Difference between revisions of "Keratin" - New World Encyclopedia

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[[Image:KeratinF9.png|thumb|right|240px|Microscopy of keratin filaments inside cells.]]
 
[[Image:KeratinF9.png|thumb|right|240px|Microscopy of keratin filaments inside cells.]]
 
'''Keratin''' is any one of the family of tough and insoluble [[fibrous protein|fibrous structural proteins]] that form the chief, hard, [[mineral|nonmineralized]] structural component of [[hair]], [[wool]], [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claw]]s, [[Hoof|hooves]], and other [[vertebrate]] tissues, as well as part of various [[invertebrate]] structures. Keratins are rivaled as [[biology|biological]] materials in toughness only by [[chitin]].
 
'''Keratin''' is any one of the family of tough and insoluble [[fibrous protein|fibrous structural proteins]] that form the chief, hard, [[mineral|nonmineralized]] structural component of [[hair]], [[wool]], [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claw]]s, [[Hoof|hooves]], and other [[vertebrate]] tissues, as well as part of various [[invertebrate]] structures. Keratins are rivaled as [[biology|biological]] materials in toughness only by [[chitin]].
  
The properties that make keratin so useful depend on the [[amino acid]] composition and sequence and the particular [[protein]] folding that results. In particular, the prevalence of the [[sulfur]]-containing amino acid [[cysteine]], with its ability to form strong covalent chemical bonds between sulfur atoms (disulfide bridge), helps confer strength and rigidity. The complexity involved in just this one type of protein is remarkable, and yet it is formed by a vast diversity of living organisms.
+
Keratins are present in all [[epithelial]] [[cell]]s, both those covering the external surfaces of organisms and those on internal surfaces, such as the lining of the [[digestive tract]]. Keratins aid the epithelial cells in maintaining their connected integrity, as the keratins typically span the full interior width of a cell and are connected indirectly to keratins in adjoining cells through cell-to-cell junctions called [[desmosomes]]. The horn, hooves, nails, hair, and other keratin-based hard, tough materials growing on animals are produced by epithelial cells adapted to growing an abundance of keratin and then dying as individual cells while leaving the keratin to help form a structure valuable to the whole animal.
 +
 
 +
Keratin's characteristic toughness and resilience depend on its [[amino acid]] composition and sequence and the particular [[protein]] folding that results. In particular, the prevalence in Keratin of the [[sulfur]]-containing [[amino acid]] [[cysteine]], with its ability to form strong [[covalent]] chemical [[bond]]s between sulfur [[atoms]] (disulfide bridge), helps confer strength and rigidity. The complexity involved in just this one type of protein is remarkable, and yet it is formed by a vast diversity of living organisms.
  
 
There are various types of keratins within a single [[animal]]. Keratin is [[nutrition]]ally useless to humans, since it is not hydrolyzed by digestive [[enzyme]]s, but it can be used as fertilizer, being slowly broken down by bacteria (Bender and Bender 2005).  
 
There are various types of keratins within a single [[animal]]. Keratin is [[nutrition]]ally useless to humans, since it is not hydrolyzed by digestive [[enzyme]]s, but it can be used as fertilizer, being slowly broken down by bacteria (Bender and Bender 2005).  
  
 
==Uses in animals==
 
==Uses in animals==
Keratins are the chief constituent of structures that grow from the skin of [[vertebrate]]s. These include:  
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Keratins are the chief constituent of structures that grow from the skin of [[vertebrate]]s. These structures include:  
 +
 
 +
* Among [[mammal]]s, the hair (including wool), horns, nails, claws, [[corn(animal)|corn]]s, and hooves, which are made primarily of ''α-keratins''
 +
* Among [[reptile]]s, the [[scale (zoology)|scales]], claws, and, in the [[chelonian]]s, such as [[tortoise]], [[turtle]], [[terrapin]], the [[animal shell|shell]]s, which are made primarily of ''β-keratins''
 +
* Among  [[bird]]s, the [[feather]]s, [[beak]]s, and claws, which are made primarily of ''β-keratins''
  
* the ''α-keratins'' in the [[hair]] (including [[wool]]), [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claw]]s, and [[Hoof|hooves]] of [[mammal]]s;
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The α-keratins are formed primarily as helical fibers, while the β-keratins are formed primarily in [[beta sheet]]s. Some beta sheets are also found in α-keratins (Kreplak et al. 2004).
* the harder ''[[β-keratins]]'' in the [[scale (zoology)|scales]] and claws of [[reptile]]s, their [[animal shell|shells]] ([[chelonian]]s, such as [[tortoise]], [[turtle]], [[terrapin]]), and in the [[feather]]s, [[beak]]s, and claws of [[birds]]. These keratins are formed primarily in [[beta sheet]]s. However, beta sheets are also found in α-keratins (Kreplak et al. 2004).
 
  
For example, hair, a filamentous outgrowth from the [[skin]] that is found only on mammals, involves fibers that comprise nonliving cells whose primary component is long chains (polymers) of [[amino acid]]s forming the protein keratin. The keratinized cells arise from cell division in the hair matrix at the base of a hair follicle and are tightly packed together.  
+
For example, hair, a filamentous outgrowth from the [[skin]] that is found only on mammals, involves fibers comprising nonliving cells whose primary component is the protein keratin, a long chain (polymer) of [[amino acid]]s that naturally forms an [[α-helix]] fiber and subsequently winds two of the α-helix fibers together to form a much stronger "coiled coil" fiber characteristic of α-keratin. The keratinized cells arise from cell division in the hair matrix at the base of a hair follicle and are tightly packed together.  
  
[[Image:Keratin.jpg|thumb|left|240px|Keratin (high molecular weight) in bile duct cell and oval cells of mouse liver]] Keratins also are a principle part of the [[cell (biology)|cells]] in the tooth enamel of mammals, and the [[baleen]] plates of filter-feeding [[whale]]s are 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.
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[[Image:Keratin.jpg|thumb|left|240px|Keratin (high molecular weight) in bile duct cell and oval cells of mouse liver]] Keratins also are a principle part of the [[cell (biology)|cells]] in the tooth enamel of mammals and the [[baleen]] plates of filter-feeding [[whale]]s. Although it is now difficult to be certain, the prevailing view among [[Paleontology|paleontologist]]s is that the scales, claws, beaks, and some [[Thyreophora|protective armor]] of [[dinosaur]]s most likely were composed of a type of keratin. In [[Crossopterygian]] fish, the outer layer of [[Scale (zoology)|cosmoid scales]] was keratin.
  
In [[invertebrate]]s, [[arthropod]]s such as [[crustacean]]s often have parts of their armor or [[exoskeleton]] made of keratin, sometimes in combination with [[chitin]]. Chitin is a hard, semitransparent [[carbohydrate#polysaccharides|polysaccharide]], and is the main component of the shells of crustaceans, such as [[crab]]s, [[lobster]]s, and [[shrimp]]. In arthropods, however, it is frequently modified by being embedded in a hardened [[protein]]aceous matrix of keratin, giving a more rigid exoskeleton, than seen in the use of chitin in the soft, more pliable body wall of a [[caterpillar]].
+
Among the [[invertebrate]]s, [[arthropod]]s such as [[crustacean]]s often have parts of their armor or [[exoskeleton]] made of keratin, sometimes in combination with [[chitin]], which is a hard, semitransparent [[carbohydrate#polysaccharides|polysaccharide]] that is the main component of the shells of crustaceans, such as [[crab]]s, [[lobster]]s, and [[shrimp]]. In arthropods, however, chitin is frequently modified by being embedded in a hardened [[protein]]aceous matrix of keratin, giving a more rigid exoskeleton, than seen, for example, in the use of [[chitin]] in the soft, more pliable body wall of a [[caterpillar]].
  
Keratins also can be integrated in the chitinophosphatic material that makes up the [[sea shell|shell]] and [[setae]] in many [[brachiopod]]s. Keratins also are found in the [[gastrointestinal tract]]s of many animals, including [[roundworm]]s (who also have an outer layer made of keratin).
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Keratins also can be integrated in the chitinophosphatic material that makes up the [[sea shell|shell]] and [[setae]] (bristles) in many [[brachiopod]]s. Keratins also are found in the [[gastrointestinal tract]]s of many animals, including [[roundworm]]s (who also have an outer layer made of keratin).
  
 
==Molecular biology and biochemistry==
 
==Molecular biology and biochemistry==
The properties that 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 [[enzyme]]s, many of which operate semi-independently, but they take on a completely dominant role in the architecture and aggregation of keratins.
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The properties that make structural proteins like keratins useful depend on their supermolecular aggregation, i.e., their pattern of protein ([[peptide|polypeptide]] strand) folding. The properties of collectives of protein strands depend on the properties of the individual 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 the disulfide bridges, are central to the architecture and aggregation of keratins.
  
 
===Disulfide bridges===
 
===Disulfide bridges===
[[Image:Cystine-skeletal.png|thumb|right|190px|[[Cystine]], showing [[disulfide bond]] joining two cysteine residues]]
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[[Image:L-cysteine-skeletal.png|thumb|right|120px|[[Cysteine]], an amino acid with the thiol functional group -SH.]]
Keratins have large amounts of the [[sulfur]]-containing amino acid [[cysteine]]. Cysteine is characterized by the presence of a thiol group (or sulfhydryl group), which is a functional group composed of a sulfur atom and a hydrogen atom (-SH). Since thiol groups can undergo reduction (redox) reactions, cysteine can undergo redox reactions. Oxidation of cysteine can produce a [[disulfide bond]] with another thiol. A disulfide bond, also called a SS-bond or disulfide bridge, is a single covalent bond derived from the coupling of thiol groups; the overall connectivity is C-S-S-C. That is, when cysteine is oxidized it can form cystine, which is two cysteine residues joined by a disulfide bond (cys-S-S-cys) between the -SH group.  
+
[[Image:Cystine-skeletal.png|thumb|right|190px|[[Cystine]], showing a [[disulfide bond]] (-S-S-) joining two cysteine residues.]]
 +
Keratins have large amounts of the [[sulfur]]-containing amino acid [[cysteine]], which is characterized by the thiol functional group, -SH, comprising a sulfur atom and a hydrogen atom. In the keratin polymer, which is originally extremely flexible, the thiol groups tend to pair up and, through [[oxidation]], form a [[covalent]] sulfur-sulfur, that is, [[disulfide]], bond with the loss of two protons and two electrons. A disulfide bond, also called a SS-bond or disulfide bridge, achieves an overall connectivity represented by C-S-S-C, in which "C" represents the immediate next carbon atom and all the remainder of the associated amino acid. Expressed more formally, when cysteine is oxidized it can form cystine, which is two cysteine residues (cys) joined by a disulfide bond (cys-S-S-cys) between the -SH group.  
  
 
[[Disulfide bond|Disulfide bridges]] 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 percent 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.
 
[[Disulfide bond|Disulfide bridges]] 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 percent 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.
+
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 wound together into superhelical or coiled-coil 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.
  
 
===Glycine and alanine===
 
===Glycine and alanine===
Keratins contain a high proportion of the smallest of the 20 [[amino acid]]s, [[glycine]], whose "[[side chain|side group]]" is a single [[hydrogen atom]]. It also contains a high proportion of 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.  
+
[[Image:Amino_acid2.png|thumbnail|The general structure of an amino acid molecule. For glycine, the simplest amino acid, R in the figure is replaced by -H. For alanine, R is replaced by the methyl group -CH3. The amino group (-NH2) is on the left and the carboxyl group (-COOH) is on the right.]]
 +
Keratins contain a high proportion of the smallest of the 20 [[amino acid]]s, [[glycine]], whose "[[side chain|side group]]" is a single [[hydrogen atom]]. They also contain a high proportion of the next smallest, [[alanine]], whose functional side group is the small and noncharged [[methyl group]]. In the case of β-sheets, this high proportion of simple and neutral side groups 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 bonding. Fibrous keratin molecules can twist around each other to form double-wound [[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 percent of the total, with 10–15 percent serine, with the rest having bulky side groups. The chains are antiparallel, with an alternating C → N orientation (Ophardt 2003). 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]].
+
The triple helix of the (unrelated) structural protein [[collagen]], found in skin, [[cartilage]], and [[bone]], likewise has a high percentage of glycine, as does the connective tissue protein [[elastin]], which also has a high percentage of alanine. Spider silk [[fibroin]], considered a β-keratin, can have glycine and alanine as 75–80 percent of its total amino acids, with an additional 10–15 percent being serine, and the rest being amino acids that have bulky side groups. The chains are antiparallel, with an alternating C → N orientation (Ophardt 2003). 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]].
  
 
==Cornification==
 
==Cornification==
 
In [[mammal]]s, 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.
 
In [[mammal]]s, 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]]).
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[[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 (such as [[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 [[molt]] 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.
 
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 [[molt]] 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.
Line 54: Line 61:
  
 
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.
 
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
 
|}
 
  
 
==Medical significance==
 
==Medical significance==
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== References ==
 
== References ==
* Australian Museum Online. 2002. [http://www.amonline.net.au/spiders/toolkit/silk/structure.htm Spiders: Silk structure]. ''Australian Museum Online''. Retrieved May 23,2008.
+
* Australian Museum Online. 2002. [http://www.amonline.net.au/spiders/toolkit/silk/structure.htm Spiders: Silk structure.] ''Australian Museum Online''. Retrieved May 23,2008.
 
 
 
* Bender, D. A., and A. E. Bender. 2005. ''A Dictionary of Food and Nutrition''. New York: Oxford University Press. ISBN 0198609612.
 
* Bender, D. A., and A. E. Bender. 2005. ''A Dictionary of Food and Nutrition''. New York: Oxford University Press. ISBN 0198609612.
 
+
* Kreplak, L. J. Doucet, P. Dumas, and F. Briki. 2004. [http://www.biophysj.org/cgi/content/abstract/87/1/640 New aspects of the alpha-helix to beta-sheet transition in stretched hard alpha-keratin fibers.] ''Biophys J'' 87(1): 640-7. Retrieved May 23, 2008.
* Kreplak, L. J. Doucet, P. Dumas, and F. Briki. 2004. [http://www.biophysj.org/cgi/content/abstract/87/1/640 New aspects of the alpha-helix to beta-sheet transition in stretched hard alpha-keratin fibers]. ''Biophys J'' 87(1): 640-7. Retrieved May 23, 2008.
 
 
 
 
* Levit, E. K., and R. E. Boissy, R. E. 2001. Chapter 6. Basic science of the nail unit. In R. K. Freinkel, and D. T. Woodley. ''The Biology of the Skin''. New York: Parthenon Pub. Group. ISBN 1850700060.
 
* Levit, E. K., and R. E. Boissy, R. E. 2001. Chapter 6. Basic science of the nail unit. In R. K. Freinkel, and D. T. Woodley. ''The Biology of the Skin''. New York: Parthenon Pub. Group. ISBN 1850700060.
 
+
* Ophardt, C. E. 2003. [http://elmhurst.edu/~chm/vchembook/566secprotein.html Secondary protein--structure.] ''Virtual Chembook''. Retrieved May 23, 2008.
* Ophardt, C. E. 2003. [http://elmhurst.edu/~chm/vchembook/566secprotein.html Secondary protein - structure]. ''Virtual Chembook''. Retrieved May 23, 2008.
 
  
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]

Latest revision as of 14:48, 19 September 2008

Microscopy of keratin filaments inside cells.

Keratin is any one of the family of tough and insoluble fibrous structural proteins that form the chief, hard, nonmineralized structural component of hair, wool, horns, nails, claws, hooves, and other vertebrate tissues, as well as part of various invertebrate structures. Keratins are rivaled as biological materials in toughness only by chitin.

Keratins are present in all epithelial cells, both those covering the external surfaces of organisms and those on internal surfaces, such as the lining of the digestive tract. Keratins aid the epithelial cells in maintaining their connected integrity, as the keratins typically span the full interior width of a cell and are connected indirectly to keratins in adjoining cells through cell-to-cell junctions called desmosomes. The horn, hooves, nails, hair, and other keratin-based hard, tough materials growing on animals are produced by epithelial cells adapted to growing an abundance of keratin and then dying as individual cells while leaving the keratin to help form a structure valuable to the whole animal.

Keratin's characteristic toughness and resilience depend on its amino acid composition and sequence and the particular protein folding that results. In particular, the prevalence in Keratin of the sulfur-containing amino acid cysteine, with its ability to form strong covalent chemical bonds between sulfur atoms (disulfide bridge), helps confer strength and rigidity. The complexity involved in just this one type of protein is remarkable, and yet it is formed by a vast diversity of living organisms.

There are various types of keratins within a single animal. Keratin is nutritionally useless to humans, since it is not hydrolyzed by digestive enzymes, but it can be used as fertilizer, being slowly broken down by bacteria (Bender and Bender 2005).

Uses in animals

Keratins are the chief constituent of structures that grow from the skin of vertebrates. These structures include:

  • Among mammals, the hair (including wool), horns, nails, claws, corns, and hooves, which are made primarily of α-keratins
  • Among reptiles, the scales, claws, and, in the chelonians, such as tortoise, turtle, terrapin, the shells, which are made primarily of β-keratins
  • Among birds, the feathers, beaks, and claws, which are made primarily of β-keratins

The α-keratins are formed primarily as helical fibers, while the β-keratins are formed primarily in beta sheets. Some beta sheets are also found in α-keratins (Kreplak et al. 2004).

For example, hair, a filamentous outgrowth from the skin that is found only on mammals, involves fibers comprising nonliving cells whose primary component is the protein keratin, a long chain (polymer) of amino acids that naturally forms an α-helix fiber and subsequently winds two of the α-helix fibers together to form a much stronger "coiled coil" fiber characteristic of α-keratin. The keratinized cells arise from cell division in the hair matrix at the base of a hair follicle and are tightly packed together.

Keratin (high molecular weight) in bile duct cell and oval cells of mouse liver

Keratins also are a principle part of the cells in the tooth enamel of mammals and the baleen plates of filter-feeding whales. Although it is now difficult to be certain, the prevailing view among paleontologists is that the scales, claws, beaks, and some protective armor of dinosaurs most likely were composed of a type of keratin. In Crossopterygian fish, the outer layer of cosmoid scales was keratin.

Among the invertebrates, arthropods such as crustaceans often have parts of their armor or exoskeleton made of keratin, sometimes in combination with chitin, which is a hard, semitransparent polysaccharide that is the main component of the shells of crustaceans, such as crabs, lobsters, and shrimp. In arthropods, however, chitin is frequently modified by being embedded in a hardened proteinaceous matrix of keratin, giving a more rigid exoskeleton, than seen, for example, in the use of chitin in the soft, more pliable body wall of a caterpillar.

Keratins also can be integrated in the chitinophosphatic material that makes up the shell and setae (bristles) in many brachiopods. Keratins also are found in the gastrointestinal tracts of many animals, including roundworms (who also have an outer layer made of keratin).

Molecular biology and biochemistry

The properties that make structural proteins like keratins useful depend on their supermolecular aggregation, i.e., their pattern of protein (polypeptide strand) folding. The properties of collectives of protein strands depend on the properties of the individual polypeptide strands, which depend in turn on their amino acid composition and sequence. The α-helix and β-sheet motifs, and the disulfide bridges, are central to the architecture and aggregation of keratins.

Disulfide bridges

Cysteine, an amino acid with the thiol functional group -SH.
Cystine, showing a disulfide bond (-S-S-) joining two cysteine residues.

Keratins have large amounts of the sulfur-containing amino acid cysteine, which is characterized by the thiol functional group, -SH, comprising a sulfur atom and a hydrogen atom. In the keratin polymer, which is originally extremely flexible, the thiol groups tend to pair up and, through oxidation, form a covalent sulfur-sulfur, that is, disulfide, bond with the loss of two protons and two electrons. A disulfide bond, also called a SS-bond or disulfide bridge, achieves an overall connectivity represented by C-S-S-C, in which "C" represents the immediate next carbon atom and all the remainder of the associated amino acid. Expressed more formally, when cysteine is oxidized it can form cystine, which is two cysteine residues (cys) joined by a disulfide bond (cys-S-S-cys) between the -SH group.

Disulfide bridges confer additional strength and rigidity by permanent, thermally-stable crosslinking—a role sulfur bridges also play in vulcanized rubber. Human hair is approximately 14 percent 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 or reducing 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-coiled single protein strands (with regular intra-chain H-bonding), which are then further wound together into superhelical or coiled-coil 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.

Glycine and alanine

The general structure of an amino acid molecule. For glycine, the simplest amino acid, R in the figure is replaced by -H. For alanine, R is replaced by the methyl group -CH3. The amino group (-NH2) is on the left and the carboxyl group (-COOH) is on the right.

Keratins contain a high proportion of the smallest of the 20 amino acids, glycine, whose "side group" is a single hydrogen atom. They also contain a high proportion of the next smallest, alanine, whose functional side group is the small and noncharged methyl group. In the case of β-sheets, this high proportion of simple and neutral side groups allows sterically-unhindered hydrogen bonding between the amino and carboxyl groups of peptide bonds on adjacent protein chains, facilitating their close alignment and strong bonding. Fibrous keratin molecules can twist around each other to form double-wound helical intermediate filaments.

The triple helix of the (unrelated) structural protein collagen, found in skin, cartilage, and bone, likewise has a high percentage of glycine, as does the connective tissue protein elastin, which also has a high percentage of alanine. Spider silk fibroin, considered a β-keratin, can have glycine and alanine as 75–80 percent of its total amino acids, with an additional 10–15 percent being serine, and the rest being amino acids that have bulky side groups. The chains are antiparallel, with an alternating C → N orientation (Ophardt 2003). A preponderance of amino acids with small, nonreactive side groups is characteristic of structural proteins, for which H-bonded close packing is more important than chemical specificity.

Cornification

In mammals, there are soft epithelial keratins, the cytokeratins, and harder hair keratins. As certain skin cells differentiate and become cornified, pre-keratin polypeptides are incorporated into intermediate filaments. Eventually, the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized.

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 keratin to proliferate with the formation of protective calluses—useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced (such as dandruff).

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.

Another example of keratinzed cells are nails. According to Levit and Boissy (2001), the nail plate is composed of "closely packed, fully keratinized, multilayered lamellae of cornified cells" (Levit and Boissy 2001). Essentially, cells in the epidermis contain a structural matrix of keratin.

The nail matrix cells differentiate and create the nail plate by flattening, broadening, and by nuclear fragmentation, with an accumulation of cytoplasmic microfibrils (Levit and Boissy 2001). As skin cells become cornified, and the nucleus and cytoplasmic organelles disappear and metabolism ceases, the cells become fully keratinized. Hard structures are formed by intercellular cementing of fibers formed from the dead, cornified cells.The keratins in the nail plate are believed to be held in place by surrounding globular matrix proteins with a high concentration of disulfide bonds between cystine (rather than by means of calcium, as in bones), creating the rigid structure (Levit and Boissy 2001).

Silk

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

Silk found in insect pupae, and in spider webs and egg casings, also has twisted β-pleated sheets incorporated into fibers wound into larger supermolecular aggregates. The structure of the spinnerets on spiders’ tails, and the contributions of their interior glands, provide remarkable control of fast extrusion. Spider silk is typically about 1 to 2 micrometers (µm) thick, compared with about 60 µm for human hair, and more for some mammals. (Hair, or fur, occurs only in mammals.) The biologically and commercially useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, crystalline regions of varying size, alternating with flexible, amorphous regions where the chains are randomly coiled (AMO 2002).

A somewhat analogous situation occurs with synthetic polymers such as nylon, developed as a silk substitute. Silk from the hornet cocoon contains doublets about 10 µm across, with cores and coating, and may be arranged in up to 10 layers; also in plaques of variable shape. Adult hornets also use silk as a glue, as do spiders.

Medical significance

Some infectious fungi, such as those that cause athlete's foot, ringworm, and the amphibian disease chytridiomycosis (caused by the chytrid fungus, Batrachochytrium dendrobatidis), feed on keratin.

Diseases caused by mutations in the keratin genes include:

  • Epidermolysis bullosa simplex
  • Ichthyosis bullosa of Siemens
  • Epidermolytic hyperkeratosis
  • Steatocystoma multiplex

Although keratin is insoluble and is not easily hydrolysed by digestive enzymes (Bender and Bender 2005), it can be used for coating pills designed to be dissolved when in the intestine. A supplement for ruminants also is made from steamed feather meal (Bender and Bender 2005).

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