This model includes the Kunitz-type chymotrypsin inhibitors SCI-III and SCI-I, and similar proteins in insects. SCI-III and SCI-I inhibit chymotrypsin, avoiding the accidental chymotrypsin-mediated activation of prophenoloxidase. This enzyme is required by the insect immune system to produce melanin which is used to engulf foreign objects. This subfamily also includes Kunitz-type male accessory gland peptide with protease inhibitory activity, synthesized and secreted by male accessory glands of Drosophila funebris; it may play a role as an acrosin inhibitor involved in reproduction. These proteins are similar to Kunitz-type proteinase inhibitors such as BPTI (bovine pancreatic trypsin inhibitor) that shows an alpha/beta fold with irregular secondary structure stabilized by three disulfide bonds.
Jiyao W et al.(2023). "The conserved domain database in 2023.",