Effects of naturally occurring osmolytes on protein stability and solubility: issues important in protein crystallization

Protein solubility and stability are issues of consideration in attempts to crystallize proteins. These two properties of proteins are also at issue in the cells of organisms that have adapted to water stress conditions that could ordinarily denature or inactivate some proteins. Most organisms that have adapted to environmental stresses have done so by production and accumulation of certain small organic molecules, known as osmolytes, that arose by natural selection and have the ability to stabilize intracellular proteins against the environmental stress. Here, concepts developed to understand the special properties of the naturally occurring osmolytes in effecting protein stability and solubility, and the principles that have come from studies of these compounds have been presented. Along with excluded volume and preferential interaction parameters, identification of the osmophobic effect and the attenuation of this effect by favorable interactions of solute with side-chains appear to contribute to the full set of effects protecting osmolytes have on protein stability and solubility. With these concepts in mind and the fact that urea interacts favorably with the peptide backbone we note that: (1) osmolyte-induced effects on protein stability ranging from denaturation to forcing proteins to fold can be achieved experimentally and the underlying principles understood at near molecular-level detail, and (2) osmolyte-mediated solubility effects ranging from protein precipitation to protein solubilization are predictable based on these principles. These effects are contrasted and compared with effects of 2-methyl-2,4-pentanediol and polyethylene glycol on proteins, and how the principles found for the naturally occurring osmolytes can be applied to these two commonly used protein crystallizing agents.