Biology:Trypsinogen
Trypsinogen (/ˌtrɪpˈsɪnədʒən, -ˌdʒɛn/[1][2]) is the precursor form (or zymogen) of trypsin, a digestive enzyme. It is produced by the pancreas and found in pancreatic juice, along with amylase, lipase, and chymotrypsinogen. It is cleaved to its active form, trypsin, by enteropeptidase, which is found in the intestinal mucosa. Once activated, the trypsin can cleave more trypsinogen into trypsin, a process called autoactivation. Trypsin cleaves the peptide bond on the carboxyl side of basic amino acids such as arginine and lysine.
Function
Trypsinogen is the proenzyme precursor of trypsin. Trypsinogen (the inactive form) is stored in the pancreas so that it may be released when required for protein digestion. The pancreas stores the inactive form trypsinogen because the active trypsin would cause severe damage to the tissue of the pancreas. Trypsinogen is released by the pancreas into the second part of the duodenum, via the pancreatic duct, along with other digestive enzymes.[3]
Activation of trypsinogen
Trypsinogen is activated by enteropeptidase (also known as enterokinase). Enteropeptidase is produced by the mucosa of duodenum and it cleaves the peptide bond of trypsinogen after residue 15, which is a lysine. The N-terminal peptide is discarded, and a slight rearrangement of the folded protein occurs. The newly formed N-terminal residue (residue 16) inserts into a cleft, where its α-amino group forms an ion pair with the aspartate near the active site serine, and results in the conformational rearrangement of other residues. The amino group of Gly 193 orientates itself into the correct position, which completes the oxyanion hole in active site, thereby activating the protein.[4] Since trypsin also cleaves the peptide bond after an arginine or a lysine, it can cleave other trypsinogen, and the activation process therefore becomes autocatalytic.
Safeguards against trypsinogen activation
Trypsin is produced, stored and released as the inactive trypsinogen to ensure that the protein is only activated in the appropriate location. Premature trypsin activation can be destructive and may trigger a series of events that lead to pancreatic self-digestion. In normal pancreas, around 5% of trypsinogens are thought to get activated[citation needed], therefore there are a number of defenses against such inappropriate activation. Trypsinogen is stored in intracellular vesicles in the pancreas called zymogen granules whose membranous walls are thought to be resistant to enzymatic degradation. A further safeguard against inappropriate trypsin activation is the presence of inhibitors such as bovine pancreatic trypsin inhibitor (BPTI) and serine protease inhibitor Kazal-type 1 (SPINK1), which binds to any trypsin formed. Trypsin autocatalytic activation of trypsinogen is also a slow process due to the presence of a large negative charge on the conserved N-terminal hexapeptide of trypsinogen, which repels the aspartate on the back of trypsin's specificity pocket.[5] Trypsin may also inactivate other trypsin by cleavage.
Serum trypsinogen
Serum trypsinogen is measured using a blood test. High levels are seen in acute pancreatitis[6] and cystic fibrosis.[7]
Trypsinogen isoforms
Three isoforms of trypsinogens may be found in human pancreatic juice. These are the cationic, anionic, and meso trypsinogen, and they account for 23.1%, 16%, and 0.5% of total pancreatic secretory proteins, respectively.[8] Other forms of trypsinogen have been found in other organisms.
Diseases
The inappropriate activation of trypsinogen in the pancreas can lead to pancreatitis. Some type of pancreatitis may be associated with mutant forms of trypsinogen. A mutation at Arg 117, a trypsin-sensitive site, in cationic trypsinogen has been implicated in hereditary pancreatitis, a rare form of early-onset genetic disorder. Arg 117 may be a fail-safe mechanism by which trypsin, when activated within the pancreas, may become inactivated, and loss of this cleavage site would result in a loss of control and permit autodigestion resulting in pancreatitis.[9] Other mutations have also been found that are linked to pancreatitis.[10]
References
- ↑ "Trypsinogen". Trypsinogen. Oxford University Press. http://www.lexico.com/definition/Trypsinogen.
- ↑ "Trypsinogen". Dictionary.com Unabridged. Random House. https://www.dictionary.com/browse/Trypsinogen.
- ↑ Mayer, J; Rau, B; Schoenberg, MH; Beger, HG (September 1999). "Mechanism and role of trypsinogen activation in acute pancreatitis". Hepatogastroenterology 46 (29): 2757–2763. PMID 10576341.
- ↑ Thomas E Creighton (1993). Proteins: Structures and Molecular Properties (2nd ed.). W H Freeman and Company. pp. 434. ISBN 0-7167-2317-4. https://archive.org/details/proteinsstructur0000crei/page/434.
- ↑ Voet; Voet (1995). Biochemistry (2nd ed.). John Wiley & Sons. pp. 399–400. ISBN 0-471-58651-X. https://archive.org/details/biochemistry00voet_0/page/399.
- ↑ "Genetics, Cell Biology, and Pathophysiology of Pancreatitis". Gastroenterology 156 (7): 1951–1968.e1. May 2019. doi:10.1053/j.gastro.2018.11.081. PMID 30660731.
- ↑ "Cystic Fibrosis". Pediatr Rev 42 (2): 55–67. February 2021. doi:10.1542/pir.2019-0212. PMID 33526571.
- ↑ "Characterization of human exocrine pancreatic proteins by two-dimensional isoelectric focusing/sodium dodecyl sulfate gel electrophoresis". Gastroenterology 80 (3): 461–73. March 1981. doi:10.1016/0016-5085(81)90007-X. PMID 6969677.
- ↑ "Hereditary pancreatitis is caused by a mutation in the cationic trypsinogen gene". Nature Genetics 14 (2): 141–5. 1996. doi:10.1038/ng1096-141. PMID 8841182.
- ↑ "An overview of hereditary pancreatitiss". Digestive and Liver Disease 44 (1): 8–15. 2011. doi:10.1016/j.dld.2011.08.003. PMID 21907651.
External links
- Trypsinogen at the US National Library of Medicine Medical Subject Headings (MeSH)
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