Folding pathway of a circular form of bovine pancreatic trypsin inhibitor

The pathway of unfolding and refolding of a circular form of bovine pancreatic trypsin inhibitor, in which the termini were linked together in a peptide bond, has been examined by trapping and identifying the disulphide-containing intermediates, as was done previously for the unmodified protein. The folding pathway of the circular protein was essentially the same as that of the unmodified inhibitor, although there were differences in the distribution of intermediates that accumulated and in the rates of some steps. The effects of the cross-link between the termini on the stabilities of the folding intermediates and the native state were determined by measuring the rates of the interconversions making up the folding transition, and comparing them with those measured for the unmodified protein. The major effect of the cross-link was to stabilize an intermediate containing two native disulphides, (30-51, 14-38), but lacking the disulphide nearest the termini, 5-55. The native conformation was not measurably stabilized by the cross-link, in spite of the expected reduction of entropy of the unfolded state, indicating that the native state of the circular protein had a slightly strained conformation. The stabilities of the major one-disulphide intermediates were not significantly affected by the cross-link, suggesting that the termini of bovine pancreatic trypsin inhibitor do not tend to interact during the early stage of folding.