Proinsulin

Proinsulin is made up of 86 residues in humans (81 in cows),{{UniProt Full|P01308| INS_HUMAN}} and formed by three distinct chains.{{cite journal | vauthors = Nolan C, Margoliash E, Peterson JD, Steiner DF | title = The structure of bovine proinsulin | journal = The Journal of Biological Chemistry | volume = 246 | issue = 9 | pages = 2780–95 | date = May 1971 | doi = 10.1016/S0021-9258(18)62252-5 | pmid = 4928892 | url = http://www.jbc.org/content/246/9/2780 | doi-access = free }} The A chain, B chain, and the area connecting the two named the C peptide. The correct structure of proinsulin is crucial for the correct folding of mature insulin, as the placement of the C peptide sets the molecule up to create correctly positioned disulfide bonds in and between the A and B chains.{{cite journal | vauthors = Snell CR, Smyth DG | title = Proinsulin: a proposed three-dimensional structure | journal = The Journal of Biological Chemistry | volume = 250 | issue = 16 | pages = 6291–5 | date = August 1975 | doi = 10.1016/S0021-9258(19)41065-X | pmid = 808541 | url = http://www.jbc.org/content/250/16/6291 | doi-access = free }} There are three disulfide bonds that are necessary for mature insulin to be the correct structure. Two of these disulfide bonds are between the A and B chains, and one is an intra-A chain bond. The disulfide bonds occur between the seventh residues of the A and B chain, the 20th residue of the A chain and the 19th residue of the B chain, and the 6th and 11th residues of the A chain.{{cite journal | vauthors = Weiss MA | title = Proinsulin and the genetics of diabetes mellitus | journal = The Journal of Biological Chemistry | volume = 284 | issue = 29 | pages = 19159–63 | date = July 2009 | pmid = 19395706 | pmc = 2740536 | doi = 10.1074/jbc.R109.009936 | doi-access = free }}

The C peptide is between the A and B chains of proinsulin. The connection between the A chain and C peptide is much more stable than the junction between the C peptide and B chain, with alpha helical features being exhibited near the C peptide-A chain connection.{{cite journal | vauthors = Yang Y, Hua QX, Liu J, Shimizu EH, Choquette MH, Mackin RB, Weiss MA | title = Solution structure of proinsulin: connecting domain flexibility and prohormone processing | journal = The Journal of Biological Chemistry | volume = 285 | issue = 11 | pages = 7847–51 | date = March 2010 | pmid = 20106974 | pmc = 2832934 | doi = 10.1074/jbc.c109.084921 | doi-access = free }} The C peptide-A chain junction occurs between residues 64 and 65 of proinsulin. These are lysine and arginine molecules, respectively. The C peptide-B chain connection is between two arginine residues at positions 31 and 32 of proinsulin.

There is conservation of much of the structure of proinsulin among mammalian species, with much of the residue changes seen from one species to another present in the C peptide.{{cite journal | vauthors = Bell GI, Pictet RL, Rutter WJ, Cordell B, Tischer E, Goodman HM | title = Sequence of the human insulin gene | journal = Nature | volume = 284 | issue = 5751 | pages = 26–32 | date = March 1980 | pmid = 6243748 | doi = 10.1038/284026a0 | bibcode = 1980Natur.284...26B | s2cid = 4363706 }} That said, the residues of the C peptide that are conserved across species interact with similarly conserved residues on the A and B chains. Thus, it is hypothesized that these conserved residues are important for the functionality of mature insulin.

Proinsulin is synthesized on membrane associated ribosomes found on the rough endoplasmic reticulum, where it is folded and its disulfide bonds are oxidized. It is then transported to the Golgi apparatus where it is packaged into secretory vesicles, and where it is processed by a series of proteases to form mature insulin. Mature insulin has 35 fewer amino acids; 4 are removed altogether, and the remaining 31 form the C-peptide. The C-peptide is abstracted from the center of the proinsulin sequence; the two other ends (the B chain and A chain) remain connected by disulfide bonds.{{cn|date=November 2023}}

The post translational modification of proinsulin to mature insulin only occurs in the beta cells of the pancreatic islets.{{cite journal | vauthors = Groskreutz DJ, Sliwkowski MX, Gorman CM | title = Genetically engineered proinsulin constitutively processed and secreted as mature, active insulin | journal = The Journal of Biological Chemistry | volume = 269 | issue = 8 | pages = 6241–5 | date = February 1994 | doi = 10.1016/S0021-9258(17)37593-2 | pmid = 8119968 | url = http://www.jbc.org/content/269/8/6241.full.pdf | doi-access = free }} When proinsulin is transported through the Golgi apparatus the C-peptide is cleaved. This cleavage occurs with the aid of two endoproteases.{{cite journal | vauthors = Kaufmann JE, Irminger JC, Halban PA | title = Sequence requirements for proinsulin processing at the B-chain/C-peptide junction | journal = The Biochemical Journal | volume = 310 | issue = 3 | pages = 869–74 | date = September 1995 | pmid = 7575420 | pmc = 1135976 | doi = 10.1042/bj3100869 }} Type I endoproteases, PC1 and PC3, disrupt the C peptide-B chain connection. PC2, a type II endoprotease, cleaves the C peptide-A chain bond. The resulting molecule, now mature insulin, is stored as a hexamer in secretory vesicles and is stabilized with $Zn^\{2+\}$ ions until it is secreted.

File:Proinsulin evolution.png

When insulin was originally purified from bovine or porcine pancreata, all the proinsulin was not fully removed.{{cite journal | vauthors = Wilson RM, Douglas CA, Tattersall RB, Reeves WG | title = Immunogenicity of highly purified bovine insulin: a comparison with conventional bovine and highly purified human insulins | journal = Diabetologia | volume = 28 | issue = 9 | pages = 667–70 | date = September 1985 | pmid = 3905477 | doi = 10.1007/BF00291973 | doi-access = free }}{{cite web | url = http://www.endotext.org/diabetes/diabetes17/diabetes17.htm | title = Insulin Pharmacology, Type of Regimens and Adjustments | vauthors = Tanyolac S, Goldfine ID, Kroon L | date = | website = | publisher = Endotext.com | access-date = 2011-03-18 | archive-url = https://web.archive.org/web/20110725130334/http://www.endotext.org/diabetes/diabetes17/diabetes17.htm | archive-date = 2011-07-25 | url-status = dead }} When some people used these insulins, the proinsulin may have caused the body to react with a rash, to resist the insulin, or even to make dents or lumps in the skin at the place where the insulin was injected. This can be described as an iatrogenic injury due to slight differences between the proinsulin of different species. Since the late 1970s, when highly purified porcine insulin was introduced, and the level of insulin purity reached 99%, this ceased to be a significant clinical issue.{{cite journal | vauthors = Home PD, Alberti KG | title = The new insulins. Their characteristics and clinical indications | journal = Drugs | volume = 24 | issue = 5 | pages = 401–13 | date = November 1982 | pmid = 6756879 | doi = 10.2165/00003495-198224050-00003 | s2cid = 28616749 | url = https://www.semanticscholar.org/paper/05663202b5ffa42441c735516788e4fa9eeece68 }} With respect to their influence on insulin pharmacokinetics, moderate concentrations of certain insulin antibodies may be of positive advantage to all diabetics without endogenous insulin secretion (e.g. people with type 1 diabetes) because insulin binding antibodies effectively increase the insulin's clearance rate and distribution space and help to prolong its pharmacological and biological half lives.{{cite journal | vauthors = Gray RS, Cowan P, di Mario U, Elton RA, Clarke BF, Duncan LJ | title = Influence of insulin antibodies on pharmacokinetics and bioavailability of recombinant human and highly purified beef insulins in insulin dependent diabetics | journal = British Medical Journal | volume = 290 | issue = 6483 | pages = 1687–91 | date = June 1985 | pmid = 3924216 | pmc = 1416075 | doi = 10.1136/bmj.290.6483.1687 }}{{Clarify|date=August 2009}}

Historically, the focus of many insulin related metabolic diseases has focused on mature insulin. However, in recent years the importance of studying the structure and function of proinsulin or proinsulin:insulin ratio{{cite journal | vauthors = Mezza T, Ferraro PM, Sun VA, Moffa S, Cefalo CM, Quero G, Cinti F, Sorice GP, Pontecorvi A, Folli F, Mari A, Alfieri S, Giaccari A | display-authors = 6 | title = Increased β-Cell Workload Modulates Proinsulin-to-Insulin Ratio in Humans | journal = Diabetes | volume = 67 | issue = 11 | pages = 2389–2396 | date = November 2018 | pmid = 30131390 | doi = 10.2337/db18-0279 | hdl = 2434/587996 | s2cid = 52058695 | hdl-access = free }} in relation to these diseases has become increasingly clear.

Increased levels of proinsulin in the circulatory system relative to mature insulin concentrations can indicate impending insulin resistance and the development of type 2 diabetes.{{cite journal | vauthors = Mykkänen L, Haffner SM, Hales CN, Rönnemaa T, Laakso M | title = The relation of proinsulin, insulin, and proinsulin-to-insulin ratio to insulin sensitivity and acute insulin response in normoglycemic subjects | journal = Diabetes | volume = 46 | issue = 12 | pages = 1990–5 | date = December 1997 | pmid = 9392485 | doi = 10.2337/diab.46.12.1990 | s2cid = 44874023 }} Additional problems with proinsulin that can lead to diabetes include mutations in the number of cysteines present, which could affect correct folding. If the mutation causes only a mild change it could simply stress the endoplasmic reticulum’s ability to properly fold the protein. This stress, after a while, would lead to a decrease in the number of β-cells producing mature insulin, and would then lead to diabetes mellitus.

Postnatal proinsulin is crucial for metabolic regulation. However, proinsulin in neonates is important for normal development of the nerves of the eye, development of the heart, and general survival of embryonic cells.{{cite journal | vauthors = Hernández-Sánchez C, Mansilla A, de la Rosa EJ, de Pablo F | title = Proinsulin in development: New roles for an ancient prohormone | journal = Diabetologia | volume = 49 | issue = 6 | pages = 1142–50 | date = June 2006 | pmid = 16596360 | doi = 10.1007/s00125-006-0232-5 | doi-access = free | hdl = 10261/72608 | hdl-access = free }} Regulation of the concentration of proinsulin during embryonic development is crucial, as too much or too little of the peptide can cause defects and death of the fetus. Thus far in the study of neonatal diabetes mellitus, only amino acid change mutations found in the B domain lead to the disease.

• insulin
• preproinsulin
• signal peptide
• signal peptide peptidase
• proprotein convertase 1 (PC1)
• proprotein convertase 2 (PC2)

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Category:Peptides

Category:Diabetes