Keratin
43377
223032001
2008-07-02T07:31:31Z
AkhtaBot
6756213
robot Modifying: [[ar:كيراتين]]
{{Distinguish|kerogen|carotene|creatine|keratan sulphate}}
[[Image:KeratinF9.png|thumb|right|370px|Microscopy of keratin filaments inside cells.]]
'''Keratins''' are a family of [[fibrous protein|fibrous structural proteins]]; tough and insoluble, they form the hard but [[mineral|nonmineralized]] structures found in [[reptile]]s, [[bird]]s, [[amphibian]]s and [[mammal]]s. They are rivaled as [[biology|biological]] materials in toughness only by [[chitin]].
There are various types of keratins within a single [[animal]].
==Variety of animal uses==
Keratins are the main constituent of structures that grow from the skin:
* the ''α-keratins'' in the [[hair]] (including [[wool]]), [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claw]]s and [[Hoof|hooves]] of mammals{{Verify source|date=November 2007}}<!-- aren't "reptile" scales mainly alpha too? -->
* the harder ''[[β-keratins]]'' found in nails and in the [[scale (zoology)|scales]] and claws of [[reptiles]], their [[animal shell|shells]] ([[chelonian]]s, such as [[tortoise]], [[turtle]], [[terrapin]]), and in the [[feather]]s, [[beak]]s, and claws of [[birds]].{{Verify source|date=November 2007}} (These keratins are formed primarily in [[beta sheet]]s. However, beta sheets are also found in α-keratins.)<ref>{{cite journal |author=Kreplak L, Doucet J, Dumas P, Briki F |title=New aspects of the alpha-helix to beta-sheet transition in stretched hard alpha-keratin fibers |journal=Biophys J |volume=87 |issue=1 |pages=640–7 |year=2004 |pmid=15240497 | doi = 10.1529/biophysj.103.036749 <!--Retrieved from CrossRef by DOI bot-->}}</ref><!-- what exactly does this reference reference? Not the distribution of a- and b-keratins it seems -->
[[Arthropod]]s such as [[crustacea]]ns often have parts of their [[armour (zoology)|armor]] or [[exoskeleton]] made of keratin, sometimes in combination with [[chitin]].
The [[baleen]] plates of filter-feeding [[whale]]s are made of keratin.
They can be integrated in the chitinophosphatic material that makes up the [[sea shell|shell]] and [[setae]] in many [[brachiopod]]s.
Keratins are also found in the [[gastrointestinal tract]]s of many animals, including [[roundworm]]s (who also have an outer layer made of keratin).
Although it is now difficult to be certain, the scales, claws, some [[Thyreophora|protective armour]] and the beaks of [[dinosaur]]s would, almost certainly, have been composed of a type of keratin.
In [[Crossopterygian]] fish, the outer layer of [[Scale (zoology)|cosmoid scales]] was keratin.
==Cornification==
In mammals there are soft [[epithelial]] keratins, the [[cytokeratin]]s, and harder [[hair keratin]]s. As certain skin cells [[cellular differentiation|differentiate]] and become [[cornification|cornified]], pre-keratin [[polypeptide]]s are incorporated into [[intermediate filament]]s. Eventually the [[cell nucleus|nucleus]] and [[cytoplasm]]ic [[organelle]]s disappear, [[metabolism]] ceases and cells undergo a [[apoptosis|programmed death]] as they become fully keratinized.
[[Cell (biology)|Cells]] in the [[epidermis (skin)|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 keratin to proliferate with the formation of protective [[callus]]es — useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced (see [[dandruff]]).
These hard, [[Integumentary system|integument]]ary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by [[sebaceous glands|specialized beds]] deep within the skin. Hair grows continuously and feathers [[moult]] and regenerate. The constituent [[protein]]s may be [[phylogenetics|phylogenetically]] [[homology (biology)|homologous]] but differ somewhat in [[chemical compound|chemical]] structure and super[[molecule|molecular]] organization. The [[evolution]]ary relationships are complex and only partially known. Multiple [[gene]]s have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.
==Molecular biology and biochemistry==<!-- This section is linked from [[Nylon]] -->
The properties which make structural proteins like keratins useful depend on their supermolecular aggregation. These depend on the properties of the individual [[peptide|polypeptide]] strands, which depend in turn on their [[amino acid]] composition and sequence. The [[alpha helix|α-helix]] and [[beta sheet|β-sheet]] motifs, and disulfide bridges, are crucial to the [[protein structure#Secondary structure elements|conformations]] of [[globular protein|globular, functional proteins]] like [[enzymes]], many of which operate semi-independently, but they take on a completely dominant role in the architecture and aggregation of keratins.
===Glycine and alanine===
Keratins contain a high proportion of the smallest of the 20 amino acids, [[glycine]], whose "[[side chain|side group]]" is a single [[hydrogen atom]]; also the next smallest, [[alanine]], with a small and noncharged [[methyl group]]. In the case of β-sheets, this allows [[steric effects|sterically-unhindered]] [[hydrogen bond]]ing between the [[amine|amino]] and [[carboxyl group]]s of [[peptide bond]]s on adjacent protein chains, facilitating their close alignment and strong binding. Fibrous keratin molecules can twist around each other to form [[helix|helical]] intermediate filaments.
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.[http://elmhurst.edu/~chm/vchembook/566secprotein.html] A preponderance of amino acids with small, [[chemical reaction|nonreactive]] side groups is characteristic of structural proteins, for which H-bonded close packing is more important than [[chemical specificity]].
===Disulfide bridges===
In addition to intra- and intermolecular hydrogen bonds, keratins have large amounts of the [[sulfur]]-containing amino acid [[cysteine]], required for the [[disulfide bond|disulfide bridges]] that confer additional strength and rigidity by permanent, thermally-stable [[cross-link|crosslinking]]—a role sulfur bridges also play in [[vulcanization|vulcanized]] [[rubber]]. 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 in[[soluble|solubility]] of keratins, except in [[dissociation (chemistry)|dissociating]] or [[redox|reducing]] agents.
The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian [[fingernail]]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 [[alpha helix|α-helically]]-coiled single protein strands (with regular intra-chain [[hydrogen bond|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.
==Silk==
The [[silk]] [[fibroins]] produced by [[insect]]s and [[spider]]s are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins.
Silk found in insect [[pupa]]e, and in [[spider web]]s and egg casings, also has twisted β-pleated sheets incorporated into fibers wound into larger supermolecular aggregates. The structure of the [[spinneret]]s on spiders’ tails, and the contributions of their interior [[gland]]s, provide remarkable control of fast [[extrusion]]. Spider silk is typically about 1 to 2 micrometres (µm) thick, compared with about 60 µm for human hair, and more for some mammals. (Hair, or [[fur]], occurs only in mammals.) The [[biology|biologically]] and [[commerce|commercially]] useful properties of [[spider silk#Properties of spider silk|silk fibers]] depend on the organization of multiple adjacent protein chains into hard, [[crystal]]line regions of varying size, alternating with flexible, [[amorphous]] regions where the chains are [[random coil|randomly coiled]].<ref>[http://www.amonline.net.au/spiders/toolkit/silk/structure.htm Spiders - Silk structure<!-- Bot generated title -->]</ref> A somewhat analogous situation occurs with [[chemical synthesis|synthetic]] [[polymer]]s such as [[nylon]], developed as a silk substitute. Silk from the [[hornet]] [[Pupa#Cocoon|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.
==Pairing==
{|border="1" cellspacing="0" cellpadding="5"
|bgcolor=#ABCDEF|'''A''' (neutral-basic)
|bgcolor=#ABCDEF|'''B''' (acidic)
|bgcolor=#ABCDEF|'''Occurrence'''
|----
| [[keratin 1]], [[keratin 2]]
| [[keratin 9]], [[keratin 10]]
| [[stratum corneum]], [[keratinocyte]]s
|----
| [[keratin 3]]
| [[keratin 12]]
| [[cornea]]
|----
| [[keratin 4]]
| [[keratin 13]]
| [[stratified epithelium]]
|----
| [[keratin 5]]
| [[keratin 14]], [[keratin 15]]
| [[stratified epithelium]]
|----
| [[keratin 6]]
| [[keratin 16]], [[keratin 17]]
| [[squamous epithelium]]
|----
| [[keratin 7]]
| [[keratin 19]]
| ductal epithelia
|----
| [[keratin 8]]
| [[keratin 18]], [[keratin 20]]
| simple epithelium
|}
==Clinical significance==
Some [[infection|infectious]] [[fungus|fungi]], such as those which cause [[athlete's foot]], [[ringworm]] or the ''[[Batrachochytrium dendrobatidis]]'' (Chytrid fungus) which is killing amphibians all over the world, feed on keratin.
Diseases caused by mutations in the keratin genes include
*[[Epidermolysis bullosa simplex]]
*[[Ichthyosis bullosa of Siemens]]
*[[Epidermolytic hyperkeratosis]]
*[[Steatocystoma multiplex]]
==See also==
*[[Acne vulgaris|Acne]]
*[[Keratosis pilaris]]
*[[Intermediate filament]]
*[[Desmosome]]
*[[Tinea versicolor]]
==Additional images==
<gallery>
Image:Keratin.jpg|Keratin (high molecular weight) in bile duct cell and oval cells of mouse liver.
</gallery>
== References ==
<references/>
==External links==
*[http://www.elmhurst.edu/~chm/vchembook/566secprotein.html Composition and β-sheet structure of silk]
*[http://www.amonline.net.au/spiders/toolkit/silk/structure.htm Spider silk fiber structure]
*[http://www.hair-science.com/_int/_en/topic/topic_sousrub.aspx?tc=ROOT-HAIR-SCIENCE^PORTRAIT-OF-AN-UNKNOWN-ELEMENT^SUPERB-CHEMISTRY&cur=SUPERB-CHEMISTRY Hair-Science.com's entry on the miroscopic elements of hair]
*[http://www.exactantigen.com/review/Keratin.html keratin antibody review]
{{Fibrous proteins}}
[[Category:Keratins]]
[[Category:Cytoskeleton]]
[[am:ኬራቲን]]
[[ar:كيراتين]]
[[bg:Кератин]]
[[ca:Ceratina]]
[[cs:Keratin]]
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[[eo:Keratino]]
[[fr:Kératine]]
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[[hr:Keratin]]
[[io:Keratino]]
[[it:Cheratina]]
[[he:קרטין]]
[[lt:Keratinas]]
[[nl:Keratine]]
[[ja:ケラチン]]
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[[oc:Queratina]]
[[pl:Keratyna]]
[[pt:Queratina]]
[[ru:Кератины]]
[[simple:Keratin]]
[[su:Keratin]]
[[fi:Keratiini]]
[[sv:Keratin]]
[[ta:நகமியம்]]
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[[ur:قرنین]]