Kinetics of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by α-chymotrypsin in aqueous solutions of alkyltrimethylammonium bromides

The rate of N-glutaryl-L-phenylalanine p-nitroanilide hydrolysis catalyzed by a-chymotripsin has been measured in aqueous solutions of cetyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and dodecyltrimethylammonium bromide at concentrations below and above their critical micellar concentrations (CMC). For the three surfactants considered superactivity was observed, with maximum catalytic efficiencies taking place near the corresponding CMCs. The effect of the surfactants after the CMCs is mostly due to a decreased thermodynamic activity of the substrate due to its incorporation into the micelles. After addition of the surfactants, the Michaelis constant values (corrected to take into account the free substrate concentration) tend to decrease, passing through an ill defined minimum, afterwards reaching a constant value. The catalytic rate constants show the same profiles that the catalytic efficiency, being maxima near the surfactants CMCs. This maximum is more important for the surfactant having the shorter tail. This result is explained by considering that the hydrophobicity of the surfactant influences more the CMC than its association to the enzyme. (c) 2007 Elsevier Inc. All rights reserved.