Correlation of lysozyme activity and stability in the presence of Hofmeister series anions.

Enzymatic activity and stability of lysozyme in the presence of salts have been studied by fluorescence spectroscopy and differential scanning calorimetry, respectively. The effect of sodium salts of sulfate, acetate, chloride, bromide, thiocyanate, and perchlorate on lysozyme properties depends on anion concentration as well as on position of anion in the Hofmeister series. Kosmotropic anions (sulfate and acetate) increase stability and activate the enzyme while chaotropic anions (bromide, thiocyanate and perchlorate) including chloride decrease stability and inhibits the enzyme activity. Strong correlation between stability and activity of lysozyme suggest the interdependence of these enzyme properties in the presence of salts. The fact that the properties of lysozyme correlate with partition coefficients of anions at hydrocarbon surface clearly indicates that Hofmeister effect of anions is mediated by their interactions with nonpolar parts of the enzyme surface despite its high positive net charge at studied conditions. The efficiency of the anions in affecting both activity and stability of lysozyme also correlates with other anion-related parameters most notably with polarizability of monovalent anions. The presented work points to a critical role of interaction of anions with nonpolar protein surface for the Hofmeister effect. Moreover, the simultaneous investigation of protein stability and activity, in the relation with the Hofmeister effect, provides important information regarding stability/rigidity of enzyme structure for its catalytic activity.