The oxidative folding pathway of the disulfide containing protein bovine pancreatic trypsin inhibitor (BPTI) was one of the first to be elucidated and has served as a basis for understanding the folding pathways of other proteins. During the oxidative folding of reduced BPTI, two intermediates (N' and N*) accumulate in significant amounts and act as kinetic traps. Both N' and N* bury their two remaining free thiols in their hydrophobic cores, which inhibits further oxidation. Historically, the rate limiting step was considered to be the intramolecular rearrangements of N' and N* to another intermediate with two free thiols, NSH. The two free thiols in NSH are solvent-exposed and easily oxidized to a disulfide, producing native protein (N). Nevertheless, our research using reduced BPTI indicated that the folding rate of N* to N was proportional to the concentration of added glutathione disulfide (GSSG), inconsistent with the slow intramolecular rearrangement of N* to NSH. To confirm our initial results, the intermediate N* was purified and refolded in the presence of GSSG. The conversion of N* to N was dependent upon the disulfide concentration and singly mixed disulfide N*(SG) was observed during folding. These results emphasize that the folding of N* can proceed via a growth type pathway, direct oxidation of the two remaining thiols in N* by an exogenous small molecule disulfide, such as GSSG, to form N. Folding of reduced BPTI via N* was performed under changing concentrations of GSSG and GSH as a function of time. The folding was improved dramatically in terms of rate and yield. Aromatic disulfides and thiols have been demonstrated to improve the folding efficiency of disulfide containing proteins including ribonuclease A (RNase A) and lysozyme. Herein, N* and N' were refolded in the presence of aromatic disulfides. Folding of the two kinetic traps with aromatic disulfides indicated that folding proceed via a growth type pathway. The singly and doubly mixed disulfide intermediates were observed during most folding reactions. The oxidative folding of reduced BPTI with aromatic disulfides and thiols were also investigated. Reduced BPTI can be folded to disulfide intermediates rapidly.

Improvement in the in vitro oxidative folding of disulfide-containing proteins, such as extracellular and pharmaceutically important proteins, is required. Traditional folding methods using small molecule aliphatic thiol and disulfide, such as glutathione (GSH) and glutathione disulfide (GSSG) are slow and low yielding. Small molecule aromatic thiols and disulfides show great potentiality because aromatic thiols have low pKa values, close to the thiol pKa of protein disulfide isomerase (PDI), higher nucleophilicity and good leaving group ability. Our studies showed that thiols with a positively charged group, quaternary ammonium salts (QAS), are better than thiols with negatively charged groups such as phosphonic acid and sulfonic acid for the folding of bovine pancreatic trypsin inhibitor (BPTI). An enhanced folding rate of BPTI was observed when the protein was folded with a redox buffer composed of a QAS thiol and its corresponding disulfide. Quaternary ammonium salt (QAS) thiols and their corresponding disulfides with longer alkyl side chains were synthesized. These QAS thiols and their corresponding disulfides are promising small molecule thiols and disulfides to fold reduced BPTI efficiently because these thiols are more hydrophobic and can enter the core of the protein. Conformational changes of disulfide-containing proteins during oxidative folding influence the folding pathway greatly. We performed the folding of BPTI using targeted molecular dynamics (TMD) simulation and investigated conformational changes along with the folding pathway. Applying a bias force to all atoms versus to only alpha carbons and the sulfur of cysteines showed different folding pathways. The formation of kinetic traps N' and N* was not observed during our simulation applying a bias force to all atoms of the starting structure. The final native conformation was obtained once the correct antiparallel β-sheets and subsequent Cys14-Cys38 distance were decreased to a bond distance level. When bias force was applied to only alpha carbons and the sulfur of cysteines, the distance between Cys14-Cys38 increased and decreased multiple times, a structure similar to the confirmation of N*, NSH were formed and native protein was ultimately obtained. We concluded that there could be multiple pathways of conformational folding which influence oxidative folding.