Hexosaminidase - Isozymes and Genes - Lysosomal A, B, and S Isozymes - Mechanism of Action

Mechanism of Action

A Michaelis complex consisting of a glutamate residue, a GalNAc residue on the G_M2 ganglioside, and an aspartate residue leads to the formation of an oxazolinium ion intermediate. A glutamate residue (alpha Glu-323/beta Glu-355) works as an acid by donating its hydrogen to the glycosidic oxygen atom on the GalNAc residue. An aspartate residue (alpha Asp-322/beta Asp-354) positions the C2-acetamindo group so that it can be attacked by the nucleophile (N-acetamido oxygen atom on carbon 1 of the substrate). The aspartate residue stabilizes the positive charge on the nitrogen atom in the oxazolinium ion intermediate. Following the formation of the oxazolinium ion intermediate, water attacks the electrophillic acetal carbon. Glutamate acts as a base by deprotonating the water leading to the formation of the product complex and the G_M3ganglioside.

The mechanism of the hydrolysis of a G_M2 ganglioside and removal of a GalNAc residue to produce G_M3 ganglioside.