Effect of urea at lower concentration on the structure of papain--formation of a stable molten globule and its characterization.

Effect of lower concentrations of urea on papain was monitored by optical spectroscopy, calorimetry and partial specific volume measurements. At lower concentrations of urea, papain exhibits a different structure and showed an increase in the intensity of circular dichroic (CD) spectra as compared to the native molecule. At lower concentrations (0.2-1.5 M) of urea, binding of 8-anilino-naphthalene sulfonic acid (ANS) to the papain molecule was higher; at 0.5 M, there was about 50% increase in ANS binding. Both calorimetric and spectroscopic studies indicated an increased thermal stability of the molecule at lower concentrations. At 0.5 M urea concentration, the apparent thermal denaturation temperature increased from a control value of 83 +/- 1 degrees C to 86 +/- 1 degrees C. At isopotential conditions, the partial specific volume of papain was found to be higher in presence of lower concentrations of urea, than the native protein or unfolded molecule. The preferential interaction parameter (deltag3/deltag2)(T,mu1,mu3) showed a negative value in the presence of lower concentrations of urea (0.2-2 M), which was maximum at 1 M urea with a value of -0.019 g/g. Above 3 M urea, the preferential interaction parameter was positive.