Keratin
Encyclopedia
Fibrous protein
Scleroproteins, or fibrous proteins, constitute one of the three main classes of proteins, alongside globular proteins and conjugated proteins.Keratin, collagen, elastin, and fibroin are all scleroproteins...
. Keratin is the key of structural material making up the outer layer of human skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
. It is also the key structural component of hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
and nails
Nail (anatomy)
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....
. Keratin monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
s assemble into bundles to form intermediate filament
Intermediate filament
Intermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin , and that of 25 nm microtubules, although they were initially designated...
s, which are tough and insoluble and form strong unmineralized
Mineralization (biology)
In biology, mineralization refers to the process where an organic substance is converted to an inorganic substance.This may also be a normal biological process which takes place during the life of an organism such as the formation of bone tissue or egg shells, largely with calcium.This term may...
tissues found in reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s, and mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s. The only other biological
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
matter known to approximate the toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
of keratinized tissue is chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
.
Function
Keratin filaments are abundant in keratinocyteKeratinocyte
Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes"...
s in the cornified layer of the epidermis; these are cells which have undergone keratinization. In addition, keratin filaments are present in epithelial cells in general. For example, mouse thymic epithelial cells (TECs) are known to react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus.
- the α-keratins in the hairHairHair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
(including woolWoolWool is the textile fiber obtained from sheep and certain other animals, including cashmere from goats, mohair from goats, qiviut from muskoxen, vicuña, alpaca, camel from animals in the camel family, and angora from rabbits....
), hornsHorn (anatomy)A horn is a pointed projection of the skin on the head of various animals, consisting of a covering of horn surrounding a core of living bone. True horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae and Bovidae...
, nailsNail (anatomy)A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....
, clawClawA claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...
s and hoovesHoofA hoof , plural hooves or hoofs , is the tip of a toe of an ungulate mammal, strengthened by a thick horny covering. The hoof consists of a hard or rubbery sole, and a hard wall formed by a thick nail rolled around the tip of the toe. The weight of the animal is normally borne by both the sole...
of mammals - the harder β-keratins found in nails and in the scalesScale (zoology)In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
and claws of reptiles, their shells (Testudines, such as tortoiseTortoiseTortoises are a family of land-dwelling reptiles of the order of turtles . Like their marine cousins, the sea turtles, tortoises are shielded from predators by a shell. The top part of the shell is the carapace, the underside is the plastron, and the two are connected by the bridge. The tortoise...
, turtleTurtleTurtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
, terrapinTerrapinA terrapin is a turtle living in fresh or brackish water.Terrapin may also refer to:* Terrapin , a transport vehicle used for amphibious assault by the Allies during the Second World War...
), and in the featherFeatherFeathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
s, beakBeakThe beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young...
s, claws of birds and quills of porcupines. (These keratins are formed primarily in beta sheetBeta sheetThe β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
s. However, beta sheets are also found in α-keratins.)
The baleen
Baleen
Baleen or whalebone is a filter-feeder system inside the mouths of baleen whales. The baleen system works when a whale opens its mouth underwater and then water pours into the whale's mouth. The whale then pushes the water out, and animals such as krill are filtered by the baleen and remain as food...
plates of filter-feeding whale
Whale
Whale is the common name for various marine mammals of the order Cetacea. The term whale sometimes refers to all cetaceans, but more often it excludes dolphins and porpoises, which belong to suborder Odontoceti . This suborder also includes the sperm whale, killer whale, pilot whale, and beluga...
s are made of keratin.
Although it is now difficult to be certain, the scales, claws, some protective armour
Thyreophora
The Thyreophora were a subgroup of the ornithischian dinosaurs...
and the beaks of dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s were likely to have been composed of keratin.
Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments, which have only been found in the genomes of chordates (vertebrates, Amphioxus, urochordates). Nematodes and many other non-chordate animals seem to only have type V intermediate filaments, lamins, which have a long rod domain (vs. a short rod domain for the keratins).
Molecular biology and biochemistry
The usefulness of keratins depends on their supermolecular aggregation. These depend on the properties of the individual polypeptidePeptide
Peptides are short polymers of amino acid monomers linked by peptide bonds. They are distinguished from proteins on the basis of size, typically containing less than 50 monomer units. The shortest peptides are dipeptides, consisting of two amino acids joined by a single peptide bond...
strands, which depend in turn on their amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
composition and sequence. The α-helix
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
and β-sheet
Beta sheet
The β sheet is the second form of regular secondary structure in proteins, only somewhat less common than the alpha helix. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a generally twisted, pleated sheet...
motifs, and disulfide bridges, are crucial to the conformations of globular, functional proteins
Globular protein
Globular proteins, or spheroproteins are one of the two main protein classes, comprising "globe"-like proteins that are more or less soluble in aqueous solutions...
like enzymes, many of which operate semi-independently, but they take on a completely dominant role in the architecture and aggregation of keratins.
The alpha keratin helix is not a true alpha helix, as it only has 3.5 residues/turn, where the normal alpha helix has 3.6 residues/turn. This is important for the different helices to form tight disulfide bonds. Also, roughly every seventh residue is a leucine, so they can line up and help the strands stick together through hydrophobic interactions.
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 transglutaminaseTransglutaminaseTransglutaminases are a family of enzymes that catalyze the formation of a covalent bond between a free amine group and the gamma-carboxamid group of protein- or peptide-bound glutamine. Bonds formed by transglutaminase exhibit high resistance to proteolytic degradation.Transglutaminases were...
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
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...
, cells become cornified as keratin protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
is incorporated into longer keratin intermediate filaments. Eventually the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
and cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...
ic organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s disappear, metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
ceases and cells undergo a programmed death
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...
as they become fully keratinized. In many other cell types, such as cells of the dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
, keratin filaments and other intermediate filaments function as part of the cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic 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
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
almost waterproof, and along with collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
and elastin
Elastin
Elastin is a protein in connective tissue that is elastic and allows many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of...
, gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis and form protective callus
Callus
A callus is an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses...
es — useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced (see dandruff
Dandruff
Dandruff is the shedding of dead skin cells from the scalp . Dandruff is sometimes caused by frequent exposure to extreme heat and cold. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and common; about 487,000 cells/cm2 get released normally after...
).
These hard, integument
Integumentary system
The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages...
ary 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 moult
Moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
and regenerate. The constituent protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s may be phylogenetically
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
but differ somewhat in chemical
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
structure and supermolecular
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
organization. The evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary relationships are complex and only partially known. Multiple gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.
Structural details
Superhelix
A superhelix is a molecular structure in which a helix is itself coiled into a helix. This is significant to both proteins and genetic material, such as overwound circular DNA....
motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
.
Limited interior space is the reason why the triple helix
Triple helix
In geometry, a triple helix is a set of three congruent geometrical helices with the same axis, differing by a translation along the axis. Structures in the form of a triple helix include:* collagen helix...
of the (unrelated) structural protein collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
, found in skin, cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
and bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
, likewise has a high percentage of glycine. The connective tissue protein elastin
Elastin
Elastin is a protein in connective tissue that is elastic and allows many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of...
also has a high percentage of both glycine and alanine. Silk fibroin
Fibroin
Fibroin is a type of protein created by Bombyx mori in the production of silk. Silk emitted by the silkworm consists of two main proteins, sericin and fibroin, fibroin being the structural center of the silk, and sericin being the sticky material surrounding it.The fibroin protein consists of...
, 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. A preponderance of amino acids with small, nonreactive
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
side groups is characteristic for structural proteins, for which H-bonded close packing is more important than chemical specificity
Chemical specificity
Chemical specificity is the ability of a protein's binding site to bind specific ligands. The fewer ligands a protein can bind, the greater its specificity....
.
Disulfide bridges
In addition to intra- and intermolecular hydrogen bonds, keratins have large amounts of the sulfurSulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
-containing amino acid cysteine
Cysteine
Cysteine is an α-amino acid with the chemical formula HO2CCHCH2SH. It is a non-essential amino acid, which means that it is biosynthesized in humans. Its codons are UGU and UGC. The side chain on cysteine is thiol, which is polar and thus cysteine is usually classified as a hydrophilic amino acid...
, required for the disulfide bridges
Disulfide bond
In chemistry, a disulfide bond is a covalent bond, usually derived by the coupling of two thiol groups. The linkage is also called an SS-bond or disulfide bridge. The overall connectivity is therefore R-S-S-R. The terminology is widely used in biochemistry...
that confer additional strength and rigidity by permanent, thermally-stable crosslinking
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
—a role sulfur bridges also play in vulcanized
Vulcanization
Vulcanization or vulcanisation is a chemical process for converting rubber or related polymers into more durable materials via the addition of sulfur or other equivalent "curatives." These additives modify the polymer by forming crosslinks between individual polymer chains. Vulcanized material is...
rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
. Human hair is approximately 14% cysteine. The pungent smells of burning hair and rubber are due to the sulfur compounds formed. Extensive disulfide bonding contributes to the insolubility of keratins, except in dissociating
Dissociation (chemistry)
Dissociation in chemistry and biochemistry is a general process in which ionic compounds separate or split into smaller particles, ions, or radicals, usually in a reversible manner...
or reducing
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
agents.
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. Hair and other α-keratins consist of α-helically
Alpha helix
A common motif in the secondary structure of proteins, the alpha helix is a right-handed coiled or spiral conformation, in which every backbone N-H group donates a hydrogen bond to the backbone C=O group of the amino acid four residues earlier...
-coiled single protein strands (with regular intra-chain H-bonding
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
), 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.
Filament formation
It was theorized that keratins are combined into 'hard' and 'soft,' or 'cytokeratinCytokeratin
Cytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. The term "cytokeratin" began to be used in the late 1970s when the protein subunits of keratin intermediate filaments inside cells were first being identified and...
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 filament
Intermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin , and that of 25 nm microtubules, although they were initially designated...
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, the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments.
Pairing
| A (neutral-basic) | B (acidic) | Occurrence |
|---|---|---|
| keratin 1 Keratin 1 Keratin 1 is a member of the keratin family. It is specifically expressed in the spinous and granular layers of the epidermis with family member keratin 10... , keratin 2 |
keratin 9 Keratin 9 Keratin 9 is a protein that in humans is encoded by the KRT9 gene.Keratin 9 is a type I cytokeratin. It is found only in the terminally differentiated epidermis of palms and soles. Mutations in the gene encoding this protein cause epidermolytic palmoplantar keratoderma.... , keratin 10 Keratin 10 Keratin, type I cytoskeletal 10 also known as cytokeratin-10 or keratin-10 is a protein that in humans is encoded by the KRT10 gene. Keratin 10 is a type I keratin.- Function :... |
stratum corneum Stratum corneum The stratum corneum is the outermost layer of the epidermis, consisting of dead cells that lack nuclei and organelles. The purpose of the stratum corneum is to form a barrier to protect underlying tissue from infection, dehydration, chemicals and mechanical stress... , keratinocyte Keratinocyte Keratinocytes are the predominant cell type in the epidermis, the outermost layer of the human skin, constituting 95% of the cells found there. Those keratinocytes found in the basal layer of the skin are sometimes referred to as "basal cells" or "basal keratinocytes"... s |
| keratin 3 Keratin 3 Keratin 3 also known as cytokeratin 3 is a protein that in humans is encoded by the KRT3 gene. Keratin 3 is a type II cytokeratin. It is specifically found in the corneal epithelium together with keratin 12.... |
keratin 12 Keratin 12 Keratin 12 is a protein that in humans is encoded by the KRT12 gene.Keratin 12 is keratin found expressed in corneal epithelia. Mutations in the gene encoding this protein lead to Meesmann corneal dystrophy.... |
cornea Cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is... |
| keratin 4 Keratin 4 Keratin, type I cytoskeletal 4 also known as cytokeratin-4 or keratin-4 is a protein that in humans is encoded by the KRT4 gene.... |
keratin 13 Keratin 13 Keratin 13 is a protein that in humans is encoded by the KRT13 gene.Keratin 13 is a type I cytokeratin, it is paired with keratin 4 and found in the suprabasal layers of non-cornified stratified epithelia... |
stratified epithelium Stratified epithelium Stratified epithelium can refer to:* Stratified squamous epithelium* Stratified cuboidal epithelium* Stratified columnar epithelia... |
| keratin 5 Keratin 5 Keratin, type II cytoskeletal 5 also known as KRT5 is a protein that in human is encoded by the KRT5 gene.- Function :The protein encoded by this gene is a member of the keratin gene family... |
keratin 14 Keratin 14 Keratin, type I cytoskeletal 14 also known as cytokeratin-14 or keratin-14 is a protein that in humans is encoded by the KRT14 gene.... , keratin 15 Keratin 15 Keratin 15 is a protein that in humans is encoded by the KRT15 gene.Keratin 15 is a type I cytokeratin. It is found in some progenitor basal cells within complex epithelia.... |
stratified epithelium Stratified epithelium Stratified epithelium can refer to:* Stratified squamous epithelium* Stratified cuboidal epithelium* Stratified columnar epithelia... |
| keratin 6 | keratin 16 Keratin 16 Keratin 16 is a protein that in humans is encoded by the KRT16 gene.Keratin 16 is a type I cytokeratin. It is paired with keratin 6 in a number of epithelial tissues, including nail bed, esophagus, tongue, and hair follicles... , keratin 17 Keratin 17 Keratin, type I cytoskeletal 17 is a protein that in humans is encoded by the KRT17 gene.Keratin 17 is a type I cytokeratin. It is found in nail beds, hair follicles, sebaceous glands, and other epidermal appendages... |
squamous epithelium Squamous epithelium In anatomy, squamous epithelium is an epithelium characterised by its most superficial layer consisting of flat, scale-like cells called squamous epithelial cells... |
| keratin 7 Keratin 7 Keratin, type I cytoskeletal 7 also known as cytokeratin-7 or keratin-7 or sarcolectin is a protein that in humans is encoded by the KRT7 gene. Keratin 7 is a type II keratin... |
keratin 19 Keratin 19 Keratin, type I cytoskeletal 19 also known as cytokeratin-19 or keratin-19 is a protein that in humans is encoded by the KRT19 gene. Keratin 19 is a type I keratin.- Function :Keratin 19 is a member of the keratin family... |
ductal epithelia |
| keratin 8 Keratin 8 Keratin, type I cytoskeletal 8 also known as cytokeratin-8 or keratin-8 is a keratin protein that in human is encoded by the KRT8 gene. It is often paired with keratin 18.-Utility as an immunohistochemical stain:... |
keratin 18 Keratin 18 Keratin 18 is a type I cytokeratin. It is, together with its filament partner keratin 8, perhaps the most commonly found products of the intermediate filament gene family. They are expressed in single layer epithelial tissues of the body. Mutations in this gene have been linked to cryptogenic... , keratin 20 Keratin 20 Keratin 20, often abbreviated CK20, is a protein that in humans is encoded by the KRT20 gene.Keratin 20 is a type I cytokeratin. It is a major cellular protein of mature enterocytes and goblet cells and is specifically found in the gastric and intestinal mucosa.... |
simple epithelium |
The entries KRT23
KRT23
Keratin, type I cytoskeletal 23 is a protein that in humans is encoded by the KRT23 gene.-Further reading:...
, KRT24
KRT24
KRT24 is a keratin gene....
, KRT25
KRT25
KRT25 is a keratin gene....
, KRT26
KRT26
KRT26 is a keratin gene....
, KRT27
KRT27
KRT27 is a keratin gene....
, KRT28
KRT28
KRT28 is a keratin gene....
, KRT31
KRT31
Keratin, type I cuticular Ha1 is a protein that in humans is encoded by the KRT31 gene.-Further reading:...
, KRT32
KRT32
Keratin, type I cuticular Ha2 is a protein that in humans is encoded by the KRT32 gene.-Further reading:...
, KRT33, KRT33A
KRT33A
Keratin, type I cuticular Ha3-I is a protein that in humans is encoded by the KRT33A gene.-Further reading:...
, KRT34
KRT34
Keratin, type I cuticular Ha4 is a protein that in humans is encoded by the KRT34 gene.-Further reading:...
, KRT35
KRT35
KRT35 is a keratin gene....
, KRT36
KRT36
Keratin, type I cuticular Ha6 is a protein that in humans is encoded by the KRT36 gene.-Further reading:...
, KRT37
KRT37
KRT37 is a keratin gene....
, KRT38
KRT38
KRT38 is a keratin gene....
, KRT39
KRT39
KRT39 is a keratin gene....
, KRT40
KRT40
KRT40 is a keratin gene....
, KRT71
KRT71
KRT71 is a keratin gene....
, KRT72
KRT72
KRT72 is a keratin gene....
, KRT73
KRT73
KRT73 is a keratin gene....
, KRT74
KRT74
KRT74 is a keratin gene....
, KRT75
KRT75
KRT75 is a keratin gene....
, KRT76
KRT76
KRT76 is a keratin gene....
, KRT77
KRT77
KRT77 is a keratin gene....
, KRT78
KRT78
Keratin, type II cytoskeletal 78 is a protein that in humans is encoded by the KRT78 gene.-Further reading:...
, KRT79
KRT79
Keratin 79 also known as KRT79 is a protein which humans is encoded by the KRT79 gene.- Function :Keratins, such as KRT79, are filament proteins that make up one of the major structural fibers of epithelial cells-Further reading:...
, KRT8, KRT80
KRT80
Keratin 80 also known as KRT80 is a protein which humans is encoded by the KRT80 gene.- Function :Keratins, such as KRT80, are filament proteins that make up one of the major structural fibers of epithelial cells-Further reading:...
, KRT81
KRT81
Keratin, type II cuticular Hb1 is a protein that in humans is encoded by the KRT81 gene.-Further reading:...
, KRT82
KRT82
KRT82 is a keratin gene. This is a type II keratin and appears to be a hair cuticle-specific....
, KRT83
KRT83
Keratin 83, also known as KRT83, is a protein which humans is encoded by the KRT83 gene.- Function :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 keratins to form hair and nails...
, KRT84
KRT84
KRT84 is a keratin gene, for a type II hair keratin contained primarily in the filiform tongue papilla....
, KRT85
KRT85
Keratin, type II cuticular Hb5 is a protein that in humans is encoded by the KRT85 gene.-Further reading:...
and KRT86
KRT86
Keratin, type II cuticular Hb6 is a protein that in humans is encoded by the KRT86 gene.-Further reading:...
have been used to describe keratins past 20.
Silk
The silkSilk
Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...
fibroins produced by insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s and spider
Spider
Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...
s are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins.
Silk found in insect pupa
Pupa
A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago...
e, and in spider web
Spider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....
s 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 gland
Gland
A gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...
s, provide remarkable control of fast extrusion
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
. Spider silk is typically about 1 to 2 micrometres (µm) thick, compared with about 60 µm for human hair, and more for some mammals. The biologically
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
and commercially
Commerce
While business refers to the value-creating activities of an organization for profit, commerce means the whole system of an economy that constitutes an environment for business. The system includes legal, economic, political, social, cultural, and technological systems that are in operation in any...
useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
line regions of varying size, alternating with flexible, amorphous regions where the chains are randomly coiled
Random coil
A random coil is a polymer conformation where the monomer subunits are oriented randomly while still being bonded to adjacent units. It is not one specific shape, but a statistical distribution of shapes for all the chains in a population of macromolecules...
. A somewhat analogous situation occurs with synthetic
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...
polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s such as nylon
Nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...
, developed as a silk substitute. Silk from the hornet
Hornet
Hornets are the largest eusocial wasps; some species can reach up to in length. The true hornets make up the genus Vespa and are distinguished from other vespines by the width of the vertex , which is proportionally larger in Vespa and by the anteriorly rounded gasters .- Life cycle :In...
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
Adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
, as do spiders.
Clinical significance
Some infectiousInfection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
, such as those that cause athlete's foot
Athlete's foot
Athlete's foot is a fungal infection of the skin that causes scaling, flaking, and itch of affected areas. It is caused by fungi in the genus Trichophyton and is typically transmitted in moist areas where people walk barefoot, such as showers or bathhouses...
and ringworm, or Batrachochytrium dendrobatidis
Batrachochytrium dendrobatidis
Batrachochytrium dendrobatidis is a chytrid fungus that causes the disease chytridiomycosis. In the decade after it was first discovered in amphibians in 1998, the disease devastated amphibian populations around the world, in a global decline towards multiple extinctions, part of the Holocene...
(Chytrid fungus), feed on keratin.
Diseases caused by mutations in the keratin genes include
- Epidermolysis bullosa simplexEpidermolysis bullosa simplexEpidermolysis bullosa simplex is a disorder resulting from mutations in the genes encoding keratin 5 or keratin 14.Blister formation of EBS occurs at the dermoepidermal junction. Sometimes EBS is called epidermolytic.-Subtypes:...
- Ichthyosis bullosa of SiemensIchthyosis bullosa of SiemensIchthyosis bullosa of Siemens is a rare skin disorder which is a type of familial, autosomal dominant ichthyosis. It is also known as bullous congenital ichthyosiform erythroderma of Siemens or ichthyosis exfoliativa...
- Epidermolytic hyperkeratosisEpidermolytic hyperkeratosisEpidermolytic hyperkeratosis, is a rare skin disease in the ichthyosis family affecting around 1 in 250,000 people.It involves the clumping of keratin filaments.-Presentation:At birth, affected babies are called "enfant...
- Steatocystoma multiplexSteatocystoma multiplexSteatocystoma multiplex is a benign, autosomal dominant congenital condition resulting in multiple cysts on a person's body....
- Keratosis pharyngisKeratosis pharyngisKeratosis pharyngis is a medical condition where keratin grows on the surface of the pharynx, that is the part of the throat at the back of the mouth...
- Rhabdoid cell formation in Large cell lung carcinoma with rhabdoid phenotypeLarge cell lung carcinoma with rhabdoid phenotypeLarge-cell lung carcinoma with rhabdoid phenotype , sometimes referred to simply as "rhabdoid carcinoma", is a rare histological variant of large-cell lung carcinoma , wherein at least 10% of the malignant cells making up the tumor contain distinctive eosinophilic whorled perinuclear inclusions...
Furthermore, keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinoma
Carcinoma
Carcinoma is the medical term for the most common type of cancer occurring in humans. Put simply, a carcinoma is a cancer that begins in a tissue that lines the inner or outer surfaces of the body, and that generally arises from cells originating in the endodermal or ectodermal germ layer during...
s, thymoma
Thymoma
Thymoma is a tumor originating from the epithelial cells of the thymus. Thymoma is an uncommon tumor, best known for its association with the neuromuscular disorder myasthenia gravis. Thymoma is found in 15% of patients with myasthenia gravis. Once diagnosed, thymomas may be removed surgically...
s, sarcoma
Sarcoma
A sarcoma is a cancer that arises from transformed cells in one of a number of tissues that develop from embryonic mesoderm. Thus, sarcomas include tumors of bone, cartilage, fat, muscle, vascular, and hematopoietic tissues...
s 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 carcinoma
Hepatocellular carcinoma
Hepatocellular carcinoma is the most common type of liver cancer. Most cases of HCC are secondary to either a viral hepatitide infection or cirrhosis .Compared to other cancers, HCC is quite a rare tumor in the United States...
s typically expresse K8 and K18, and cholangiocarcinoma
Cholangiocarcinoma
Cholangiocarcinoma is a cancer of the bile ducts which drain bile from the liver into the small intestine. Other biliary tract cancers include pancreatic cancer, gallbladder cancer, and cancer of the ampulla of Vater...
s express K7, K8 and K18, while metastases of colorectal carcinomas express K20, but not K7.
See also
- CorneousCorneousCorneous is a biological and medical term meaning horny, in other words made out of a substance similar to that of horns and hooves in some mammals....
- CytokeratinCytokeratinCytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. The term "cytokeratin" began to be used in the late 1970s when the protein subunits of keratin intermediate filaments inside cells were first being identified and...
- DesmosomeDesmosomeA desmosome , also known as macula adherens , is a cell structure specialized for cell-to-cell adhesion...
- Intermediate filamentIntermediate filamentIntermediate filaments are a family of related proteins that share common structural and sequence features. Intermediate filaments have an average diameter of 10 nanometers, which is between that of 7 nm actin , and that of 25 nm microtubules, although they were initially designated...
- Keratosis pilarisKeratosis pilarisKeratosis pilaris is a common, autosomal dominant, genetic follicular condition that is manifested by the appearance of rough bumps on the skin...
- Pachyonychia congenitaPachyonychia congenitaPachyonychia congenita, also called Jadassohn-Lewandowski Syndrome or simply pachyonychia, is an autosomal dominant skin disorder.-Symptoms:Common symptoms include:*Excess keratin in nail beds and thickening of the nails...
- Tinea versicolorTinea versicolorTinea versicolor is a condition characterized by a rash on the trunk and proximal extremities. Recent research has shown that the majority of Tinea versicolor is caused by the Malassezia globosa fungus, although Malassezia furfur is responsible for a small number of cases...