Understanding Ion-Specific "Hofmeister" Effects in Enzyme Catalysis Through Using RNase A as a Paradigm Model.

Biophysical studies in the last two decades demonstrate that salts affect biomolecules in an ion-specific manner. Diverse biological processes such as protein folding, protein precipitation, protein coacervation and phase separation, and protein oligomerization, all show that this ion specificity directly relates to how individual ions interact with biomolecular surfaces. Interestingly, although ion-specific effects upon enzyme catalytic processes are well-known in the literature, a molecular level description of these effects is not yet available. This work addresses this need by investigating ion-specific effects upon the enzymatic activity and stability of RNase A. We have developed a robust framework to analyze and quantify ion-specific effects upon the RNase A catalyzed phosphate ring opening reaction of cCMP. Both the folding thermodynamics and the Michaelis-Menten kinetic parameters of this enzyme show ion-specificity. However, these effects are not necessarily directly related to each other. Ion-specific effects observed in protein folding reflects mostly how an individual ion interacts with the overall protein surface; while alternatively, ion-specific effects on enzyme activity indicate how a given ion interacts with the enzyme active site surface or alternatively, how ions interact with the substrate molecule as represented by changes in the substrate thermodynamic activity coefficient.