C-13 NMR analysis of electrostatic interactions between NAD(+) and active site residues of UDP-galactose 4-epimerase: Implications for the activation induced by uridine nucleotidest

摘要

UDP-galactose 4-epimerase contains the coenzyme NAD(+) bound tightly at the active site. NAD(+) functions as the coenzyme for the interconversion of UDP-galactose and UDP-glucose by reversibly mediating their dehydrogenation to the common intermediate UDP-4-ketohexopyranoside. The epimerase structure and spectrophotometric data indicate that NAD(+) may engage in electrostatic interactions with amino acid side chains that may regulate the reactivity of NAD(+). In this work, we carried out NMR studies of [nicotinamide-4-C-13]NAD(+) bound to wild-type epimerase and epimerases mutated at amino acid residues in contact with NAD(+). The 4-C-13 NMR chemical shifts revealed the following: The 4-C-13 chemical shift in wild-type epimerase is 149.9 ppm; mutation of Ser 124 to Ala changes it slightly by 0.2 ppm to 150.1 ppm; mutation of Tyr 149 to Phe results in a downfield perturbation of 2.7 ppm to 152.6 ppm; and the simultaneous mutation of Ser 124 to Ala and Tyr 149 to Phe also causes a downfield perturbation of 2.8 ppm to 152.7 ppm. Mutation of Lys 1.53 to Met results in a C-13 chemical shift of 150.8 ppm, which is 0.9 ppm downfield from that of wild type and 1.8 ppm upfield from that of Y149F-epimerase. The 13C chemical shifts of nicotinamide C4 of NAD(+) in these epimerases are correlated with their respective reactivities with NaBH3CN. In addition, reactivity of NAD(+) in wild-type and S124A-epimerases displays pH dependence, with higher rates at lower pH where Tyr 149 in these two enzymes is protonated. The results support an electrostatic model in which repulsion between positively charged Lys 153 and N1 of the nicotinamide ring increases the reactivity of NAD(+), while the phenolate of Tyr 149 opposes the positive electrostatic field and attenuates the reactivity of NAD(+). Ser 124 has very little effect on the electron distribution within the nicotinamide ring or the reactivity of NAD(+). The effects of binding the substrate analogue P-1-uridyl-P-2-methyl diphosphate (Me-UDP) on the 4-13C chemical shifts are opposite to those induced by the mutations. MeUDP perturbs the 4-13C chemical shift 2.9 ppm downfield in the wild-type and S124A-epimerases but has little or no effect in the cases of Y149F- or K153M-epimerases. The results support the postulate that NAD+ activation induced by uridine nucleotides is brought about by a conformational change of epimerase that repositions Tyr 149 at an increased distance from nicotinamide N1 of NAD(+) while maintaining the electrostatic repulsion between Lys 153 and nicotinamide N1 of NAD(+).