Investigation of guanidine hydrochloride induced unfolding of apolipoprotein A-I_Milano

Abstract

A guanidine hydrochloride (GuHCl) induced unfolding study of apolipoprotein A-I_Milano (apo A-I_M) has been performed. The unfolding was followed by circular dichroism (CD) measurements at 222 nm and by isothermal titration (ITC) calorimetry at 25°C. In the ITC experiments enthalpies of transfer were determined for apo A-I_M from aqueous sodium phosphate buffer solution into solutions of different concentrations of GuHCl. The CD data and the ITC data give complementary and consistent results of the complex unfolding process. Analytical ultracentrifugation experiments were made on apo A-I_M in sodium phosphate buffer and in 0.6 M GuHCl, respectively. Analyses of the obtained sedimentation velocity data show that apo A-I_M is highly aggregated in sodium phosphate buffer. The aggregates are almost completely dissociated in 0.6 M GuHCl. Aggregation of the protein in sodium phosphate buffer solution induces an increase in α-helical content. The loss of α-helical secondary structural element of the protein upon dissociation of the aggregates destabilises the protein resulting in a low GuHCl concentration of unfolding, [GuHCl]_m=1.1 M. The unfolded protein has a significant α-helical content at the unfolded state. From ITC- and CD data we suggest that increased binding of GuHCl to the unfolded protein results in a disruption of the residual secondary structure.