Kunitz soybean trypsin inhibitor is a type of protein contained in legume seeds which functions as a protease inhibitor.Kunitz-type Soybean Trypsin Inhibitors are usually specific for either trypsin or chymotrypsin. They are thought to protect seeds against consumption by animal predators.

Two types of trypsin inhibitors are found in soy: the Kunitz trypsin inhibitor (KTI) and the Bowman-Birk inhibitor (BBI). KTI is a large (20,100 daltons), strong inhibitor of trypsin, while BBI is much smaller (8,000 daltons) and inhibits both trypsin and chymotrypsin. Both inhibitors have significant anti-nutritive effects in the body, affecting digestion by hindering protein hydrolysis and activation of other enzymes in the gut. In soy, KTI is found in much larger concentrations than BBI is soy, however, to achieve the highest nutritional value from this ingredient, both of these inhibitors must be denatured in some way. Whole soybeans have been reported to contain 17–27 mg of trypsin inhibitor per gram.

Proteins from the Kunitz family contain from 170 to 200 amino acid residues and one or two intra-chain disulfide bonds. The best conserved region is found in their N-terminal section. The crystal structures of soybean trypsin inhibitor (STI), trypsin inhibitor DE-3 from the Kaffir tree Erythrina caffra (ETI) and the bifunctional proteinase K/alpha-amylase inhibitor from wheat (PK13) have been solved, showing them to share the same beta trefoil fold structure as those of interleukin 1 and heparin-binding growth factors.

Despite the structural similarity, STI shows no interleukin-1 bioactivity, presumably as a result of their primary sequence disparities. The active inhibitory site containing the scissile bond is located in the loop between beta-strands 4 and 5 in STI and ETI.

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