Esterase activity of glycosylasparaginase

Glycosylasparaginase (GA) catalyzes the hydrolysis of the N ‐glycosylic bond in β‐ N ‐acetylglucosaminyl‐L‐asparagine to give N ‐acetylglucosamine, aspartic acid and ammonium ion in the catabolism of N‐linked glycoproteins. A deficiency or absence of activity of GA gives rise to the most common inherited disorder of glycoprotein metabolism, aspartylglycosaminuria. The mechanism for GA has been proposed to be analogous to that of serine proteases, involving an acylation reaction to an acyl‐enzyme intermediate, followed by a deacylation reaction. Studies of the mechanism thus far show that some properties of GA are similar to properties of serine proteases, although differences exist in some of the properties. Serine proteases have esterase activity where the rate‐limiting step in the hydrolysis of substituted phenyl esters is the deacylation step. The esterase activity of GA was investigated using O 4 ‐(parasubstituted phenyl)‐L‐aspartate esters synthesized for the first time. The release of the parasubstituted phenolate was measured upon incubation with GA from human amniotic fluid. The rates of hydrolysis of the esters were dependent on the electronic properties of the parasubstitution. The rate‐determining step in the hydrolysis of the esters by GA is the acylation step, opposite to that of serine proteases. The analogy of the mechanisms for serine proteases and GA is not absolute. The research was supported in part by funds provided by The University of North Carolina at Charlotte.