Hyperactivation of α-chymotrypsin by the Hofmeister effect

The enzyme activity of α-chymotrypsin (ChT) has been known to increase with the addition of polyelectrolytes and amine compounds that combine with oppositely charged substrates. Enzyme hyperactivation is thought to occur by changing the electrostatic field around the enzyme, leading to favorable interactions between the enzyme and substrate. Inspired by the enzyme hyperactivation system, this paper focuses on inorganic salts as additives that specifically affect the enzyme activity of ChT toward the small, hydrophobic substrate phenylalanine-p-nitroanilide. The enzyme activity of ChT increased linearly with increasing concentration of kosmotropes; k_cat/K_M of ChT in the presence of 1.5 M sodium sulfate was 18-fold higher than in the absence of salts. In contrast, the enzyme activity of ChT was decreased by sodium perchlorate and sodium thiocyanate due to denaturation. Enzyme kinetic analysis showed that the increased activity of ChT caused by sodium sulfate results from both increasing k_cat and decreasing K_M. Kosmotropes enhanced both the structural stability of the native state and hydrophobic interactions between the enzyme and substrate. This simple yet effective enzyme activation mechanism provides biotechnological applications as well as new insight into the enzymology of ChT.