Keratin

File:Male impala profile.jpg of the impala are made of keratin covering a core of bone.]]

Alpha-keratins (α-keratins) are found in all vertebrates. They form the hair (including wool), the outer layer of skin, horns, nails, claws and hooves of mammals, and the slime threads of hagfish. The baleen plates of filter-feeding whales are also made of keratin. Keratin filaments are abundant in keratinocytes in the hornified layer of the epidermis; these are proteins which have undergone keratinization. They are also present in epithelial cells in general. For example, mouse thymic epithelial cells react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of mouse thymic epithelial cells in genetic studies of the thymus.

The harder beta-keratins (β-keratins) are found only in the sauropsids, i.e., all living reptiles and birds. They are found in the nails, scales, and claws of reptiles, in some reptile shells (Testudines), and in the feathers, beaks, and claws of birds.{{Cite book | author1=Hickman, Cleveland Pendleton | author2=Roberts, Larry S. | author3=Larson, Allan L. | title=Integrated principles of zoology | year=2003 | publisher=McGraw-Hill | location=Dubuque, IA | isbn=978-0-07-243940-3 | page=[https://archive.org/details/isbn_9780072930283/page/538 538] | url-access=registration | url=https://archive.org/details/isbn_9780072930283/page/538 }} These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins.{{cite journal|last1=Kreplak|first1=L.|last2=Doucet|first2=J.|last3=Dumas|first3=P.|last4=Briki|first4=F.|title=New Aspects of the α-Helix to β-Sheet Transition in Stretched Hard α-Keratin Fibers|journal=Biophysical Journal|date=July 2004|volume=87|issue=1|pages=640–647|doi=10.1529/biophysj.103.036749|pmid=15240497|pmc=1304386|bibcode=2004BpJ....87..640K}}

Recent scholarship has shown that sauropsid β-keratins are fundamentally different from α-keratins at a genetic and structural level. The new term corneous beta protein (CBP) has been proposed to avoid confusion with α-keratins.{{cite journal |last1=Alibardi |first1=Lorenzo |title=Sauropsids Cornification is Based on Corneous Beta-Proteins, a Special Type of Keratin-Associated Corneous Proteins of the Epidermis |journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution |date=September 2016 |volume=326 |issue=6 |pages=338–351 |doi=10.1002/jez.b.22689 |pmid=27506161 |bibcode=2016JEZB..326..338A }}

Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments that have been found only in chordates (vertebrates, amphioxi, urochordates). Nematodes and many other non-chordate animals seem to have only type VI intermediate filaments, fibers that structure the nucleus.

{{Cleanup section|reason=Not particularly helpful to dump a big list of KRT genes here. Using the source a bit more to explain what each gene and each zone of genes mean will be helpful, as we currently have no particular examples of a hair keratin.|date=October 2022}}

File:Chromosome 12.svg

File:Chromosome 17.svg

The human genome encodes 54 functional keratin genes, located in two clusters on chromosomes 12 and 17. This suggests that they originated from a series of gene duplications on these chromosomes.{{cite journal |last1=Moll |first1=Roland |last2=Divo |first2=Markus |last3=Langbein |first3=Lutz |title=The human keratins: biology and pathology |journal=Histochemistry and Cell Biology |date=June 2008 |volume=129 |issue=6 |pages=705–733 |doi=10.1007/s00418-008-0435-6 |pmid=18461349 |pmc=2386534 }}

The keratins include the following proteins of which KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36, KRT37, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20.{{cite journal |vauthors=Schweizer J, Bowden PE, Coulombe PA, etal |title=New consensus nomenclature for mammalian keratins |journal=J. Cell Biol. |volume=174 |issue=2 |pages=169–74 |date=July 2006 |pmid=16831889 |pmc=2064177 |doi=10.1083/jcb.200603161 }}

File:Human Keratins 1-8 Protein Alignment Rod Domain.tif

The first sequences of keratins were determined by Israel Hanukoglu and Elaine Fuchs (1982, 1983).{{cite journal |last1=Hanukoglu |first1=Israel |last2=Fuchs |first2=Elaine |title=The cDNA sequence of a human epidermal keratin: Divergence of sequence but conservation of structure among intermediate filament proteins |journal=Cell |date=November 1982 |volume=31 |issue=1 |pages=243–252 |doi=10.1016/0092-8674(82)90424-x |pmid=6186381 }}{{cite journal |last1=Hanukoglu |first1=Israel |last2=Fuchs |first2=Elaine |title=The cDNA sequence of a type II cytoskeletal keratin reveals constant and variable structural domains among keratins |journal=Cell |date=July 1983 |volume=33 |issue=3 |pages=915–924 |doi=10.1016/0092-8674(83)90034-x |pmid=6191871 }} These sequences revealed that there are two distinct but homologous keratin families, which were named type I and type II keratins. By analysis of the primary structures of these keratins and other intermediate filament proteins, Hanukoglu and Fuchs suggested a model in which keratins and intermediate filament proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation. This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.{{cite journal |last1=Lee |first1=Chang-Hun |last2=Kim |first2=Min-Sung |last3=Chung |first3=Byung Min |last4=Leahy |first4=Daniel J |last5=Coulombe |first5=Pierre A |title=Structural basis for heteromeric assembly and perinuclear organization of keratin filaments |journal=Nature Structural & Molecular Biology |date=July 2012 |volume=19 |issue=7 |pages=707–715 |doi=10.1038/nsmb.2330 |pmid=22705788 |pmc=3864793 }}

The human genome has 54 functional annotated keratin genes, of which 28 are type I keratins and 26 are type II keratins.{{cite book |doi=10.1016/B978-0-12-819460-7.00037-2 |quote=Type I and type II IFs are part of the keratin (or cytokeratin) family of proteins found in all epithelia. The human genome features 54 functional keratin genes, with 28 type I and 26 type II keratin genes (see Table 1). Type I keratins tend to be smaller and acidic compared to the larger, neutral–basic type II keratins. |chapter=Cytoskeleton | Intermediate Filaments |title=Encyclopedia of Biological Chemistry III |date=2021 |last1=Bernot |first1=Kelsie M. |last2=Coulombe |first2=Pierre A. |last3=Zaher |first3=Hani |pages=193–199 |isbn=978-0-12-822040-5 }}

File:Keratin.jpg cell and oval cells of horse liver.]]

Fibrous keratin molecules supercoil to form a very stable, left-handed superhelical motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer.{{Cite book |page=158 |quote=Fibrous proteins are characterized by a single type of secondary structure: a keratin is a left-handed coil of two a helices |chapter-url=http://biochem118.stanford.edu/Papers/Protein%20Papers/Voet%26Voet%20chapter6.pdf |archive-url=https://web.archive.org/web/20060917080333/http://biochem118.stanford.edu/Papers/Protein%20Papers/Voet%26Voet%20chapter6.pdf |archive-date=2006-09-17 |url-status=live |chapter=Proteins: Three-Dimensional Structure |last1=Voet |first1=Donald |last2=Voet |first2=Judith G. |last3=Pratt |first3=Charlotte W. |title=Fundamentals of Biochemistry |date=1998 |publisher=Wiley |isbn=978-0-471-58650-0 }}

The major force that keeps the coiled-coil structure is hydrophobic interactions between apolar residues along the keratin's helical segments.{{cite journal |last1=Hanukoglu |first1=Israel |last2=Ezra |first2=Liora |title=Proteopedia entry: Coiled-coil structure of keratins: Multimedia in Biochemistry and Molecular Biology Education |journal=Biochemistry and Molecular Biology Education |date=January 2014 |volume=42 |issue=1 |pages=93–94 |doi=10.1002/bmb.20746 |pmid=24265184 |doi-access=free }}

Limited interior space is the reason why the triple helix of the (unrelated) structural protein collagen, found in skin, cartilage and bone, likewise has a high percentage of glycine. The connective tissue protein elastin also has a high percentage of both glycine and alanine. Silk fibroin, considered a β-keratin, can have these two as 75–80% of the total, with 10–15% serine, with the rest having bulky side groups. The chains are antiparallel, with an alternating C → N orientation.{{cite web|url=http://elmhurst.edu/~chm/vchembook/566secprotein.html |title=Secondary Protein |publisher=Elmhurst.edu |access-date=2010-09-23 |url-status=dead |archive-url=https://web.archive.org/web/20100922111144/http://elmhurst.edu/~chm/vchembook/566secprotein.html |archive-date=2010-09-22 }} A preponderance of amino acids with small, nonreactive side groups is characteristic of structural proteins, for which H-bonded close packing is more important than chemical specificity.

In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking{{cite web|title=What is Keratin?|url=http://www.wisegeek.org/what-is-keratin.htm|publisher=WiseGEEK|access-date=11 May 2014|archive-date=13 May 2014|archive-url=https://web.archive.org/web/20140513010609/http://www.wisegeek.org/what-is-keratin.htm|url-status=live}}—in much the same way that non-protein sulfur bridges stabilize vulcanized rubber. Human hair is approximately 14% cysteine. The pungent smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the insolubility of keratins, except in a small number of solvents such as dissociating or reducing agents.

File:Toe nail.jpg that fell off after a small trauma.]]

The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian fingernails, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes.{{cite journal |last1=H Bragulla |first1=Hermann |last2=G Homberger |first2=Dominique |title=Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia |journal=Journal of Anatomy |year=2009 |volume=214 |issue=4 |pages=516–559 |doi=10.1111/j.1469-7580.2009.01066.x |pmid=19422428 |pmc=2736122 }} Hair and other α-keratins consist of α-helically coiled single protein strands (with regular intra-chain H-bonding), which are then further twisted into superhelical ropes that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.

Thiolated polymers (thiomers) can form disulfide bridges with cysteine substructures of keratins getting covalently attached to these proteins.{{cite journal |last1=Leichner |first1=C |last2=Jelkmann |first2=M |last3=Bernkop-Schnürch |first3=A |title=Thiolated polymers: Bioinspired polymers utilizing one of the most important bridging structures in nature |journal=Adv Drug Deliv Rev |date=2019 |volume=151-152 |pages=191–221 |doi=10.1016/j.addr.2019.04.007 |pmid=31028759 }} Thiomers therefore exhibit high binding properties to keratins found in hair,{{cite patent |inventor1-last=Hawkins |inventor1-first=Geoffrey |inventor2-last=Afriat |inventor2-first=Isabelle R |inventor3-last=Xavier |inventor3-first=Jean Harry |inventor4-last=Popescu |inventor4-first=Lavinia C |title=Cosmetic compositions containing thiomers for hair color retention |country=US |number=20110229430A1 |pubdate=22 September 2011 }} on skin{{cite journal |last1=Grießinger |first1=JA |last2=Bonengel |first2=S |last3=Partenhauser |first3=A |last4=Ijaz |first4=M |last5=Bernkop-Schnürch |first5=A |title=Thiolated polymers: Evaluation of their potential as dermoadhesive excipients |journal= Drug Dev. Ind. Pharm. |date=2017 |volume=43 |issue=2 |pages=204–212 |doi=10.1080/03639045.2016.1231809 |pmid=27585266 }}{{cite journal |last1=Partenhauser |first1=A |last2= Zupančič |first2=O |last3=Rohrer |first3=J |last4=Bonengel |first4=S |last5=Bernkop-Schnürch |first5=A |title=Thiolated silicone oils as adhesive skin protectants for improved barrier function |journal= Int. J. Cosm. Sci. |date=2015 |volume=38 |issue=3 |pages=257–265|doi=10.1111/ics.12284 | pmid=26444859 }} and on the surface of many cell types.{{cite journal |last1=Le-Vinh |first1=B |last2=Steinbring |first2=C |last3=Nguyen Le |first3=NM |last4= Matuszczak |first4=B |last5=Bernkop-Schnürch |first5=A |title= S-Protected thiolated chitosan versus thiolated chitosan as cell adhesive biomaterials for tissue engineering. |journal= ACS Appl Mater Interfaces |date=2023 |volume=15 |issue=34 |pages=40304–40316 |doi=10.1021/acsami.3c09337 | pmid=37594415|pmc=10472333 }}

It has been proposed that keratins can be divided into 'hard' and 'soft' forms, or 'cytokeratins' and 'other keratins'.{{Clarify|date=May 2010}}{{dubious|reason=Not supported by source. Every keratin is cytokeratin until it gets squeezed out by cell death, which happens a bit more often for hair/nail (hard) stuff. The ref does not recommend the cyto- name at all.|date=October 2022}} That model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account.

Keratin filaments are intermediate filaments. Like all intermediate filaments, keratin proteins form filamentous polymers in a series of assembly steps beginning with dimerization; dimers assemble into tetramers and octamers and eventually, if the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments.

Cornification is the process of forming an epidermal barrier in

stratified squamous epithelial tissue. At the cellular level,

cornification is characterised by:

• production of keratin
• production of small proline-rich (SPRR) proteins and transglutaminase which eventually form a cornified cell envelope beneath the plasma membrane
• terminal differentiation
• loss of nuclei and organelles, in the final stages of cornification

Metabolism ceases, and the cells are almost completely filled by keratin. During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton to mechanically stabilize the cell against physical stress. It does this through connections to desmosomes, cell–cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures.

Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis and form protective calluses, which are useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced.

These hard, integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by specialized beds deep within the skin. Hair grows continuously and feathers molt and regenerate. The constituent proteins may be phylogenetically homologous but differ somewhat in chemical structure and supermolecular organization. The evolutionary relationships are complex and only partially known. Multiple genes have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.

{{More citations needed|date=January 2022}}

The silk fibroins produced by insects and spiders are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins.

Silk found in insect pupae, and in spider webs and egg casings, also has twisted β-pleated sheets incorporated into fibers wound into larger supermolecular aggregates. The structure of the spinnerets on spiders' tails, and the contributions of their interior glands, provide remarkable control of fast extrusion. Spider silk is typically about 1 to 2 micrometers (μm) thick, compared with about 60 μm for human hair, and more for some mammals. The biologically and commercially useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, crystalline regions of varying size, alternating with flexible, amorphous regions where the chains are randomly coiled.{{cite web|author=Australia |url=http://www.amonline.net.au/spiders/toolkit/silk/structure.htm |title=Spiders – Silk structure |publisher=Amonline.net.au |access-date=2010-09-23 |url-status=dead |archive-url=https://web.archive.org/web/20090508161836/http://www.amonline.net.au/spiders/toolkit/silk/structure.htm |archive-date=2009-05-08 }} A somewhat analogous situation occurs with synthetic polymers such as nylon, developed as a silk substitute. Silk from the hornet cocoon contains doublets about 10 μm across, with cores and coating, and may be arranged in up to 10 layers, also in plaques of variable shape. Adult hornets also use silk as a glue, as do spiders.

Abnormal growth of keratin can occur in a variety of conditions including keratosis, hyperkeratosis and keratoderma.

Mutations in keratin gene expression can lead to, among others:

• Alopecia areata
• Epidermolysis bullosa simplex
• Ichthyosis bullosa of Siemens
• Epidermolytic hyperkeratosis
• Steatocystoma multiplex
• Keratosis pharyngis
• Rhabdoid cell formation in large cell lung carcinoma with rhabdoid phenotype{{cite journal |last1=Shiratsuchi |first1=Hideki |last2=Saito |first2=Tsuyoshi |last3=Sakamoto |first3=Akio |last4=Itakura |first4=Eijun |last5=Tamiya |first5=Sadafumi |last6=Oshiro |first6=Yumi |last7=Oda |first7=Yoshinao |last8=Toh |first8=Satoshi |last9=Komiyama |first9=Sohtaro |last10=Tsuneyoshi |first10=Masazumi |title=Mutation Analysis of Human Cytokeratin 8 Gene in Malignant Rhabdoid Tumor: A Possible Association with Intracytoplasmic Inclusion Body Formation |journal=Modern Pathology |date=February 2002 |volume=15 |issue=2 |pages=146–153 |doi=10.1038/modpathol.3880506 |pmid=11850543 |doi-access=free }}{{cite journal |last1=Itakura |first1=Eijun |last2=Tamiya |first2=Sadafumi |last3=Morita |first3=Keisuke |last4=Shiratsuchi |first4=Hideki |last5=Kinoshita |first5=Yoshiaki |last6=Oshiro |first6=Yumi |last7=Oda |first7=Yoshinao |last8=Ohta |first8=Shigeru |last9=Furue |first9=Masutaka |last10=Tsuneyoshi |first10=Masazumi |title=Subcellular Distribution of Cytokeratin and Vimentin in Malignant Rhabdoid Tumor: Three-Dimensional Imaging with Confocal Laser Scanning Microscopy and Double Immunofluorescence |journal=Modern Pathology |date=September 2001 |volume=14 |issue=9 |pages=854–861 |doi=10.1038/modpathol.3880401 |pmid=11557780 |doi-access=free }}

Several diseases, such as athlete's foot and ringworm, are caused by infectious fungi that feed on keratin.{{cite journal |last1=Mercer |first1=Derry K |last2=Stewart |first2=Colin S |title=Keratin hydrolysis by dermatophytes |journal=Medical Mycology |date=1 January 2019 |volume=57 |issue=1 |pages=13–22 |doi=10.1093/mmy/myx160 |pmid=29361043 }}

Keratin is highly resistant to digestive acids if ingested. Cats regularly ingest hair as part of their grooming behavior, leading to the gradual formation of hairballs that may be expelled orally or excreted. In humans, trichophagia may lead to Rapunzel syndrome, an extremely rare but potentially fatal intestinal condition.

Keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas, thymomas, sarcomas and trophoblastic neoplasms. Furthermore, the precise expression-pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases. For example, hepatocellular carcinomas typically express CK8 and CK18, and cholangiocarcinomas express CK7, CK8 and CK18, while metastases of colorectal carcinomas express CK20, but not CK7.{{cite journal |last1=Omary |first1=M. Bishr |last2=Ku |first2=Nam-On |last3=Strnad |first3=Pavel |last4=Hanada |first4=Shinichiro |title=Toward unraveling the complexity of simple epithelial keratins in human disease |journal=Journal of Clinical Investigation |date=1 July 2009 |volume=119 |issue=7 |pages=1794–1805 |doi=10.1172/JCI37762 |pmid=19587454 |pmc=2701867 }}

• Keratin-associated proteins (KRTAPs)
• List of cutaneous conditions caused by mutations in keratins
• List of keratins expressed in the human integumentary system
• List of keratins
• Keratinase

{{reflist|30em}}

{{Americana Poster}}

• [https://web.archive.org/web/20060220123505/http://www.elmhurst.edu/~chm/vchembook/566secprotein.html Composition and β-sheet structure of silk]
• [https://web.archive.org/web/20060113171639/http://www.hair-science.com/_int/_en/topic/topic_sousrub.aspx?tc=ROOT-HAIR-SCIENCE%5EPORTRAIT-OF-AN-UNKNOWN-ELEMENT%5ESUPERB-CHEMISTRY&cur=SUPERB-CHEMISTRY Hair-Science.com's entry on the microscopic elements of hair]
• [http://www.proteopedia.org/w/Keratins Proteopedia page on keratins]

{{Fibrous proteins}}

{{Cytoskeletal proteins}}

{{Authority control}}

Category:Keratins

Category:Cytoskeleton

Category:Skin anatomy