Thermodynamics of unfolding of the α-amylase inhibitor tendamistat: Correlations between accessible surface area and heat capacity

Abstract

Unfolding of the small α-amylase inhibitor tendamistat (74 residues, 2 disulfide bridges) has been characterized thermodynamically by high sensitivity scanning microcalorimetry. To link the stability parameters with structural information we use heat capacity group parameters and water accessible surface areas to calculate the change in heat capacity on unfolding of tendamistat. Our results show that both the group parameter and surface area approaches provide a reasonable, though not perfect, basis for ΔC p calculations. When using the experimentally determined temperature-independent heat capacity increase of 2.89 kJ mol −1 K −1 tendamistat exhibits convergence of thermodynamic parameters at about 140 °C, in agreement with recent predictions of the temperature at which the hydrophobic hydration is supposed to disappear. Despite the apparent support of this new view of the hydrophobic effect, there are inconsistencies in the interpretation of the thermodynamic parameters and these are addressed in the Discussion. The specific stability of tendamistat is similar to that of modified bovine pancreatic trypsin inhibitor, with only two of the native three disulfide bridges intact. This observation confirms our previous conclusion that disulfide bridges affect significantly the enthalpy and entropy of unfolding. The recent study by Doig & Williams provides additional convincing support for this conclusion. The predictive scheme proposed by these authors permits a fair estimate of the Gibbs free energy and enthalpy changes of these two proteins.