REFOLDING OF BOVINE PANCREATIC TRYPSIN-INHIBITOR VIA NONNATIVE DISULFIDE INTERMEDIATES

The disulphide folding pathway of bovine pancreatic trypsin inhibitor (BPTI), especially at the two-disulphide stage, has been dissected by replacing one or two particular cysteine residues by serine. This rest ricts which disulphide species are possible, and the observed kinetics of disulphide-coupled folding reveal the roles of the remaining speci es. The results obtained confirm the kinetic roles in the original BPT I pathway of the two specific two-disulphide intermediates with non-na tive second disulphide bonds, (30-51, 5-14) and (30-51, 5-38). Moreove r, the rates of folding through each of these intermediates are shown to account quantitatively for the rate of folding of the normal protei n; therefore, essentially all the molecules refold through these two p articular intermediates. They are amongst the most productive on the f olding pathway, and their roles are readily explicable on the basis of their conformations.