Transition-State Analysis of a V_max Mutant of AMP Nucleosidase by the Application of Heavy-Atom Kinetic Isotope Effects

The transition state of the V_max mutant of AMP nucleosidase from Azotobacter υinelandii [Leung, H. B., & Schramm, V. L. (1981) J. Biol. Chem. 256, 12823−12829] has been characterized by heavy-atom kinetic isotope effects in the presence and absence of MgATP, the allosteric activator. The enzyme catalyzes hydrolysis of the N-glycosidic bond of AMP at approximately 2% of the rate of the normal enzyme with only minor changes in the K_m for substrate, the activation constant for MgATP, and the K_i for formycin 5′-phosphate, a tight-binding competitive inhibitor. Isotope effects were measured as a function of the allosteric activator concentration that increases the turnover number of the enzyme from 0.006 s⁻¹ to 1.2 s⁻¹. The kinetic isotope effects were measured with the substrates [l′-³H]AMP, [2′-²H]AMP, [2′-²H]AMP, [9-¹⁵N]AMP, and [l′,9-¹⁴C, ¹⁵N]AMP. All substrates gave significant kinetic isotope effects in a pattern that establishes that the reaction expresses intrinsic kinetic isotope effects in the presence or absence of MgATP. The kinetic isotope effect with [9-¹⁵N]AMP decreased from 1.034 ± 0.002 to 1.021 ± 0.002 in response to MgATP. The [l′-³H]AMP isotope effect increased from 1.086 ± 0.003 to 1.094 ± 0.002, while the kinetic isotope effect for [l′,9-¹⁴C, ¹⁵N]AMP decreased from 1.085 ± 0.003 to 1.070 ± 0.004 in response to allosteric activation with MgATP. Kinetic isotope effects with [l′-¹⁴C]AMP and [2′-²H]AMP were 1.041 ± 0.006 and 1.089 ± 0.002 and were not changed by addition of MgATP. Transition-state analysis using bond-energy and bond-order vibrational analysis indicated that the transition state for the mutant enzyme has a similar position in the reaction coordinate compared to that for the normal enzyme. The mutant enzyme is less effective in stabilizing the carbocation-like intermediate and in the ability to protonate N7 of adenine to create a better leaving group. This altered transition-state structure was confirmed by an altered substrate specificity for the mutant protein.