Electrostatic potential surfaces of the transition state for AMP deaminase and for (R)-coformycin, a transition state inhibitor

The transition state for the hydrolysis of AMP by AMP deaminase has been characterized by heavy atom kinetic isotope effects (Merkler, D. J., Kline, P. C., Weiss, P., and Schramm, V. L. (1993) Biochemistry 32, 12993-13001). The experimentally established transition state includes a bond order of 0.8 to the attacking water nucleophile, a full bond order to the exocyclic 6- amino group, rehybridization of C-6 of the purine ring to sp³ and protonation of N-1 by Glu⁶³³. The transition state is on the path to formation of an unstable tetrahedral intermediate in which the exocyclic amine undergoes rapid protonation followed by its departure. In this mechanism, the highest energetic barrier on the reaction coordinate is the attack of the zinc-activated water. In a further test of this transition state structure, the electrostatic potential surface for the purine ring of the transition state has been determined by molecular orbital calculations and compared to that of the base of (R)-coformycin 5'-monophosphate, a slow onset, tight binding inhibitor of AMP deaminase that binds with an overall dissociation constant of 10^-11 M. The electrostatic potential surfaces of the aglycones of the transition state and (R)-coformycin are compared to the adenine ring of the substrate and to an alternative transition state structure in which the transition state is late, with fully bonded hydroxyl and fully protonated exocyclic amine. The results indicate a near-match of the electrostatic potential surfaces for the early transition state and (R)- coformycin. The electrostatic nature of the late transition state with a protonated amine leaving group differs both from the transition state determined by kinetic isotope effects and from that of (R)-coformycin analogues. The results provide evidence that the nature of the enzyme- stabilized transition state for adenine deamination involves an early transition state with a partially bonded hydroxyl group. The observed tight binding inhibition by (R)-coformycin analogues as transition state inhibitors results from the similarity of the partial charges on the inhibitors to that of the enzymatic transition state stabilized by AMP deaminase.