Glycosylasparaginase Activity Requires the α-Carboxyl Group, but Not the α-Amino Group, on N4-(2-Acetamido- 2-deoxy-β-d-glucopyranosyl)-l-asparagine

Glycosylasparaginase catalyzes the hydrolysis of the N-glycosylic bond in N4-(2-acetamido-2-deoxy-β-d-glucopyranosyl)-l-asparagine in the catabolism of N-linked oligosaccharides. A deficiency, or absence, of enzyme activity gives rise to aspartylglycosaminuria, the most common disorder of glycoprotein metabolism. The enzyme catalyzes the hydrolysis of a variety of asparagine and aspartyl compounds containing a free α-carboxyl group and a free α-amino group; computational studies suggest that the α-amino group actively participates in the catalytic mechanism. In order to study the importance of the α-carboxyl group and the α-amino group on the natural substrate to the reaction catalyzed by the enzyme, 14 analogues of the natural substrate were studied where the structure of the aspartyl group of the substrate was changed. The incremental binding energy (ΔΔGb) for those analogues that were substrates was calculated. The results show that the α-amino group may be substituted with a group of comparable size, for the α-amino group contributes little, if any, to the transition state binding energy of the natural substrate. The α-amino group position acts as an “anchor” in the binding site for the substrate. On the other hand, the α-carboxyl group is necessary for enzyme activity; removal of the α-carboxyl group or changing it to an α-carboxamide group results in no hydrolysis reaction. Also, N-acetyl-d-glucosamine is not sufficient for binding to the active site for efficient hydrolysis by the enzyme. These results provide supporting evidence for a proposed intramolecular autoproteolytic activation reaction for the enzyme. However, the results raise a question as to an important role for the α-amino group in the catalytic mechanism as indicated in computational studies.