Correlation between disulfide reduction and conformational unfolding in bovine pancreatic trypsin inhibitor

The native-like two-disulfide intermediate of bovine pancreatic trypsin inhibitor (BPTI), with the disulfide between Cys14 and Cys38 reduced, plays a particularly important role in the disulfide-coupled folding pathway of BPTI because of its participation in the rate-determining step of the reaction [Creighton & Goldenberg (1984) J. Mol. Biol. 179, 497-526; Weissman & Kim (1991) Science 253, 1386-1393]. In order to study directly the relationship between conformational stability and reductive unfolding kinetics, and to gain insight concerning the rate-limiting transition state in the thiol/disulfide-mediated folding/unfolding reaction of BPTI, BPTI variants based on a native-like two-disulfide analog of this intermediate, BPTI(Ala14)Ala38, were examined. The amino acid replacements introduced into BPTI(Ala14)Ala38 rendered it thermodynamically less stable. The kinetic stability, with respect to reduction by dithiothreitol, of the disulfides in these BPTI(Ala14)Ala38 variants was also decreased by the substitutions. The stabilization free energy (deltaG), obtained from chemical denaturation measurements, and the activation energy of the conformational transition (deltaG(++)conf), from the reductive unfolding reaction for this series of variants, were highly correlated. The observed correlation implies a direct coupling of disulfide reduction to conformational stability in this set of protein variants. It also strongly suggests that the transition state in the rate-limiting step of the reductive unfolding reaction involves a highly unfolded conformation of the protein. These data are consistent with a conformation-coupled redox folding pathway for BPTI(Ala14)Ala38 involving two parallel paths with unfolded (30-51) and unfolded (5-55) as the reactive species. Furthermore, the results provide a theoretical explanation for the observed 1000-fold diminution in the rate of 5-55 disulfide bond formation, relative to that of 14-38 bond formation, from the one-disulfide (30-51) intermediate in the wild-type BPTI refolding reaction. The data fit a general paradigm for protein disulfide formation during protein folding whereby native-like structure in folding intermediates accelerates formation of solvent-exposed disulfides but inhibits formation of core disulfides. This model predicts that a "rearrangement" mechanism (i.e., with non-native disulfides involved in the rate-limiting step) to form buried disulfides at a late stage in the folding reaction may be a common feature of redox folding pathways for surface disulfide-containing proteins of high stability.