Amino acid replacement that eliminates kinetic traps in the folding pathway of pancreatic trypsin inhibitor.

The disulfide-coupled folding pathway of a bovine pancreatic trypsin inhibitor (BPTI) variant, in which Tyr 35 is replaced by Leu, was determined and compared with that of the wild-type protein. Two of the most highly populated intermediates in the refolding of the wild-type protein, [30-51, 14-38] and [5-55, 14-38], did not accumulate to detectable levels during folding of this variant. The absence of these native-like intermediates was associated with a substantially increased rate of overall folding, consistent with previous results indicating that these species act as kinetic traps. As in the folding of the wild-type protein, the kinetically preferred folding pathway for the mutant protein includes intramolecular rearrangements and intermediates with nonnative disulfide bonds. These results suggest that the predominance of the rearrangement mechanism is not simply the consequence of the stability of the kinetically trapped species. Rather, the rearrangements appear to arise because of conformational constraints in earlier intermediates.