The Role of Halide Anions in the Modulation of Colloidal Chemical Properties of Lysozyme and Its Catalytic Function

The effect of F−, Cl−, Br−, and I− halide ions on the behavior of lysozyme in an aqueous−toluene system and the bacteriolytic activity of the enzyme has been studied. The addition of NaF causes lysozyme molecules to form a denser layer at the interfaces due to the stabilization of protein globules and the enhancement of hydrophobic interactions. As a result, the addition of NaF does not affect the enzymatic activity of lysozyme. A different effect has been found upon the addition of NaCl at the same molar concentration. It decreases both adsorption and catalytic activity of the enzyme. The probable reason is the efficient screening of electrostatic interactions, as well as the mutual compensation of hydrophobic attraction and steric repulsion. Chaotropic anions enhance the intermolecular attraction in the adsorption layer, but, at the same time, reduce the catalytic activity. In the case of a strong chaotrope (NaI), this is explained by the destabilization of the network of hydrogen bonds both in the solvent and in the structure of the protein itself, thus increasing the affinity of the protein for the hydrophobic surface; however, it negatively affects its enzymatic function. The key factor determining the catalytic parameters (vmax and Km) in the presence of cosmotropic anions and a weak chaotrope (NaBr) is the molar concentration of a salt: as it increases, the parameter values decrease. At the same time, the vmax/Km ratio for these anions remains almost independent of the concentration, while bromide anions appear to be most favorable for maintaining lysozyme activity among the studied salts.