Trypsin Inhibition by a Peptide Hormone: Crystal Structure of Trypsin–Vasopressin Complex

The large variety of serine protease inhibitors, available from various sources such as tissues, microorganisms, plants, etc., play an important role in regulating the proteolytic enzymes. The analysis of protease–inhibitor complexes helps in understanding the mechanism of action, as well as in designing inhibitors. Vasopressin, an anti-diuretic nonapeptide hormone, is found to be an effective inhibitor of trypsin, with a Ki value of 5 nM. The crystal structure of the trypsin–vasopressin complex revealed that vasopressin fulfils all the important interactions for an inhibitor, without any break in the scissile peptide bond. The cyclic nature due to a disulfide bridge between Cys1 and Cys6 of vasopressin provides structural rigidity to the peptide hormone. The trypsin-binding site is located at the C terminus, while the neurophysin-binding site is at the N terminus of vasopressin. This study will assist in designing new peptide inhibitors. This study suggests that vasopressin inhibition of trypsin may have unexplored biological implications.