In this paper a general thermodynamic analysis of the Gibbs energy change associated with the two-state denaturation process of small globular proteins is presented. The proposed "parabolic approximation" to the Gibbs energy change, apart from an analytical relationship for calculating the hot and cold denaturation temperatures, emphasizes the limiting thermodynamic mechanisms that a globular protein can exploit to increase its thermostability. These mechanisms are critically discussed, by stressing the fact that, actually, they are mutually dependent, due to the strong temperature-dependence of denaturation enthalpy and entropy changes.