Chymotrypsinogen

Chymotrypsinogen is a precursor (zymogen) of the digestive enzyme chymotrypsin.

This molecule is inactive and must be cleaved by trypsin, and then by other chymotrypsin molecules, before it can reach its full activity. Its function is to convert proteins to smaller peptides. The active site of chymotrypsinogen is covered by a 6-amino-acid-long mask. It is only when this mask is removed - when the chymotrypsinogen molecule enters the lumen of the intestine and comes into contact with trypsin molecules - that the enzyme becomes active. This is a very useful safety feature for a protein-digesting enzyme. If chymotrypsinogen were not inactivated in this way, it would digest the pancreas, where it is produced.