Thermodynamics of the folding of D-glyceraldehyde-3-phosphate dehydrogenase assisted by protein disulfide isomerase studied by microcalorimetry

Abstract

Thermodynamics of the refolding of denatured D-glyceraldehyde 3-phosphate dehydrogenase (GAPDH) assisted by protein disulfide isomerase (PDI), a molecular chaperone, has been studied by isothermal microcalorimetry at different molar ratios of PDI/GAPDH and temperatures using two thermodynamic models proposed for chaperone-substrate binding and chaperone-assisted substrate folding, respectively. The binding of GAPDH folding intermediates to PDI is driven by a large favorable enthalpy decrease with a large unfavorable entropy reduction, and shows strong enthalpy-entropy compensation and weak temperature dependence of Gibbs free energy change. A large negative heat-capacity change of the binding, 156 kJ·mol-1·K-1, at all temperatures examined indicates that hydrophobic interaction is a major force for the binding. The binding stoichiometry shows one dimeric GAPDH intermediate per PDI monomer. The refolding of GAPDH assisted by PDI is a largely exothermic reaction at 15.0-25.0°C. With increasing temperature from 15.0 to 37.0°C, the PDI-assisted reactivation yield of denatured GAPDH upon dilution decreases. At 37.0°C, the spontaneous reactivation, PDI-assisted reactivation and intrinsic molar enthalpy change during the PDI-assisted refolding of GAPDH are not detected.