Computational evidence for the substrate-assisted catalytic mechanism of O-GlcNAcase. A DFT investigation

A DFT computational investigation of the catalytic mechanism of O-GlcNAcase shows the existence of a substrate-assisted reaction pathway similar to that proposed in the literature on the basis of experimental evidence: the carbonyl oxygen of the N-acyl group bonded at C2 of the substrate pyranose ring attacks the anomeric carbon affording a bicyclic oxazoline intermediate and causing the breaking of the glycosidic bond and the expulsion of the aglycon. This occurs in a single kinetic step where the transfer of a proton from Asp-243 (behaving as a general base) to the leaving group is simultaneous to the cycle formation and departure of the aglycon (an activation barrier E(a) of 16.5 kcal mol(-1)). Even if the other component of the catalytic dyad (Asp-242) is not actually involved in a proton transfer (as commonly suggested), it plays an important role in the catalysis through a complex network of hydrogen bonds that contribute to lower the activation barrier. The transition state of the process resembles an oxocarbenium ion (half chair conformation with an approximately planar sp(2) anomeric carbon). Following the lines of previous experiments aimed to demonstrate the existence of a substrate-assisted mechanism, it is found that the computed E(a) increases when the hydrogen atoms of the N-acetyl group are replaced with one, two and three F atoms and that a good linear correlation exists between the activation barrier E(a) and the σ* Taft electronic parameter of the fluoro-substituted N-acetyl groups.