Modulation of global stability, ligand binding and catalytic properties of trypsin by anions

Specific salts effect is well-known on stability and solubility of proteins, however, relatively limited knowledge is known regarding the effect on catalytic properties of enzymes. Here, we examined the effect of four sodium anions on thermal stability and catalytic properties of trypsin and binding of the fluorescent probe, p-aminobenzamidine (PAB), to the enzyme. We show that the specific anions effect on trypsin properties agrees with the localization of the anions in the Hofmeister series. Thermal stability of trypsin, Tm, the affinity of the fluorescent probe to the binding site, Kd, and the rate constant, kcat, of trypsin-catalyzed hydrolysis of the substrate N-benzoyl-L-arginine ethyl ester (BAEE) increase with increasing kosmotropic character of anions in the order: perchlorate<bromide<chloride<sulfate, while the value of Michaelis constant, KM, decreases. Correlations between the values of Tm, Kd for PAB, kcat, and KM for BAEE in the presence of 1M studied salts suggest interrelation among these parameters of the enzyme. Global stabilization as well as increased rigidity of trypsin is accompanied by strengthening of interaction with fluorescent probe PAB and in accordance with decreasing values of KM for the substrate BAEE.  Strong correlations between parameters characterizing the trypsin properties with the charge densities of anions clearly indicate direct electrostatic interaction as a basis of the specific anion effect on the conformational and functional properties of the enzyme.