Thermodynamics of denaturation of barstar: evidence for cold denaturation and evaluation of the interaction with guanidine hydrochloride

Abstract

Isothermal guandine hydrochloride (GdnHCL)-included denaturation curves obtained at 14 different temperatures in the range 273-323 K have been used in conjunction with thermally-induced denaturation curves obtained in the presence of 15 different concentrations of GdnHCL to characterize the thermodynamics of cold and heat denaturation of barstar. The linear free energy model has been used to determine the excess changes in free energy, enthalpy, entropy, and heat capacity that occur on denaturation. The stability of barstar in water decreases as the temperature is decreased from 300 to 273 K. This decrease in stability is not accompanied by a change in structure as monitored by measurement of the mean residue ellipticities at both 222 and 275 nm. When GdnHCL is present at concentrations between 1.2 and 2.0 M, the decrease in stability with decrease in temperature is however so large that the protein undergoes cold denaturation. The structural transition accompanying the cold denaturation process has been monitored by measuring the mean residue ellipticity at 222 nm. The temperature dependence of the change in free energy, obtained in the presence of 10 different concentrations of GdnHCL in the range 0.2-2.0 M, shows a decrease in stability with a decrease as well as an increase in temperature from 300 K. Values of the thermodynamic parameters governing the cold and the heat denaturation of barstar have been obtained with high precision by analysis of these bell-shaped stability curves. The change in heat capacity accompanying the unfolding reaction. ΔC_p, has a value of 1460±70 cal mol⁻¹ K⁻¹ in water. The dependencies of the changes in enthalpy, entropy, free energy, and heat capacity on GdnHCL concentration have been analyzed on the basis of the linear free energy model. The changes in enthalpy (ΔH₁) and entropy (ΔS₁), which occur on preferential binding of GdnHCL to the unfolded state, vis-a-vis the folded state, both have a negative value at low temperatures. With an increase in temperature ΔH₁ makes a less favorable contribution, while ΔS₁ makes a more favorable contribution to the change in free energy (ΔG₁) due to this interaction. The change in heat capacity (ΔC_pi) that occurs on preferential interaction of GdnHCL with the unfolded form has a value of only 53±36 cal mol⁻¹K⁻¹M⁻¹. The data validate the linear free energy model that is commonly used to analyze protein stability.