4-Oxalocrotonate tautomerase (4-OT) converts 2-oxo-4E-hexenedioate (1) to 2-oxo-3Ehexenedioate (3) through the dienol intermediate, 2-hydroxy-2,4-hexadiene-1,6-dioate (2). Previous studies established that the isomerization of 1 to 3 is primarily a suprafacial process. It was also suggested that the 6-carboxylate group of the substrate maintains the regio-and stereochemical fidelity of the reaction by anchoring the substrate at the active site. A subsequent study suggested an additional role for the 6-carboxylate group in the mechanism: the enzyme may utilize the binding energy of the carboxylate group to facilitate catalysis. In order to explore the role of the carboxylate group in the mechanism further, the nonenzymatic rate constants for mono-and dicarboxylated substrates were measured and compared to the rates obtained for the corresponding enzymatic reactions. The results show that the missing carboxylate group has a profound effect on enzymatic catalysis as evidenced by the significant decreases (a 10 4 -and a 10 5 -fold reduction) in the values of k cat /K m observed for the two monocarboxylated substrates. A comparison of the nonenzymatic rate constants indicates that the reduced k cat /K m values cannot be explained on the basis of the chemical reactivities. The stereochemical course of the 4-OT-catalyzed reaction was also determined using 2-hydroxy-2,4Z-heptadiene-1,7-dioate. The stereochemical analysis reveals that the presence of the carboxylate group improves the stereoselectivity of the enzyme-catalyzed ketonization of 2-hydroxy-2,4Z-heptadiene-1,7-dioate to 2-oxo-[3-2 H]-4Z-heptene-1,7-dioate in 2 H 2 O-a result that is consistent with its previously assigned role. These findings provide further evidence that the substrate's carboxylate group contributes to the mechanism of the enzyme in two ways: it anchors the substrate at the active site and it facilitates catalysis by destabilizing the substrate or by stabilizing the transition state.