Editing Proinsulin

{{short description|Precursor protein in humans}}
{{infobox protein
| Name = insulin
| caption = Insulin undergoes extensive posttranslational modification along the production pathway. Production and secretion are largely independent; prepared insulin is stored awaiting secretion. Both C-peptide and mature insulin are biologically active. Cell components and proteins in this image are not to scale.
| image = Insulin path.svg
| width = 220px
| HGNCid = 6081
| Symbol = INS
| AltSymbols =
| EntrezGene = 3630
| OMIM =  176730
| RefSeq = NM_000207
| UniProt =  P01308
| PDB =
| ECnumber =
| Chromosome = 11
| Arm = p
| Band = 15.5
| LocusSupplementaryData =
}}
'''Proinsulin''' is the [[prohormone]] precursor to [[insulin]] made in the [[beta cell]]s of the [[Pancreatic islets|Pancreatic Islets]], specialized regions of the [[pancreas]]. In humans, proinsulin is encoded by the ''INS'' [[gene]].<ref name="entrez">{{cite web | title = Entrez Gene: INS insulin| url =https://www.ncbi.nlm.nih.gov/sites/entrez?Db=gene&Cmd=ShowDetailView&TermToSearch=3630| access-date = }}</ref><ref name="pmid6243748">{{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 }}</ref> The pancreatic islets only secrete between 1% and 3% of proinsulin intact.<ref name="Mayo_Medical">{{Cite web |url= http://www.mayomedicallaboratories.com/interpretive-guide/?alpha=P&unit_code=80908 |title= Interpretation for 80908 Proinsulin, Plasma |website= www.mayomedicallaboratories.com |access-date= 2017-03-09 |archive-date= 2017-10-04 |archive-url= https://web.archive.org/web/20171004085449/https://www.mayomedicallaboratories.com/interpretive-guide/?alpha=P&unit_code=80908 |url-status= dead }}</ref> However, because proinsulin has a longer half life than insulin, it can account for anywhere from 5–30% of the insulin-like structures circulating in the blood.<ref name="Mayo_Medical" /> There are higher concentrations of proinsulin after meals and lower levels when a person is fasting.<ref name="Mayo_Medical" /> Additionally, while proinsulin and insulin have structural differences, proinsulin does demonstrate some affinity for the [[insulin receptor]]. Due to the relative similarities in structure, proinsulin can produce between 5% and 10% of the metabolic activity similarly induced by insulin.<ref name="Mayo_Medical" />

Proinsulin is the final single chain protein structure secreted by cells before cleavage into mature insulin.<ref>{{cite journal | vauthors = Assali NS, Clark KE, Zugaib M, Brinkman CR, Nuwayhid B | title = Effects of estrogenic hormones on uteroplacental hemodynamics and progesterone production in the sheep | journal = International Journal of Fertility | volume = 23 | issue = 3 | pages = 219–23 | year = 1995 | pmid= 40897 | pmc = 8333766 }}</ref> Proinsulin was discovered by Professor [[Donald F. Steiner]] of the University of Chicago in 1967.<ref>{{cite journal | vauthors = Philipson LH, Bell G, Polonsky KS | title = Donald F. Steiner MD, 1930-2014: discoverer of proinsulin | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 4 | pages = 940–1 | date = January 2015 | pmid = 25561547 | pmc = 4313841 | doi = 10.1073/pnas.1423774112 | bibcode = 2015PNAS..112..940P | doi-access = free }}</ref>

== Structure ==
Proinsulin is made up of 86 [[Residue (amino acid)|residues]] in humans (81 in cows),<ref>{{UniProt Full|P01308| INS_HUMAN}}</ref> and formed by three distinct chains.<ref name="Nolan_1971">{{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 }}</ref> The A chain, B chain, and the area connecting the two named the [[C-peptide|C peptide]].<ref name="Nolan_1971" /> 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.<ref name="Nolan_1971" /><ref name="Snell_1975">{{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 }}</ref> 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.<ref name="Nolan_1971" />  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.<ref name="Weiss_2009">{{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 }}</ref>

The C peptide is between the A and B chains of proinsulin.<ref name="Nolan_1971" /> 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.<ref name="Yang_2010">{{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 }}</ref> The C peptide-A chain junction occurs between residues 64 and 65 of proinsulin. These are [[lysine]] and [[arginine]] molecules, respectively.<ref name="Yang_2010" /> The C peptide-B chain connection is between two arginine residues at positions 31 and 32 of proinsulin.<ref name="Yang_2010" />

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.<ref name="Snell_1975" /><ref>{{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 }}</ref> That said, the residues of the C peptide that are conserved across species interact with similarly conserved residues on the A and B chains.<ref name="Snell_1975" /> Thus, it is hypothesized that these conserved residues are important for the functionality of mature insulin.<ref name="Snell_1975" />
{| class="wikitable"
|-
| [[File:Proinsulin 3.png|frameless|center|upright=1.5|class=skin-invert-image]]
|-
| [[File:Proinsulin annotated.png|frameless|center|upright=1.5]]
|-
| 3D Model of proinsulin - A chain is in blue, b chain in red, c peptide in orange. The dibasic cleavage for c peptide and a chain is in green KR (lysine and arginine), the one for c peptide and b chain is in cyan RR (arginine).|3D Model of proinsulin - A chain is in blue, b chain in red, c peptide in orange. The dibasic cleavage for c peptide and a chain is in green KR (lysine and arginine), the one for c peptide and b chain is in cyan RR (arginine).
|}

== Synthesis and Post-translational Modification ==

Proinsulin is synthesized on membrane associated ribosomes found on the [[rough endoplasmic reticulum]], where it is folded and its [[disulfide bond]]s 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|post translational modification]] of proinsulin to mature insulin only occurs in the beta cells of the pancreatic islets.<ref>{{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 }}</ref> When proinsulin is transported through the Golgi apparatus the C-peptide is cleaved.<ref name="Weiss_2009"/> This cleavage occurs with the aid of two endoproteases.<ref name="Kaufmann_1995">{{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 }}</ref> Type I [[endoprotease]]s, [[Proprotein convertase 1|PC1]] and PC3, disrupt the C peptide-B chain connection.<ref name="Kaufmann_1995" /> PC2, a type II endoprotease, cleaves the C peptide-A chain bond.<ref name="Kaufmann_1995" /> The resulting molecule, now mature insulin, is stored as a hexamer in [[secretory vesicles]] and is stabilized with <math>Zn^{2+} </math> ions until it is secreted.<ref name="Weiss_2009"/>
[[File:Proinsulin evolution.png|frameless|center|upright=2.5|class=skin-invert-image]]

== Immunogenicity ==

When insulin was originally purified from [[bovine]] or [[porcine]] pancreata, all the proinsulin was not fully removed.<ref name=Purified>{{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 }}</ref><ref name=Endotext>{{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 }}</ref> 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.<!-- The differences are in the amino acid composition of the respective insulins.  While some skin injuries were due to the non-highly purified insulins causing immunogenic reactions, these issues are still present due to various poor injection techniques, and can occur with any that is poorly used. http://www.endocrine-abstracts.org/ea/0012/ea0012p40.htm  These same amino acid differences in insulin species can cause longer duration which can be a "positive" aspect in some cases. Bovine Ultralente is peakless and lasts longer than Lantus because of this<ref name="pmid3698778">{{cite journal | vauthors = Rizza RA, O'Brien PC, Service FJ | title = Use of beef ultralente for basal insulin delivery: plasma insulin concentrations after chronic ultralente administration in patients with IDDM | journal = Diabetes Care | volume = 9 | issue = 2 | pages = 120–3 | year = 1986 | pmid = 3698778 | doi = 10.2337/diacare.9.2.120 }}</ref> But I question whether this belongs on a page about proinsulin.-->  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.<ref name="pmid6756879">{{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 }}</ref> 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.<ref name="pmid3924216">{{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 }}</ref>{{Clarify|date=August 2009}}

== Medical Relevance ==
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<ref>{{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 }}</ref> in relation to these diseases has become increasingly clear.

=== [[Diabetes mellitus|Diabetes Mellitus]] ===
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]].<ref>{{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 }}</ref> Additional problems with proinsulin that can lead to diabetes include mutations in the number of cysteines present, which could affect correct folding.<ref name="Weiss_2009"/> If the mutation causes only a mild change it could simply stress the endoplasmic reticulum’s ability to properly fold the protein.<ref name="Weiss_2009" /> This stress, after a while, would lead to a decrease in the number of [[Beta cell|β-cells]] producing mature insulin, and would then lead to diabetes mellitus.<ref name="Weiss_2009" />

=== [[Neonatal diabetes mellitus|Neonatal 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.<ref name="Hernández-Sánchez_2006">{{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 }}</ref> 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.<ref name="Hernández-Sánchez_2006" /> Thus far in the study of neonatal diabetes mellitus, only amino acid change mutations found in the B domain lead to the disease.<ref name="Weiss_2009"/>

== See also ==
* [[insulin]]
* [[preproinsulin]]
* [[signal peptide]]
* [[signal peptide peptidase]]
* [[proprotein convertase 1]] (PC1)
* [[proprotein convertase 2]] (PC2)

== References ==
{{reflist}}

[[Category:Peptides]]
[[Category:Diabetes]]