Chymotrypsin is a digestive enzyme that breaks down proteins in the small intestine. It is synthesized as a larger molecule in the pancreas and transported in an inactive form. Once in the small intestine, it is activated by another digestive enzyme, trypsin.
The digestion of food involves the breakdown of large molecules to small ones, which can be absorbed into the blood. Proteins are made up of chains of amino acids, linked by peptide bonds. Enzymes are proteins that accelerate the rate of reactions, and proteases speed up the breakdown of proteins. There are a number of different proteases acting in digestion, that act on different types and regions of proteins.
Chymotrypsin is a type of protease that is in a family known as serine proteases, also known as serine endopeptidases. They have that name because they have the amino acid serine at their active site. Serine proteases are synthesized in an inactive, larger form, which allows them to be transported to their site of activity without causing any tissue damage. The inactive form of chymotrypsin is known as chymotrypsinogen.
After limited proteolysis by trypsin to cleave off part of chymotrypsinogen, a two-molecule combination of chymotrypsin is formed. The two molecules activate each other, and then can catalyze the breakdown of proteins. Chymotrypsin has a preference for acting on certain types of amino acids. It tends to degrade the peptide bond by using phenylalanine, tryptophan, or tyrosine. These are all aromatic amino acids, and are so named because they have a cyclic ring structure in them.
The chymotrypsin mechanism involves hydrolysis — splitting a molecule of water in two, and adding each fragment to a different part of the peptide bond. This cleaves the bond. Once the protein has been fragmented by this protease, other proteases and peptidases continue degrading it, until it has been broken down to amino acids. After digestion has been completed, there is a class of natural serine protease inhibitors in the body called serpins that inactivate the enzymes.
Many plants make a chymotrypsin inhibitor. Protease inhibitors in general are particularly common in legume seeds, and can be a defense mechanism against insects, limiting their absorption of nutrients. Also, many of the bacteria that live in our intestines produce protease inhibitors against digestive enzymes to help them survive without being digested.
Both chymotrypsin and trypsin activities are assayed medically to test for cystic fibrosis and pancreatic insufficiency. Normally, both enzymes are found in the stool. If they are not, further testing is indicated.