Linear free-energy model description of the conformational stability of uracil-DNA glycosylase inhibitor - A thermodynamic characterization of interaction with denaturant and cold denaturation

The equilibrium unfolding of uracil DNA glycosylase inhibitor (Ugi), a smal l acidic protein of molecular mass 9474 Da, has been studied by a combinati on of thermal-induced and guanidine hydrochloride (GdnCl)-induced denaturat ion. The analysis of the denaturation data provides a measure of the change s in conformational Free energy, enthalpy, entropy and heat capacity Delta C-p that accompany the equilibrium unfolding of Ugi over a wide range of te mperature and GdnCl concentration. The unfolding of Ugi is a simple two-sta te, reversible process. The protein undergoes both low-temperature and high -temperature unfolding even in the absence of GdnCl but more so in the pres ence of denaturant. The data are consistent with the linear free-energy mod el and with a temperature independent Delta C-p over the large temperature range of unfolding. The small Delta C-p (6.52 kJ.mol(-1).K-1) for the unfol ding of Ugi, is perhaps a reflection of a relatively small, buried hydropho bic core in the folded form of this small monomeric protein. Despite a rela tively low value of Delta G((H2O)), 7.40 kJ.mol(-1) at pH 8.3. Ugi displays considerable stability with the temperature of maximum stability being 301 .6 K.