Studying salt effects on protein stability using ribonuclease t1 as a model system

Abstract

Salt ions affect protein stability in a variety of ways. In general, these effects have either been interpreted from a charge solvation/charge screening standpoint or they have been considered to be the result of ion-specific interactions with a particular protein. Recent theoretical work suggests that a major contribution to salt effects on proteins is through the interaction of salt ions that are located near the protein surface and their induced point image charges that are located in the low-dielectric protein cavity. These interactions form the basis of "salting-out" interactions. Salt ions induce an image charge of the same sign in the low dielectric protein medium. The interaction between the induced charge and its mirror charge is repulsive and consequently thermodynamically destabilizing. However, a folded protein that has a much smaller surface area will be less destabilized than the unfolded state. Consequently, the folded state will be stabilized relative to the unfolded state. This work analyzes salt effects in the model enzyme ribonuclease t1, and demonstrates that interactions between salt ions and their induced point charges provide a major contribution to the observed salt-induced increase in protein stability. This work also demonstrates that in the case of weakly-binding ions (ions with binding constants that are in the order of 50 M(-1) and less), salting-out effects should still be considered in order to provide a more realistic interpretation of ion binding. These results should therefore be considered when salt effects are used to analyze electrostatic contributions to protein structure or are used to study the thermodynamics of proteins associated with halophillic organisms.