Inactivation before significant conformational change during denaturation of papain by guanidine hydrochloride

During denaturation by GuHCl, papain shows a rapid decrease in activity with increasing concentrations of the denaturant followed by an intermediate stage of relatively little change from 1 to 2 M before complete inactivation at 4 M GuHCl. At GuHCl concentrations lower than 2 M, enzyme activity is more sensitive to GuHCl than noticeable conformation changes as followed by fluorescence and CD measurements. Kinetics of GuHCl inactivation were studied by following the substrate reaction in the presence of denaturant and the apparent rate constants thus obtained were found to be only slightly higher than those for conformational changes. However, apparent inactivation rate constants obtained in the presence of saturating concentration of substrate are actually inactivation constants for the ES complex. The inactivation rates at different substrate concentrations were, therefore, followed and the microscopic inactivation rate constants for the free enzyme obtained (Tsou, C.L. (1988) Adv. Enzymol. 61, 381-436). It was found that substrate protects strongly against inactivation and at the same GuHCl concentration, the inactivation rate of the free enzyme is 100-fold higher than that of unfolding. The above results show that the activity of papain is more sensitive to GuHCl than its overall conformation and like the enzymes previously studied in this laboratory, its active site is more flexible than the enzyme molecule as a whole.