Quantum chemical study of leaving group activation in T. vivax nucleoside hydrolase

Abstract

General acid catalysis is a powerful and widely used strategy in enzymatic nucleophilic displacement reactions. However, in the nucleoside hydrolase of the parasite Trypanosoma vivax, crystallographic and mutagenesis studies failed to identify a general acid. The only groups in the vicinity of the leaving group that contribute to catalysis are (i) the indole side chain of Trp260, and (ii) the 5′-group of the substrate's ribose moiety. The x-ray structure of the slow Asp10Ala mutant of nucleoside hydrolase with the substrate inosine bound in the active site displays a faceto-face aromatic stacking interaction between Trp260 and the purine base of the substrate, as well as a peculiar C4′-endo ribose pucker that allows the 5′-OH group to accept an intramolecular hydrogen bond from the C8 of the purine. The first interaction (aromatic stacking) has been shown to raise the pKa of the leaving purine. Here, we present a DFT study showing that the 5′-OH group of ribose fulfills a similar role, rather than stabilizing the oxocarbenium-like transition state. © 2005 Wiley Periodicals, Inc.