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Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''.
Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprotein, scleroproteins. ''Alpha-keratin, α-Keratin'' is a type of keratin found in vertebrates. It is the key structu ...
(α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
,
hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Ph ...

hair
, nails,
feathers Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both Bird, avian (bird) and some non-avian (non-bird) and possibly other archosauromorpha, archosauromorphs. They are consider ...

feathers
,
hornsHorns or The Horns may refer to: * The Horns (Colorado), a summit on Cheyenne Mountain * ''Horns'' (novel), a dark fantasy novel written in 2010 by Joe Hill ** ''Horns'' (film), a 2013 film adaptation of Hill's novel * "The Horns" (song), a 2015 ...
,
claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bowl in which the blood of sacrificed animals was caught", ...

claw
s,
hooves Cloven hooves of roe deer (''Capreolus capreolus''), with prominent dewclaws A hoof ( or ), plural hooves ( or ) or hoofs , is the tip of a toe Toes are the digits (fingers) of the foot of a tetrapod. Animal Animals (also called M ...
, and the outer layer of
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have differ ...

skin
among vertebrates. Keratin also protects
epithelial Epithelium is one of the four basic types of animal Tissue (biology), tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed Cell (biology), cells with little Ext ...
cells from damage or stress. Keratin is extremely insoluble in water and organic solvents. Keratin
monomer In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

monomer
s assemble into bundles to form
intermediate filament Intermediate filaments (IFs) are cytoskeletal 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking prote ...

intermediate filament
s, which are tough and form strong unmineralized epidermal appendages found in
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s,
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s,
amphibian Amphibians are ectothermic, tetrapod vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial animal, ter ...
s, and
mammal Mammals (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be i ...
s. Excessive keratinization participate in fortification of certain tissues such as in horns of cattle and rhinos, and armadillos' osteoderm. The only other
biological Biology is the natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowl ...

biological
matter known to approximate the
toughness In materials science The Interdisciplinarity, interdisciplinary field of materials science, also commonly termed materials science and engineering, covers the design and discovery of new materials, particularly solids. The intellectual origin ...
of keratinized tissue is
chitin units that repeat to form long chains in β-(1→4)-linkage. of the chitin molecule. Chitin ( carbon, C8H13O5N)n ( ) is a long-chain polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-mer, -mer'', "part") is a Chemical su ...

chitin
. Keratin comes in two types, the primitive, softer forms found in all vertebrates and harder, derived forms found only among
sauropsids Sauropsida ("lizard faces") is a clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—that is, composed of a common ancestor and al ...
(reptiles and birds). Keratin resists digestion, which is why cats regurgitate
hairball A hairball is a small collection of or formed in the of s, and uncommonly in s, that is occasionally up when it becomes too big. Hairballs are primarily a tight elongated cylinder of packed fur, but may include bits of other elements such as ...
s.
Spider silk Spider silk is a protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1 ...

Spider silk
is classified as keratin, although production of the protein may have evolved independently of the process in vertebrates.


Examples of occurrence

Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprotein, scleroproteins. ''Alpha-keratin, α-Keratin'' is a type of keratin found in vertebrates. It is the key structu ...
s (α-keratins) are found in all vertebrates. They form the
hair Hair is a protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Ph ...

hair
(including
wool Wool is the textile A textile is a flexible material made by creating an interlocking bundle of yarn Yarn is a long continuous length of interlocked fibres, suitable for use in the production of textiles, sewing, crocheting, knitti ...
), the outer layer of skin,
hornsHorns or The Horns may refer to: * The Horns (Colorado), a summit on Cheyenne Mountain * ''Horns'' (novel), a dark fantasy novel written in 2010 by Joe Hill ** ''Horns'' (film), a 2013 film adaptation of Hill's novel * "The Horns" (song), a 2015 ...
, nails,
claws A domestic cat's retractable claw in protracted position A claw is a curved, pointed appendage found at the end of a toe or finger in most amniote Amniotes (from Greek ἀμνίον ''amnion'', "membrane surrounding the fetus", earlier "bo ...

claws
and
hooves Cloven hooves of roe deer (''Capreolus capreolus''), with prominent dewclaws A hoof ( or ), plural hooves ( or ) or hoofs , is the tip of a toe Toes are the digits (fingers) of the foot of a tetrapod. Animal Animals (also called M ...
of mammals, and the slime threads of
hagfish Hagfish, of the class Myxini (also known as Hyperotreti) and order Myxiniformes , are eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera Genus (plural ...

hagfish
. Keratin filaments are abundant in
keratinocyte Keratinocytes are the primary type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse ...
s in the hornified layer of the
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...

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 An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and Viral disease, viruses. The antibody recognizes a unique mo ...

antibodies
for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of mouse epithelial cells in genetic studies of the
thymus The thymus is a specialized primary lymphoid organ of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''or ...

thymus
. The harder
beta-keratin β-keratin or beta-keratin is a member of a structural protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of func ...
s (β-keratin)s are found only in the
sauropsid Sauropsida ("lizard faces") is a clade of Amniote, amniotes, broadly equivalent to the Class (biology), class Reptile, Reptilia. Sauropsida is the Sister group, sister taxon to Synapsid, Synapsida, the clade of amniotes which includes Mammal, mamma ...
s, that is all living
reptile Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptile
s and
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s. They are found in the nails,
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory)In the mathematical discipline of descriptive set theory, a scale is a certain kind of object defined on a set (mathematics), set of point (mathematics), points in some Poli ...
, and claws of
reptiles Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptiles
, in some reptile
shells Shell may refer to: Architecture and design * Shell (structure), a thin structure **Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses **Thin-shell structure, **Oil company Science Biology * Seashell ...
(
testudines Turtles are reptile Reptiles are tetrapod Tetrapods (; from Greek 'four' and 'foot') are four-limbed animals constituting the superclass Tetrapoda . It includes extant and extinct amphibians, reptiles (including dinosaurs and ther ...
, such as
tortoise Tortoises () are reptiles Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a ...

tortoise
,
turtle Turtles are an of s known as Testudines, characterized by a developed mainly from their ribs. Modern turtles are divided into two major groups, the s and which differ in the way the head retracts. There are 360 living and recently extinct ...

turtle
,
terrapin Terrapins are one of several small species of turtle Turtles are reptiles of the Order (biology), order Chelonia or Testudines . They are characterized by a special bone, bony or cartilage, cartilaginous animal shell, shell developed fro ...
), and in the
feather Feathers are epidermal growths that form a distinctive outer covering, or plumage Plumage ( "feather") is a layer of feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both avi ...

feather
s,
beak The beak, bill, and/or rostrum is an external anatomical structure found mostly in birds Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beaked jaws, the Ovi ...

beak
s, and claws of
bird Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

bird
s. These keratins are formed primarily in
beta sheet The beta sheet, (β-sheet) (also β-pleated sheet) is a common motif Motif may refer to: General concepts * Motif (chess composition), an element of a move in the consideration of its purpose * Motif (folkloristics), a recurring element that cre ...
s. However, beta sheets are also found in α-keratins. The
baleen Baleen is a filter feeder, filter-feeding system inside the mouths of baleen whales. To use baleen, the whale first opens its mouth underwater to take in water. The whale then pushes the water out, and animals such as krill are filtered by th ...

baleen
plates of filter-feeding
whale Whales are a widely distributed and diverse group of fully s. They are an informal grouping within the infraorder , which usually excludes s and s. Whales, dolphins and porpoises belong to the order , which consists of s. Their closest n ...

whale
s are made of keratin. 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. Keratins (also described as
cytokeratins Cytokeratins are keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprotein, scleroproteins. ''Alpha-keratin, α-Keratin'' is a type of keratin found in vertebrates. It is the key structural material making up Sc ...
) are
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s of type I and type II
intermediate filaments Intermediate filaments (IFs) are cytoskeleton, cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate ''Branchiostoma' ...

intermediate filaments
that have been found only in
chordate A chordate () is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...
s (
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an indiv ...
s,
amphioxus The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates A chordate () is an animal of the phylum Chordata (). During some period of their life cycl ...
,
urochordates A tunicate is a marine invertebrate Invertebrates are animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This includes all animals apart from the subphylum ...
).
Nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

Nematode
s and many other non-chordate animals seem to have only type VI
intermediate filaments Intermediate filaments (IFs) are cytoskeleton, cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate ''Branchiostoma' ...

intermediate filaments
, fibers that structure the nucleus.


Genes

The
human genome The human genome is a complete set of nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides Nucleotides are organic molecules c ...

human genome
encodes 54 functional keratin
gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mecha ...

gene
s, located in two clusters on
chromosome A chromosome is a long DNA molecule with part or all of the genome, genetic material of an organism. Most eukaryotic chromosomes include packaging proteins called histones which, aided by Chaperone (protein), chaperone proteins, bind to and ...

chromosome
s 12 and 17. This suggests that they originated from a series of gene duplications on these chromosomes. The keratins include the following proteins of which KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36,
KRT37 Keratin 37 is a protein that in humans is encoded by the KRT37 gene. KRT37 is a member of the keratin gene family. Clinical significance KRT37 is the only keratin that is regulated by androgens. This sensitivity to androgens was acquired by H ...
, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80,
KRT81 Keratin, type II cuticular Hb1 is a protein that in humans is encoded by the ''KRT81'' gene. The protein encoded by this gene is a member of the keratin gene family. As a type II hair keratin, it is a basic protein which heterodimerizes with type I ...
, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20.


Protein structure

The first sequences of keratins were determined by
Israel Hanukoglu Israel Hanukoglu ( tr, İsrael Hanukoğlu) is a Turkish-born Israel Israel (; he, יִשְׂרָאֵל; ar, إِسْرَائِيل), officially known as the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, '), is a country i ...
and
Elaine Fuchs Elaine V. Fuchs is an American cell biologist, famous for her work on the biology and molecular biology, molecular mechanisms of mammalian skin and skin diseases, who helped lead the modernization of dermatology. Fuchs pioneered reverse genetic ...
(1982, 1983). 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. 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 In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during a ...

monomer
. The major force that keeps the coiled-coil structure is
hydrophobic interactions thumbnail, 250px, A droplet of water forms a spherical shape, minimizing contact with the hydrophobic leaf. The hydrophobic effect is the observed tendency of nonpolar substances to aggregate in an aqueous solution and exclude water#Chemical and p ...

hydrophobic interactions
between
apolar In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in ...
residues along the keratins helical segments. Limited interior space is the reason why the
triple helix In the fields of geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with propertie ...

triple helix
of the (unrelated) structural protein
collagen Collagen () is the main structural protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowder ...

collagen
, found in
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have differ ...

skin
,
cartilage Cartilage (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilage
and
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
, likewise has a high percentage of
glycine Glycine (symbol Gly or G; ) is an amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond ...

glycine
. The connective tissue protein
elastin Elastin is a key protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, f ...
also has a high percentage of both glycine and
alanine Alanine (symbol Ala or A) is an α-amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are ...

alanine
.
Silk Silk is a natural Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxies, and all o ...

Silk
fibroin Fibroin is an insoluble protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew ...
, considered a β-keratin, can have these two as 75–80% of the total, with 10–15%
serine Serine (symbol Ser or S) is an α-amino acid Amino acids are organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, ...

serine
, with the rest having bulky side groups. The chains are antiparallel, with an alternating C → N orientation. A preponderance of
amino acid Amino acids are organic compound In , organic compounds are generally any s that contain - . Due to carbon's ability to (form chains with other carbon s), millions of organic compounds are known. The study of the properties, reactions, a ...

amino acid
s with small, side groups is characteristic of structural proteins, for which H-bonded close packing is more important than
chemical specificity Chemical specificity is the ability of a protein's binding site to bind specific ligand (biochemistry), ligands. The fewer ligands a protein can bind, the greater its specificity. Specificity describes the Chemical affinity, strength of binding b ...
.


Disulfide bridges

In addition to intra- and intermolecular
hydrogen bond A hydrogen bond (or H-bond) is a primarily electrostatic Electrostatics is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department ...

hydrogen bond
s, the distinguishing feature of keratins is the presence of large amounts of the
sulfur Sulfur (in nontechnical British English: sulphur) is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: th ...

sulfur
-containing amino acid
cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the chemical formula, formula HOOC-CH-(NH2)-CH2-SH. The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. The thiol is suscepti ...

cysteine
, required for the
disulfide bridges In chemistry Chemistry is the scientific discipline involved with Chemical element, elements and chemical compound, compounds composed of atoms, molecules and ions: their composition, structure, properties, behavior and the changes they underg ...
that confer additional strength and rigidity by permanent, thermally stable
crosslinkingCross-linking may refer to *Cross-link In chemistry and biology a cross-link is a bond that links one polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecule F ...
—in much the same way that non-protein sulfur bridges stabilize
vulcanized Vulcanization (British: Vulcanisation) refers to a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to i ...

vulcanized
rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much" ...

rubber
. Human hair is approximately 14% cysteine. The
pungent Pungency is the condition of having a strong, sharp smell or flavor Flavor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the Engli ...
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 agents. The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in
mammalian Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of t ...
fingernail A nail is a claw-like plate at the tip of the fingers and toes in most primate A primate ( ) (from Latin , from 'prime, first rank') is a eutheria Eutheria (; from Greek , 'good, right' and , 'beast'; ) is the clade A clade (; from ...
s, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes. Hair and other α-keratins consist of coiled single protein strands (with regular intra-chain ), which are then further twisted into superhelical
rope A rope is a group of yarns, Plying, plies, fibers or strands that are twisted or braided together into a larger and stronger form. Ropes have tensile strength and so can be used for dragging and lifting. Rope is thicker and stronger than simil ...

rope
s 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.


Filament formation

It has been proposed that keratins can be divided into 'hard' and 'soft' forms, or '
cytokeratin Cytokeratins are keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprotein, scleroproteins. ''Alpha-keratin, α-Keratin'' is a type of keratin found in vertebrates. It is the key structural material making up Sc ...
s' and 'other keratins'. 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 filament Intermediate filaments (IFs) are cytoskeletal 300px, The eukaryotic cytoskeleton. Actin filaments are shown in red, and microtubules composed of beta tubulin are in green. The cytoskeleton is a complex, dynamic network of interlinking prote ...

intermediate filament
s. 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.


Pairing


Cornification

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 cell death, 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.


Silk

The silk
fibroin Fibroin is an insoluble protein Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew ...
s 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 spinneret (spider), 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 biology, biologically and commerce, commercially useful properties of spider silk#Properties, 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 random coil, randomly coiled. A somewhat analogous situation occurs with chemical synthesis, synthetic
polymer A polymer (; Greek ''poly- Poly, from the Greek :wikt:πολύς, πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong inc ...

polymer
s such as nylon, developed as a silk substitute. Silk from the hornet Silkworm cocoons, 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 adhesive, glue, as do spiders.


Clinical significance

Some infection, infectious fungus , fungi, such as those that cause athlete's foot and ringworm (i.e. the dermatophytes) feed on keratin. Diseases caused by mutations in the keratin genes include: * Epidermolysis bullosa simplex * Ichthyosis bullosa of Siemens * Epidermolytic hyperkeratosis * Steatocystoma multiplex * Keratosis pharyngis * Rhabdoid cell formation in Large cell lung carcinoma with rhabdoid phenotype 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 metastasis , 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. Keratin is highly resistant to digestive acids if ingested, as occurs in the human disorder trichophagia. Thus, cats (which groom themselves with their tongues) regularly ingest hair, leading to the gradual formation of a
hairball A hairball is a small collection of or formed in the of s, and uncommonly in s, that is occasionally up when it becomes too big. Hairballs are primarily a tight elongated cylinder of packed fur, but may include bits of other elements such as ...
that may be vomited. Rapunzel syndrome, an extremely rare but potentially fatal intestinal condition in humans, is caused by trichophagia.


See also

* List of cutaneous conditions caused by mutations in keratins * List of keratins expressed in the human integumentary system * List of keratins


References


External links


Composition and β-sheet structure of silkHair-Science.com's entry on the microscopic elements of hair

Proteopedia page on keratins
{{Authority control Keratins Cytoskeleton Skin anatomy