Equilibrium Unfolding Mechanism of Chicken Muscle Triose Phosphate Isomerase

Triose phosphate isomerase (TIM) was prepared and purified from chicken breast muscle. The equilibrium unfolding of TIM by urea was investigated by following the changes of intrinsic fluorescence and circular dichroism spectroscopy, and the equilibrium thermal unfolding by differential scanning calorimetry (DSC). Results show that the unfolding of TIM in urea is highly cooperative and no folding intermediate was detected in the experimental conditions used. The thermodynamic parameters of TIM during its urea induced unfolding were calculated as ΔG =3.54 kcal•mol-1, and mG = 0.67 kcal•mol-1•M-1, which just reflect the unfolding of dissociated folded monomer to fully unfolded monomer transition, while the dissociation energy of folded dimer to folded monomer is probe silence. DSC results indicate that TIM unfolding follows an irreversible two-state step with a slow aggregation process. The cooperative unfolding ratio, ΔHcal/ΔHvH, was measured close to 2, indicating that the two subunits of chicken muscle TIM unfold independently. The van't Hoff enthalpy, ΔHvH, was estimated as about 200 kcal•mol-1. These results support the unfolding mechanism with a folded monomer formation before its tertiary structure and secondary structure unfolding.