Chymotrypsinogen A - Chymotrypsin precursor

Chymotrypsinogen is the practically inactive "precursor" (proenzyme, or zymogen) of chymotrypsin. In order to be activated, chymotrypsinogen has to be cleaved by trypsin between the amino acids arginine and isoleucine (R15 and I16) , which leads to structural changes and the formation of the substrate binding site (Sears 2010). Chymotrypsin is a trypsin-like serine protease.

Chymotrypsin differs from trypsin in the selectivity of the cleavage site of proteins. While trypsin cleaves proteins between the amino acids Arg and Lys, Chymotrypsin prefers large hydrophobic amino acids as cleavage site.

Unit definition: The amount of activated zymogen that causes a decrease in absorbance at 237 nm of 0.0075 per minute at 25°C resulting from the hydrolysis of ATEE (NF/USP unit).