Experimental validation of a model for α-chymotrypsin activity in aqueous solutions of surfactant aggregates

The hydrolysis of N-glutaryl-l-phenylalanine p-nitroanilide and N-succinyl-l-phenylalanine p-nitroanilide catalysed by α-chymotrypsin was studied in the presence of cationic (cetyltrimethylammonium bromide and cetyltripropylammonium bromide) and non-ionic (t-octylphenoxypolyethoxyethanol and polyoxyethylene-9-lauryl ether) surfactants. The ratio of reaction rates in surfactant rich systems to those in pure buffer solutions was simulated by a previously developed model by Viparelli et al. [Biochem. J. 344 (1999) 765] based on the pseudo-phase approach introduced by Bru et al. [Biochem. J. 259 (1989) 355] for enzymatic reactions in reverse micelles. The system was depicted by three pseudo-phases: free water, bound water and surfactant core. The substrate and enzyme concentration in the pseudo-phase was related to total substrate and enzyme concentrations in the reaction medium. ctivity occurs only in the presence of cetyltripropylammonium bromide. Plots of reaction rate ratio versus surfactant concentration were bell-shaped. Model simulation indicated that this behaviour could be attributed to the equilibrium between the enzyme confined in the free water and that in the bound water pseudo-phase, and to partition of the substrate in the pseudo-phases. The two enzyme forms must have different catalytic behaviour. The overall reaction rate depends on the two enzymatic reactions. Reasonable agreement was found between experimental results and model predictions.