A thermodynamic analysis of a family of small globular proteins: SH3 domains

The stability and folding thermodynamics of two SH3-domains, belonging to Fyn and Abl proteins, have been studied by scanning calorimetry and urea-induced unfolding. They undergo an essentially two-state unfolding with parameters similar to those of the previously studied alpha-spectrin SH3 domain. The correlations between the thermodynamic parameters (heat capacity increment, Delta C-p,C-U, the proportionality factor, m, and the Gibbs energy, Delta G(w)(298)) of unfolding and some integral structural parameters, such as polar and non-polar areas exposed upon domain denaturation, have been analyzed. The experimental data on dC(p,U) and the m-factor of the linear extrapolation model (LEM) obey the simple empirical correlations deduced elsewhere. The Gibbs energies calculated from the DSC data were compared with those found by fitting urea-unfolding curves to the LEM and the denaturant-binding model (DBM). The Delta G(w)(298) values found with DBM correlate better with the DSC data, while those obtained with LEM are systematically smaller. The systematic difference between the parameters calculated with LEM and DBM are explained by an inherent imperfection of the LEM. (C) 1999 Elsevier Science B.V. All rights reserved.