Peptide proteinase inhibitors are in many cases synthesised as part of a larger precursor protein, referred to as a propeptide or zymogen, which remains inactive until the precursor domain is cleaved off in the lysosome, the precursor domain preventing access of the substrate to the active site until necessary.
Very little is known about the endogenous function of pacifastin-like inhibitors except that they may play roles in arthropod immunity and in regulation of the physiological processes involved in insect reproduction [REF].