Three arginine residues (Arg-11, Arg-39, Arg-61) are found at the active site of 4-oxalocrotonate tautomerase in the X-ray structure of the affinity-labeled enzyme Taylor, A. B., Czerwinski, R. M., Johnson, R. M., Jr., Whitman, C. P., and Hackert, M. L. (1998) Biochemistry 37, 14692−14700. The catalytic roles of these arginines were examined by mutagenesis, kinetic, and heteronuclear NMR studies. With a 1,6-dicarboxylate substrate (2-hydroxymuconate), the R61A mutation showed no kinetic effects, while the R11A mutation decreased k cat 88-fold and increased K m 8.6-fold, suggesting both binding and catalytic roles for Arg-11. With a 1-monocarboxylate substrate (2-hydroxy-2,4-pentadienoate), no kinetic effects of the R11A mutation were found, indicating that Arg-11 interacts with the 6-carboxylate of the substrate. The stereoselectivity of the R11A-catalyzed protonation at C-5 of the dicarboxylate substrate decreased, while the stereoselectivity of protonation at C-3 of the monocarboxylate substrate increased in comparison with wild-type 4-OT, indicating the importance of Arg-11 in properly orienting the dicarboxylate substrate by interacting with the charged 6-carboxylate group. With 2-hydroxymuconate, the R39A and R39Q mutations decreased k cat by 125- and 389-fold and increased K m by 1.5- and 2.6-fold, respectively, suggesting a largely catalytic role for Arg-39. The activity of the R11A/R39A double mutant was at least 104-fold lower than that of the wild-type enzyme, indicating approximate additivity of the effects of the two arginine mutants on k cat. For both R11A and R39Q, 2D 1H−15N HSQC and 3D 1H−15N NOESY−HSQC spectra showed chemical shift changes mainly near the mutated residues, indicating otherwise intact protein structures. The changes in the R39Q mutant were mainly in the β-hairpin from residues 50 to 57 which covers the active site. HSQC titration of R11A with the substrate analogue cis,cis-muconate yielded a K d of 22 mM, 37-fold greater than the K d found with wild-type 4-OT (0.6 mM). With the R39Q mutant, cis,cis-muconate showed negative cooperativity in active site binding with two K d values, 3.5 and 29 mM. This observation together with the low K m of 2-hydroxymuconate (0.47 mM) suggests that only the tight binding sites function catalytically in the R39Q mutant. The 15Nε resonances of all six Arg residues of 4-OT were assigned, and the assignments of Arg-11, -39, and -61 were confirmed by mutagenesis. The binding of cis,cis-muconate to wild-type 4-OT upshifts Arg-11 Nε (by 0.05 ppm) and downshifts Arg-39 Nε (by 1.19 ppm), indicating differing electronic delocalizations in the guanidinium groups. A mechanism is proposed in which Arg-11 interacts with the 6-carboxylate of the substrate to facilitate both substrate binding and catalysis and Arg-39 interacts with the 1-carboxylate and the 2-keto group of the substrate to promote carbonyl polarization and catalysis, while Pro-1 transfers protons from C-3 to C-5. This mechanism, together with the effects of mutations of catalytic residues on k cat, provides a quantitative explanation of the 107-fold catalytic power of 4-OT. Despite its presence in the active site in the crystal structure of the affinity-labeled enzyme, Arg-61 does not play a significant role in either substrate binding or catalysis.