Kinetic studies on the unfolding and refolding of pepsinogen in urea. The nature of the rate-limiting step.

Kinetic studies on the unfolding of pepsinogen by urea showed that changes in absorbance and potential pepsin activity followed simple first order kinetics. Changes in these variables on refolding were more complex. A large part of the absorbance was recovered within the mixing time of these experiments, whereas the appearance of activity was a slow sigmoidal function of time. The results were interpreted to show that pepsinogen can rapidly regain a globular form, but that its activatable form is produced by a slow conformational change in the folded protein. The enthalpies of activation of this change are similar to those of the cis-trans isomerization of proline residues. If the latter reaction is involved in the folding of pepsinogen, it must occur after extensive folding has already occurred.