Effect of quaternary ammonium salts on the hydrolysis of N-glutaryl-l-phenylalanine catalysed by α-chymotrypsin

The hydrolysis of N-glutaryl-l-phenylalanine p-nitroanilide catalysed by α-chymotrypsin (α-CT) was studied in the presence of the following quaternary ammonium salts: tetrapentyl ammonium bromide (TPeABr), tetrabutyl ammonium bromide (TBABr), tetrapropyl ammonium bromide (TPABr), tetraethyl ammonium bromide (TEABr) and tetramethyl ammonium bromide (TMABr). The activity of the enzyme is strongly affected by the salts that act as activators. Superactivity has been detected in the presence of TPeABr, TBABr, TPABr and TEABr. The enzyme activity seems to depend on the molecular structure of the salts; the higher the molecular weight of the alkyl residues in the cationic ammonium group, the higher the superactivity. In the whole investigated range, the enzyme hydrolysis rate resulted to be a monotonic increasing function of the salt concentration. The model of a non-essential activator was adopted to describe the effect of the salts on the hydrolysis activity and good agreement was found between the experimental results and the model predictions. The dependence of the enzyme activity on the substrate concentration was also studied to further verify the applicability of the model. Finally, a preliminary study about the effect of these additives on α-chymotrypsin thermal stability was performed.