Soybean Trypsin Inhibitor
SBTI is a single polypeptide chain cross-linked by two disulfide bonds that forms a stable, enzymatically inactive complex with trypsin. SBTI is a reversible competitive inhibitor of trypsin and other trypsin-like proteases. It forms a 1:1 stoichiometric complex with trypsin; upon formation of this complex, trypsin may cleave a single arginine-isoleucine bond in the inhibitor forming covalent bimolecular complex of inhibitor and trypsin.
It acts as an inhibitor only when it is in its native state. Addition of increasing amounts of SBTI to a solution of trypsin decreases the proteolytic activity of the trypsin in direct proportion to the amount of inhibitor added. This reaction is practically instantaneous and is independent, within a wide range, of the pH of the solution. Chymotrypsin is only slightly inhibited by SBTI as this interaction results in the formation of a reversibly dissociable compound; SBTI has no effect on the activity of pepsin.