Inbiochemistry, azymogen (/ˈzaɪmədʒən,-moʊ-/[1][2]), also called aproenzyme (/ˌproʊˈɛnzaɪm/[3][4]), is an inactiveprecursor of anenzyme. A zymogen requires a biochemical change (such as ahydrolysis reaction revealing theactive site, or changing the configuration to reveal the active site) to become an active enzyme. The biochemical change usually occurs inGolgi bodies, where a specific part of the precursor enzyme iscleaved in order to activate it. The inactivating piece which is cleaved off can be apeptide unit, or can be independently-foldingdomains comprising more than 100residues. Although they limit the enzyme's ability, theseN-terminal extensions of the enzyme or a "prosegment" often aid in the stabilization andfolding of the enzyme they inhibit.[citation needed]
Thepancreas secretes zymogens partly to prevent the enzymes from digesting proteins in thecells where they are synthesised. Enzymes likepepsin are created in the form ofpepsinogen, an inactive zymogen. Pepsinogen is activated whenchief cells release it into thegastric acid, whosehydrochloric acid partially activates it.[5] Another partially inactivated pepsinogen completes the activation by removing a peptide, turning the pepsinogen into pepsin. Accidental activation of zymogens can happen when the secretion duct in the pancreas is blocked by agallstone, resulting in acutepancreatitis.[citation needed]
Fungi also secretedigestive enzymes into the environment as zymogens. The external environment has a differentpH than inside the fungal cell and this changes the zymogen's structure into an active enzyme.[citation needed]
Another way that enzymes can exist in inactive forms and later be converted to active forms is by activating only when acofactor, called a coenzyme, is bound. In this system, the inactive form (the apoenzyme) becomes the active form (the holoenzyme) when the coenzyme binds.
In the duodenum, the pancreatic zymogens, trypsinogen, chymotrypsinogen, proelastase and procarboxypeptidase, are converted into active enzymes by enteropeptidase and trypsin. Chymotrypsinogen, a single polypeptide chain of 245 amino acid residues, is converted to alpha-chymotrypsin, which has three polypeptide chains linked by two of the five disulfide bonds present in the primary structure of chymotrypsinogen.[6]
Examples of zymogens:
