Contribution of hydration to protein folding thermodynamics. II. The entropy and Gibbs energy of hydration.

The entropy of hydration of non-polar and polar groups upon protein unfolding has been estimated for four globular proteins in the temperature range 5 to 125 degrees C, using structural information on the groups of these proteins exposed to water in the native and unfolded states and the entropies of transfer of various model compounds from the gaseous phase to water. The latter was calculated using the molar scale. It is shown that the entropies of hydration of non-polar and polar groups are both negative, but change in different directions with increasing temperature: the entropy of hydration of polar groups increases in absolute magnitude, while the entropy of hydration of non-polar groups decreases and becomes zero at 122 degrees C for aliphatic groups and at 104 degrees C for aromatic groups, with a change in sign at higher temperature. The configurational entropy was estimated by comparing the entropy of overall hydration with the total entropy of protein unfolding. Using previously determined enthalpies of hydration of polar and non-polar groups and disruption of the internal bonds, the Gibbs energies of hydration of protein groups and breaking of the hydrogen bonds and van der Waals interactions have been estimated. It was shown that hydration of the polar groups and aromatic non-polar groups destabilizes the native protein structure, while hydration of aliphatic non-polar groups (hydrophobic hydration) stabilizes it; however, the main stabilization effect comes from the internal van der Waals interactions and hydrogen bonding. Analysis of the factors contributing to the stability of the folded protein conformation shows that it is the hydration of polar groups that is mainly responsible for the cold denaturation of proteins.