Effects of mutations of the active site arginine residues in 4-oxalocrotonate tautomerase on the pKa values of active site residues and on the pH dependence of catalysis.

Abstract

The unusually low pK(a) value of the general base catalyst Pro-1 (pK(a) = 6.4) in 4-oxalocrotonate tautomerase (4-OT) has been ascribed to both a low dielectric constant at the active site and the proximity of the cationic residues Arg-11 and Arg-39 [Stivers, J. T., Abeygunawardana, C., Mildvan, A. S., Hajipour, G., and Whitman, C. P. (1996) Biochemistry 35, 814-823]. In addition, the pH-rate profiles in that study showed an unidentified protonated group essential for catalysis with a pK(a) of 9.0. To address these issues, the pK(a) values of the active site Pro-1 and lower limit pK(a) values of arginine residues were determined by direct (15)N NMR pH titrations. The pK(a) values of Pro-1 and of the essential acid group were determined independently from pH-rate profiles of the kinetic parameters of 4-OT in arginine mutants of 4-OT and compared with those of wild type. The chemical shifts of all of the Arg Nepsilon resonances in wild-type 4-OT and in the R11A and R39Q mutants were found to be independent of pH over the range 4.9-9.7, indicating that no arginine is responsible for the kinetically determined pK(a) of 9.0 for an acidic group in free 4-OT. With the R11A mutant, where k(cat)/K(m) was reduced by a factor of 10(2.9), the pK(a) of Pro-1 was not significantly altered from that of the wild-type enzyme (pK(a) = 6.4 +/- 0.2) as revealed by both direct (15)N NMR titration (pK(a) = 6.3 +/- 0.1) and the pH dependence of k(cat)/K(m) (pK(a) = 6.4 +/- 0.2). The pH-rate profiles of both k(cat)/K(m) and k(cat) for the reaction of the R11A mutant with the dicarboxylate substrate, 2-hydroxymuconate, showed humps, i.e., sharply defined maxima followed by nonzero plateaus. The humps disappeared in the reaction with the monocarboxylate substrate, 2-hydroxy-2,4-pentadienoate, indicating that, unlike the wild-type enzyme which reacts only with the dianionic form of the dicarboxylic substrate, the R11A mutant reacts with both the 6-COOH and 6-COO(-) forms, with the 6-COOH form being 12-fold more active. This reversal in the preferred ionization state of the 6-carboxyl group of the substrate that occurs upon mutation of Arg-11 to Ala provides strong evidence that Arg-11 interacts with the 6-carboxylate of the substrate. In the R39Q mutant, where k(cat)/K(m) was reduced by a factor of 10(3), the kinetically determined pK(a) value for Pro-1 was 4.6 +/- 0.2, while the ionization of Pro-1 showed negative cooperativity with an apparent pK(a) of 7.1 +/- 0.1 determined by 1D (15)N NMR. From the Hill coefficient of 0.54, it can be shown that the apparent pK(a) value of 7.1 could result most simply from the averaging of two limiting pK(a) values of 4.6 and 8.2. Mutation of Arg-39, by altering the structure of the beta-hairpin which covers the active site, could result in an increase in the solvent exposure of Pro-1, raising its upper limit pK(a) value to 8.2. In the R39A mutant, the kinetically determined pK(a) of Pro-1 was also low, 5.0 +/- 0.2, indicating that in both the R39Q and R39A mutants, only the sites with low pK(a) values were kinetically operative. With the fully active R61A mutant, the kinetically determined pK(a) of Pro-1 (pK(a) = 6.5 +/- 0.2) agreed with that of wild-type 4-OT. It is concluded that the unusually low pK(a) of Pro-1 shows little contribution from electrostatic effects of the nearby cationic Arg-11, Arg-39, and Arg-61 residues but results primarily from a site of low local dielectric constant.