Purification and characterization of a chymotrypsin Kunitz inhibitor type of polypeptide from the venom of cobra (Naja naja naja).

A chymotrypsin Kunitz inhibitor type of polypeptide has been isolated from the venom of Naja naja naja by reverse phase HPLC and cation exchange FPLC. It is present in a considerably lower amount than that of the corresponding trypsin inhibitor. The primary structure, determined by sequence analysis of the whole molecule and its tryptic peptides, has 57 residues with an apparent molecular mass of 6.2 kDa. The main contact site with the protease (P1) has a Phe, showing the specificity of the inhibitor. Of residues considered functionally important in Kunitz-type inhibitors, Gly-36 is replaced by Ser in a segment of weak contacts with the protease.