Thermal and conformational stability of Ssh10b protein from archaeon Sulfolobus shibattae

Abstract

The secondary structure of the DNA binding protein Ssh10b is largely unaffected by change in temperature between 25 °C and 85 °C, indicating that the protein is highly thermostable. Here, we report the temperature-dependent equilibrium denaturation of Ssh10b in the presence of guanidine hydrochloride (GdnHCl). It was found that the transition midpoint values of the temperature (Tm), and changes of enthalpy (ΔHm) and entropy (ΔSm) of Ssh10b unfolding were linearly decreasing with increasing GdnHCl concentration. The true values of the thermodynamic parameters, Tm=402 K, ΔHm=590±40 kJ·mol−1 and ΔSm=1.4±0.15 kJ·T−1·mol−1, were obtained by linear extrapolation to 0 M GdnHCl. The value of the heat capacity change of Ssh10b unfolding, ΔCp=3.8±0.2 kJ·T−1·mol−1 (approx. 19 J T−1·mol residue−1), was obtained from the measured thermodynamic parameters. This is significantly smaller than that of the average value for mesophilic proteins (50 J·K−1·mol residue−1) or the value calculated from the Ssh10b structural data (64 J T−1·mol residue−1). A consequence of the small ΔCp is that the ΔG of Ssh10b is larger than that of mesophilic proteins, while the values of ΔH and T*ΔS are smaller. The small ΔCp of Ssh10b appears to result mainly from the presence of compactness in the denatured state.