This family of proteinase inhibitors belong to MEROPS inhibitor family I13, clan IG. They inhibit peptidases of the S1 and S8 families .
Potato inhibitor type I sequences are not solely restricted to potatoes but are found in other plant species for example: barley endosperm chymotrypsin inhibitor , and pumpkin trypsin inhibitor. Exceptions are found in leech's, e.g.Hirudo medicinalis (Medicinal leech), but not other metazoa . In general, the proteins have retained a specificity towards chymotrypsin-like and elastase-like proteases. Structurally these inhibitors are small (60 to 90 residues) and in contrast with other families of protease inhibitors, they lack disulphide bonds. The inhibitor is a wedge-shaped molecule, its pointed edge formed by the protease-binding loop, which contains the scissile bond. The loop binds tightly to the protease active site, subsequent cleavage of the scissile bond causing inhibition of the enzyme .
The inhibitors (designated type I and II) are
synthesised in potato tubers, increasing in concentration as the tuber develops. Synthesis of the inhibitors throughout the plant is also induced by leaf damage; this systemic response being triggered by the release of a putative plant hormone.
Examples found in the bacteria and archaea are probable false positives.