trans-3-Chloroacrylic acid dehalogenase (CaaD) converts trans-3-chloroacrylic acid to malonate semialdehyde by the addition of H2O to the C-2, C-3 double bond, followed by the loss of HCl from the C-3 position. Sequence similarity between CaaD, an (0)3 heterohexamer (molecular weight 47 547), and 4-oxalocrotonate tautomerase (4-OT), an (alpha)(6) homohexamer, distinguishes CaaD from those hydrolytic dehalogenases that form alkyl-enzyme intermediates. The recently solved X-ray structure of CaaD demonstrates that betaPro-1 (i.e., Pro-1 of the beta subunit), alphaArg-8, alphaArg-11, and alphaGlu-52 are at or near the active site, and the greater than or equal to10(3.4)-fold decreases in k(cat) on mutating these residues implicate them as mechanistically important. The effect of pH on k(cat)/K-m indicates a catalytic base with a pK(a) of 7.6 and an acid with a pK(a) of 9.2. NMR titration of N-15-labeled wild-type CaaD yielded pK(a) values of 9.3 and 11.1 for the N-terminal prolines, while the fully active but unstable alphaP1A mutant showed a pK(a) of 9.7 (for the betaPro-1), implicating betaPro-1 as the acid catalyst, which may protonate C-2 of the substrate. These results provide the first evidence for an amino-terminal proline, conserved in all known tautomerase superfamily members, functioning as a general acid, rather than as a general base as in 4-OT. Hence, a reasonable candidate for the general base in CaaD is the active site residue alphaGlu-52. CaaD has 10 arginine residues, six in the alpha-subunit (Arg-8, Arg-11, Arg-17, Arg-25, Arg-35, and Arg-43), and four in the beta-subunit (Arg-15, Arg- 21, Arg-55, and Arg-65). H-1-N-15-heteronuclear single quantum coherence (HSQC) spectra of CaaD showed seven to nine Arg-NepsilonH resonances (denoted R-A to R-I) depending on the protein concentration and pH. One of these signals (R-D) disappeared in the spectrum of the largely inactive alphaR11A mutant (deltaH = 7.11 ppm, deltaN = 89.5 ppm), and another one (R-G) disappeared in the spectrum of the inactive alphaR8A mutant (deltaH = 7.48 ppm, deltaN = 89.6 ppm), thereby assigning these resonances to alphaArg-11NepsilonH, and alphaArg-8NepsilonH, respectively. H-1-N-15-HSQC titration of the enzyme with the substrate analogue 3-chloro-2-butenoic acid (3-CBA), a competitive inhibitor (K-1(slope) = 0.35 +/- 0.06 mM), resulted in progressive downfield lope shifts of the alphaArg-8Nepsilon resonance yielding a K-D = 0.77 +/- 0.44 mM, comparable to the K-1(slope), suggestive of active site binding. Increasing the pH of free CaaD to 8.9 at 5 degreesC resulted in the disappearance of all nine Arg-NepsilonH resonances due to base-catalyzed NepsilonH exchange. Saturating the enzyme with 3-CBA (16 mM) induced the reappearance of two NepsilonH signals, those of alphaArg-8 and alphaArg-11, indicating that the binding of the substrate analogue 3-CBA selectively slows the NepsilonH exchange rates of these two arginine residues. The kinetic and NMR data thus indicate that betaPro-1 is the acid catalyst, alphaGlu-52 is a reasonable candidate for the general base, and alphaArg-8 and alphaArg-11 participate in substrate binding and in stabilizing the aci-carboxylate intermediate in a Michael addition mechanism.
