Protein-water interactions.

By recognizing the forces that drive water transport across cell membranes or across tissues, we can see how water is driven to and from proteins. We learn from examples. When a dissolved protein (bovine serum albumin) accumulates water relative to small solutes, it effectively withdraws a number of water molecules from the bath; the number of water molecules changes with the identity but not with the concentration of small solutes. When a large ionic channel (VDAC or alamethicin) opens, it withdraws water from its bathing solution; excluded solute stabilizes the closed state in proportion to activity of water, the osmotic stress created by the solute, rather than in proportion to the activity of the solute itself. Hemoglobin too acts like an osmometer whose loading of oxygen shifts with the chemical potential of water. Assemblies of many macromolecules (proteins, nucleic acids, polysaccharides, lipids), subjected to the osmotic stress of completely excluded solutes, fight dehydration with powerful, exponentially varying intermolecular forces. Should we speak of these sensitivities and responses as solute effects or water effects? Intuitive but rigorous thermodynamics, developed in a set of appendixes, provides a surprisingly practical guide to alternatives in language.