Irreversible kinetics of β-N-acetyl-d-glucosaminidase from prawn (Penaeus vannamei) inactivated by mercuric ion

Cysteine residues in prawn (Penaeus vannamei) β-N-acetyl-d-glucosaminidase (NAGase, EC 3.2.1.52) have been modified by p-chloromercuribenzoate (PCMB). The results show that sulfhydryl group is essential for the activity of the enzyme. Inactivation kinetics of the enzyme by mercuric chloride (HgCl2) has been studied using the kinetic method of the substrate reaction during inactivation of enzyme previously described by Tsou. The kinetic results show that the inactivation of the enzyme is an irreversible reaction. The microscopic rate constants for the reaction of Hg2+ with free enzyme and with the enzyme–substrate complex are determined. Comparison of these rate constants indicates that the presence of substrate offers marked protection of this enzyme against inactivation by Hg2+. The above results suggest that the cysteine residue is essential for activity.