A zymogen is the inactive precursor of an enzyme. The molecule is composed of amino acids strung together into a peptide. When the zymogen is in the presence of an enzyme specifically designed to breakdown peptides, called a protease, some of the amino acids are removed. This cleavage renders the zymogen a functional enzyme by changing the shape of the peptide and forming the active site where enzymatic action will occur. For this reason, a zymogen is also called a proenzyme.
The active site is the key feature of an enzyme. It is the place where the molecule that the enzyme acts on, called the substrate, binds and undergoes chemical change. An enzyme’s active site and overall function is dependent on the shape of the enzyme. This is determined by four structural levels.
The primary structure of an enzyme is simply the sequence of amino acids. The secondary structure represents how the peptide folds and twists on itself due to interactions between the amino acids. Secondary structures include coil-like alpha helices and beta pleated sheets, which resemble accordion folds.
The tertiary structure describes the overall folding of the whole peptide, with the secondary structures folding upon themselves to form a globular ball, the protein’s active conformation. Some proteins have a quaternary structure that describes how two or more peptides combine to form a complex protein. For example, hemoglobin, which carries oxygen in the blood, is composed of four individual peptides that are linked to create a functional molecule.
The body typically secretes zymogens rather than active enzymes because they can be stored and transported safely without harm to surrounding tissues, and released when conditions are favorable for optimal activity. For example, pepsinogen is secreted in the stomach and cleaved to form pepsin, an enzyme that breaks down the proteins ingested as food. The highly acidic conditions of the stomach actually induce cleavage of pepsinogen and promotes the activity of pepsin. Once digestion moves to the small intestine, however, the drastic change in pH inactivates pepsin and two more zymogens are released.
Chymotrysinogen and trypsinogen, also protein digesting enzymes, are key components of the digestive juice released by the pancreas. They travel through the main pancreatic duct into the duodenum of the small intestine where they are then cleaved into their active forms. By releasing zymogens instead of the active enzymes chymotrypsin and trypsin, the pancreas avoids self-digestion.
Other zymogens in the body include prothrombin and fibrinogen, both essential to clot formation. Both exist as plasma proteins. When they are needed to stem blood loss due to tissue damage, these zymogens are readily available, not having caused any damage to the circulatory system in which they are stored.