The Kunitz-type soybean trypsin inhibitor (STI) family consists mainly of proteinase inhibitors from Leguminosae seeds . They belong to MEROPS inhibitor family I3, clan IC. They exhibit proteinase inhibitory activity against serine proteinases; trypsin (MEROPS peptidase family S1, ) and subtilisin (MEROPS peptidase family S8, ), thiol proteinases (MEROPS peptidase family C1, ) and aspartic proteinases (MEROPS peptidase family A1, ) . STI has a beta-Trefoil type fold consisting of a closed barrel and a hairpin triplet, with internal pseudo threefold symmetry.
The C-terminal domain of Clostridium spp. neurotoxins have an overfold that is very similar to that of the Kunitz ST1 family. The tetanus toxin binds to the gangliosides receptor, GT1b, and is composed of light and heavy chains, where the light chain is responsible for toxicity; the Kunitz-like domain is found on the C-terminal of the heavy chain, and is responsible for binding to sensitive cells .