Studies on Turbatrix aceti beta-N-acetylglucosaminidase. 2. Kinetic studies on the active site

Abstract

The purified beta-N-acetylglucosaminidase isolated from Turbatrix aceti hydrolyzes both p-nitrophenyl 2-acetamido-2-deoxy-beta-D-gluco- and beta-D-galactopyranosides. The enzyme had Km values of 0.28 and 0.23 mM, Vmax values of 104 and 69 mumol min-1 mg protein-1, and activation energies of 11.7 and 9.9 kcal/mol for the two substrates, respectively. Several lines of experimental evidence show that both beta-N-acetylglucosaminidase and beta-N-acetylgalactosaminidase activities reside in the same molecule at a single catalytic site. Substrate analogs were synthesized in which the acetamido group of p-nitrophenyl 2-acetamido-2-deoxy-beta-D-gluco- and galactopyranoside, and their 1-thio analogs was modified by replacement of the amido-carbonyl oxygen with sulfur. These substrate analogs competitively inhibited both enzymatic activities. Analysis of the inhibition data indicates that a single catalytic site of the enzyme is responsible for both beta-N-acetylglucosaminidase and beta-N-acetylgalactosaminidase activities. Competition kinetics between the two substrates further confirm the presence of a single active site for both activities. The pH dependence of the hydrolysis of p-nitrophenyl 2-acetamido-2-deoxy-beta-D-gluco- and beta-D-galactopyranosides has been determined. pKe1 and pKe2 values of 4.7 and 5.2, determined from the dependence of log Vmax/Km on pH, suggest that two carboxyl groups are involved in the reaction mechanism. The heats of ionization of the groups further confirm the above results.