In
biochemistry
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
, a zymogen (), also called a proenzyme (), is an inactive
precursor of an
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
. A zymogen requires a biochemical change (such as a
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
reaction revealing the
active site, or changing the configuration to reveal the active site) to become an active enzyme. The biochemical change usually occurs in
Golgi bodies, where a specific part of the precursor enzyme is
cleaved in order to activate it. The inactivating piece which is cleaved off can be a
peptide
Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...
unit, or can be independently-folding
domains comprising more than 100
residues. Although they limit the enzyme's ability, these
N-terminal extensions of the enzyme or a "prosegment" often aid in the stabilization and
folding of the enzyme they inhibit.
The
pancreas
The pancreas (plural pancreases, or pancreata) is an Organ (anatomy), organ of the Digestion, digestive system and endocrine system of vertebrates. In humans, it is located in the abdominal cavity, abdomen behind the stomach and functions as a ...
secretes zymogens partly to prevent the enzymes from digesting proteins in the
cells where they are synthesised. Enzymes like
pepsin are created in the form of
pepsinogen, an inactive zymogen. Pepsinogen is activated when
chief cells release it into the
gastric acid
Gastric acid or stomach acid is the acidic component – hydrochloric acid – of gastric juice, produced by parietal cells in the gastric glands of the stomach lining. In humans, the pH is between one and three, much lower than most other a ...
, whose
hydrochloric acid partially activates it. 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 a
gallstone, resulting in acute
pancreatitis
Pancreatitis is a condition characterized by inflammation of the pancreas. The pancreas is a large organ behind the stomach that produces digestive enzymes and a number of hormone
A hormone (from the Ancient Greek, Greek participle , "se ...
.
Fungi
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
also secrete
digestive enzymes into the environment as zymogens. The external environment has a different
pH than inside the fungal cell and this changes the zymogen's structure into an active enzyme.
Another way that enzymes can exist in inactive forms and later be converted to active forms is by activating only when a
cofactor, 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.
Examples
Examples of zymogens:
*
Trypsinogen
*
Chymotrypsinogen
*
Pepsinogen
* Most proteins in the
coagulation system (examples, prothrombin, or plasminogen)
* Some of the proteins of the
complement system
*
Procaspases
*
Pacifastin
*
Proelastase
*
Prolipase
*
Procarboxypolypeptidases
See also
*
Enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
*
Protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
*
Prohormone
References
External links
{{wiktionary
Zymogens - Washington.eduMolecular mechanisms for the conversion of zymogens to active proteolytic enzymes
Enzymes
Precursor proteins