolization of the dTDP-4-ketoglucose intermediate, followed by elimination [as proposed for beta-eliminations by Gerlt, J. A., and Gassman, P. G. (1992) J. Am. Chem. Soc. 114, 5928-5934], or a concerted 5,6-elimination of water from the intermediate. To assign one of these two mechanisms, a simultaneous kinetic characterization of glucosyl C5((1)H/(2)H) solvent hydrogen and C6((16)OH/(18)OH) solvent oxygen exchange was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The reaction of the wild-type enzyme is shown to proceed through a concerted dehydration mechanism. Interestingly, mutation of Asp135, the acid catalyst, to Asn or Ala alters the mechanism, allowing enolization to occur to varying extents. While aspartic acid 135 is the acid catalyst for dehydration in the wild-type enzyme, the differential enolization capabilities of D135N and D135A dehydratases suggest an additional role for this residue. We postulate that the switch from a concerted to stepwise dehydration mechanism observed in the aspartic acid variants is due to the loss of control over the glucosyl C5-C6 bond rotation in the active site.