COAGULATION FACTOR XIIIA-INHIBITOR TRIDEGIN: ON THE ROLE OF DISULFIDE BONDS FOR FOLDING, STABILITY, AND FUNCTION.
Tridegin is a potent and specific 66mer peptide inhibitor of coagulation factor XIIIa with six cysteines involved in three disulfide bonds. Only three of the fifteen possible 3-disulfide-bonded isomers have been identified, which share a bridge between cysteines 19 and 25. This raises the question of the specific role of C19-C25 in folding, stability, and function of tridegin. We synthesized the three possible 2-disulfide-bonded analogs using a targeted protecting-group strategy to investigate the impact of the C19-C25 bond on structure and activity of tridegin. The FXIIIa inhibitory activity of the analogs was analyzed with in vitro fluorogenic activity and whole blood clotting assays, both revealed no diminished inhibitory capacity for these peptides. Molecular dynamics simulations of wild type tridegin and the analogs were performed to elucidate the impact of the C19-C25 bond on conformational stability and integrity. Molecular docking studies with peptide models and the FXIIIa crystal structure revealed insights into their structure-activity relationships. The strategy of selectively reducing disulfide bonds to facilitate large-scale synthesis while retaining the functionality of disulfide bonded peptides has been demonstrated with our present study.