The effect of low concentrations of alcohols on protein stability: a cold and heat denaturation study of yeast frataxin

Although alcohols are well known to be protein denaturants when present at high concentrations, their effect on proteins at low concentrations is much less well characterized. In this paper, we present a study of the effects of alcohols on protein stability using Yfh1, the yeast ortholog of the human protein frataxin. Exploiting the unusual property of this protein of undergoing cold denaturation around 0 °C without any ad hoc destabilization, we determined the stability curve on the basis of both high and low temperature unfolding in the presence of three commonly used alcohols: trifluoroethanol, ethanol and methanol. In all cases, we observed an extended temperature range of protein stability as determined by a modest increase of the high temperature of unfolding but an appreciable decrease in the low temperature of unfolding. Based on simple thermodynamics considerations, we are able to interpret the literature on the effects of alcohols on proteins and to generalise our findings. We suggest that alcohols, at low concentration and physiological pH, stabilize proteins by greatly widening the range of temperatures over which the protein is stable. Our results also clarify the molecular mechanism of the interaction and validate the current theoretical interpretation of the mechanism of cold denaturation.