Betalactoglobulin as a tool for dissecting the determinants of structural stabilization of proteins by polyols

INTRODUCTION Polyols have long been known to impact stability of proteins folding, but the exact mechanism of their stabilizing action remains much debated. Beta-lactoglobulin (BLG) is a relatively small protein with a well known denaturation behaviour. BLG is a major food allergen, and also has been proposed as a drug carrier. Denaturation of BLG by chemical and physical agents is known to occur through a number of steps, affecting separate regions of the protein. OBJECTIVES In this study, BLG was used to assess the effects of high concentrations of sucrose, sorbitol, glycerol, or trehalose on the unfolding of various structural regions in BLG. MATERIALS AND METHODS Stabilizing effects towards physical denaturation were assessed by near-UV circular dichroism, by evaluating the reactivity of residues that are hidden in the native state (such as Cys121), and by monitoring changes in surface hydrophobicity as detected by non-covalent binding of suitable fluorescent probes. Effects on the dissociation equilibrium of the BLG native dimer were assessed by estimating a diffusion coefficient through NMR spectroscopy. DISCUSSION AND RESULTS All polyols do not affect the functional properties of BLG, as the ligand binding ability of the protein is preserved even at the highest polyol concentrations. Different polyols had region-specific stabilizing effects, suggesting a different and specific mechanism of action for some of them. CONCLUSIONS The different nature and extent of the stabilizing effects reported here are discussed in terms of the preferential exclusion theory (polyols interacting only with the solvent and not with the protein), but also in terms of the effects of individual polyols on water activity, that was assessed by independent measurements.