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 and ...
, a zymogen (), also called a proenzyme (), is an inactive
precursor
Precursor or Precursors may refer to:
* Precursor (religion), a forerunner, predecessor
** The Precursor, John the Baptist
Science and technology
* Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
of an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
. 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, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolysi ...
reaction revealing the
active site, or changing the configuration to reveal the active site) for it to become an active enzyme. The biochemical change usually occurs in
Golgi bodies
The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle found in most eukaryotic cells. Part of the endomembrane system in the cytoplasm, it packages proteins into membrane-bound vesicles insid ...
, 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. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. ...
unit, or can be independently-folding
domains comprising more than 100
residues. Although they limit the enzyme's ability, these
N-terminal
The N-terminus (also known as the amino-terminus, NH2-terminus, N-terminal end or amine-terminus) is the start of a protein or polypeptide, referring to the free amine group (-NH2) located at the end of a polypeptide. Within a peptide, the ami ...
extensions of the enzyme or a “prosegment” often aid in the stabilization and
folding of the enzyme they inhibit.
The
pancreas
The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
secretes zymogens partly to prevent the enzymes from digesting proteins in the
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
in which 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, whose
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dige ...
partially activates it. Another partially inactivated pepsinogen completes the activation by removing the 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 hormones. There are two main types: acute pancreatitis, and chronic p ...
.
Fungi
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
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, is single polypeptide chain of 245 amino acids residues, is converted to alpha-chymotrypsin, which has three polypeptide chains linked by two of the five disulfide bond 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
The complement system, also known as complement cascade, is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and ...
*
Procaspases
Caspases (cysteine-aspartic proteases, cysteine aspartases or cysteine-dependent aspartate-directed proteases) are a family of protease enzymes playing essential roles in programmed cell death. They are named caspases due to their specific cystei ...
*
Pacifastin
*
Proelastase
*
Prolipase
*
Procarboxypolypeptidases
*
Proinsulin
See also
*
Enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
*
Protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
References
External links
{{wiktionary
Zymogens - Washington.eduMolecular mechanisms for the conversion of zymogens to active proteolytic enzymes
Enzymes
Precursor proteins