Elucidation of cosolvent effects thermostabilizing water-soluble and membrane proteins

Abstract Thermostability of a water-soluble protein becomes higher with an increase in the gain of translational, configurational entropy of the solvent upon protein folding at 298 K. The gain, which is significantly large when the solvent is water, can be made even larger by the cosolvent addition. In an earlier study, we proposed a measure evaluating the enhancement of thermostability achieved by the cosolvent addition. The larger the measure is, the stronger the enhancement is. The measure becomes larger as the total packing fraction of the solvent increases. In this study, we predict that the measure follows the order, sucrose > glucose ~ mannitol > erythritol > glycerol. On the other hand, the addition of 2-propanol, a monohydric alcohol, lowers the thermostability. These theoretical predictions, which are almost independent of the water-soluble protein species, coincide with the experimental observations. We then argue that they are applicable to membrane proteins for which not only the hydrocarbon groups in nonpolar chains of lipid molecules or in surfactant molecules of detergents but also water molecules play essential roles in protein folding. For membrane proteins solubilized in detergents, for example, closer packing of water and cosolvent molecules is also accompanied by that of the hydrocarbon groups, enlarging the entropy gain for the hydrocarbon groups upon protein folding and enhancing the thermostability. We experimentally examine the above argument for two representative membrane proteins. It is corroborated for both of them that the thermostability is actually enhanced by the sugar or polyol addition and the degree of enhancement follows the order described above, and it is lowered by the addition of 2-propanol. The cosolvent effects, especially those leading to enhanced thermostability, can be comprehended for both of water-soluble and membrane proteins within the same theoretical framework.