A third native one-disulphide intermediate in the folding of bovine pancreatic trypsin inhibitor.

A previously unidentified intermediate has been detected in the early stages of the oxidative folding of bovine pancreatic trypsin inhibitor (BPTI). The intermediate contains one disulphide bond between residues 14 and 38 and is denoted [14-38]. The 14-38 disulphide bond is also found in native BPTI. Although the other native one-disulphide intermediates, [30-51] and [5-55], are thermodynamically more stable, [14-38] can be populated substantially at the early stages of BPTI folding. Moreover, initial characterization of the kinetic properties of this intermediate strongly suggest that a substantial fraction of BPTI molecules fold by way of the [14-38] intermediate. Our results emphasize the importance of native-like tendencies in protein folding.