Thermodynamic Effects of Disulfide Bond on Thermal Unfolding of the Starch-Binding Domain of Aspergillus niger Glucoamylase

Abstract

The thermodynamic effects of the disulfide bond of the fragment protein of the starch-binding domain of Aspergillus niger glucoamylase was investigated by measuring the thermal unfolding of the wild-type protein and its two mutant forms, Cys3Gly/Cys98Gly and Cys3Ser/Cys98Ser. The circular dichroism spectra and the thermodynamic parameters of binding with β-cyclodextrin at 25 °C suggested that the native structures of the three proteins are essentially the same. Differential scanning calorimetry of the thermal unfolding of the proteins showed that the unfolding temperature t_1⁄2 of the two mutant proteins decreased by about 10 °C as compared to the wild-type protein at pH 7.0. At t_1⁄2 of the wild-type protein (52.7 °C), the mutant proteins destabilized by about 10 kJ mol⁻¹ in terms of the Gibbs energy change. It was found that the mutant proteins were quite stabilized in terms of enthalpy, but that a higher entropy change overwhelmed the enthalpic effect, resulting in destabilization.