Parallel Change with Temperature of Water Structure and Protein Behavior

Abstract

Dome-shaped temperature dependencies have been observed widely for the unfolding of proteins and for ligand binding by proteins. These curves have been rationalized in terms of hydrophobic interactions. Similar dependencies have been found for transfer equilibria of many nonpolar molecules to water, and these too have been ascribed to hydrophobic effects. Less well-recognized is the variation with temperature of the ionization constants of formic acid and of related small carboxylic acids, which are also dome-shaped; it is difficult to attribute such behavior in these molecules to hydrophobic interactions. What all of these equilibria do have in common is that they are taking place in water. In protein studies, little attention has been paid to the variation with temperature of the structure of pure liquid water. Vibrational spectroscopic studies of water upon warming above 4 °C and upon cooling the supercooled liquid have disclosed and tracked similar changes in the molecular structure with the rise or with the fall of temperature. Furthermore, the densities of liquid water from −31 to +60 °C fit strikingly on a dome-shaped curve, which is also a manifestation of structural changes. If one adopts a phenomenological formulation of unfolding transformations that couples the conformational rearrangement in a protein with the equilibria between closely-packed and more open forms of water, one can derive an equation for protein unfolding that incorporates the dome-shaped temperature behavior of the solvent water. The equation fits the experimental observations over the full range of temperatures from cold denaturation to warm denaturation and is concordant with a dome-shaped temperature dependence. This outcome appears without any consideration of the effect of temperature on the interactions of specific residues of the protein with solvent molecules.