KERATIN (/ˈkɛrətɪn/ ) is one of a family of fibrous structural
* 1 Etymology * 2 Examples of occurrence * 3 Genes
* 4.1 Disulfide bridges * 4.2 Filament formation * 4.3 Pairing
* 5 Cornification
EXAMPLES OF OCCURRENCE
Horns such as those of the impala are made up of keratin covering a core of live bone
* the α-keratins in the hair (including wool ), stratum corneum , horns , nails , claws and hooves of mammals and the hagfish slime threads. * the harder β-keratins found in nails and in the scales and claws of reptiles , their shells ( Testudines , such as tortoise , turtle , terrapin ), and in the feathers , beaks , claws of birds and quills of porcupines. (These keratins are formed primarily in beta sheets . However, beta sheets are also found in α-keratins.)
The baleen plates of filter-feeding whales are made 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 VI intermediate filaments , lamins , which have a long rod domain (vs. a short rod domain for the keratins).
The neutral-basic keratins are found on chromosome 12 (12q13.13). While the acidic keratins are found on chromosome 17 (17q21.2).
The human genome encodes 54 functional keratin genes which are located in two clusters on chromosomes 12 and 17. This suggests that they have originated from a series of gene duplications on these chromosomes.
The keratins include the following proteins of which
KRT71 , KRT72
The first sequences of keratins were determined by Hanukoglu and
Fuchs. These sequences revealed that there are two distinct but
homologous keratin families which were named as
Type I keratin and
Type II keratins. By analysis of the primary structures of these
keratins and other intermediate filament proteins, Hanukoglu and Fuchs
suggested a model that keratins and intermediate filament proteins
contain a central ~310 residue domain with four segments in α-helical
conformation that are separated by three short linker segments
predicted to be in beta-turn conformation. This model has been
confirmed by the determination of the crystal structure of a helical
domain of keratins.
Fibrous keratin molecules supercoil to form a very stable, left-handed superhelical motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer .
The major force that keeps the coiled-coil structure is hydrophobic interactions between apolar residues along the keratins helical segments.
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
In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of large amounts of the sulfur -containing amino acid cysteine , required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking —in much the same way that non-protein sulfur bridges stabilize vulcanized rubber . Human hair is approximately 14% cysteine. The pungent smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the insolubility of keratins, except in a small number of solvents such as dissociating or reducing agents.
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.
It was theorized that keratins are combined into 'hard' and 'soft,' or 'cytokeratins ' and 'other keratins'. That model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account.
A (NEUTRAL-BASIC) B (ACIDIC) OCCURRENCE
keratin 1 , keratin 2 keratin 9 , keratin 10 stratum corneum , keratinocytes
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
Cornification is the process of forming an epidermal barrier in stratified squamous epithelial tissue. At the cellular level, cornification is characterised by:
* production of keratin * production of small proline-rich (SPRR) proteins and transglutaminase which eventually form a cornified cell envelope beneath the plasma membrane * terminal differentiation * loss of nuclei and organelles, in the final stages of cornification
Metabolism ceases, and the cells are almost completely filled by keratin. During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton to mechanically stabilize the cell against physical stress. It does this through connections to desmosomes, cell-cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures.
Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin, gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis and form protective calluses — useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced.
These hard, integumentary structures are formed by intercellular
cementing of fibers formed from the dead, cornified cells generated by
specialized beds deep within the skin.
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.
Some infectious fungi , such as those that cause athlete\'s foot and ringworm (i.e. the dermatophytes ), or Batrachochytrium dendrobatidis (Chytrid fungus), feed on keratin.
Diseases caused by mutations in the keratin genes include
* Epidermolysis bullosa simplex * Ichthyosis bullosa of Siemens * Epidermolytic hyperkeratosis * Steatocystoma multiplex * Keratosis pharyngis * Rhabdoid cell formation in Large cell lung carcinoma with rhabdoid phenotype
Furthermore, keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas , thymomas , sarcomas and trophoblastic neoplasms . Furthermore, the precise expression pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases. For example, hepatocellular carcinomas typically expresse K8 and K18, and cholangiocarcinomas express K7, K8 and K18, while metastases of colorectal carcinomas express K20, but not K7.
* ^ "Keratin". Online Etymology Dictionary.
"Horn". Online Etymology Dictionary.
* ^ "kerato-". Online Etymology Dictionary.
"Horn". Online Etymology Dictionary. * ^ "-in/-ine chemical
suffix". Online Etymology Dictionary.
* ^ Hickman, Cleveland Pendleton; Roberts, Larry S.; Larson, Allan
L. (2003). Integrated principles of zoology. Dubuque, IA: McGraw-Hill.
p. 538. ISBN 0-07-243940-8 .
* ^ Kreplak, L.; Doucet, J.; Dumas, P.; Briki, F. (July 2004). "New
Aspects of the α-Helix to β-Sheet Transition in Stretched Hard