Chymotrypsinogen

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

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