Effects of cryoprotectants on enzyme structure

The interaction between organic cosolvents and proteins is considered, especially from the point of view of effects on protein stability. It is concluded that each protein-cosolvent system constitutes a unique situation, making generalized predictions of expected effects difficult. Two classes of cosolvents are distinguished, based on the nature of their interactions with the protein surface. The thermodynamic instability to the system introduced by the presence of the cosolvent can be accommodated (i) by preferential exclusion of the cosolvent from the vicinity of the protein, (ii) by major structural changes of the protein, or (iii) by aggregation. Polyols tend to undergo preferential exclusion due to unfavorable interactions with nonpolar surface groups, whereas monohydric alcohols and other more hydrophobic cosolvents may undergo preferential exclusion due to adverse interactions with charged groups on the protein surface. Typical cosolvent effects on the structural and catalytic properties of enzymes are illustrated with data for ribonuclease and beta-lactamase with alcohol cosolvents. The relative hydrophobicity of the cosolvent is the major determinant of the effect of a cryosolvent on the enzyme stability and properties. Thus the position of the unfolding transition in cryosolvent will be decreased more by a more nonpolar cosolvent. Different cosolvents can have significantly different effects on the catalytic and structural properties of the same enzyme. Conversely the same cosolvent can have significantly different effects on similar proteins. The number and distribution of the nonpolar and charged groups on the protein's surface probably are the major determinants of the protein contribution to the solvent-protein interaction. The large temperature dependence of the rates of protein unfolding and refolding can be beneficially utilized in cryoprotectant studies of living cells.