Keratin

Keratin (

Spider silk is classified as keratin,^[9] although production of the protein may have evolved independently of the process in vertebrates.

Examples of occurrence

Keratins (also described as

Genes

This section may require cleanup to meet Wikipedia's quality standards. The specific problem is: Not particularly helpful to dump a big list of KRT genes here. Using the source a bit more to explain what each gene and each zone of genes mean will be helpful, as we currently have no particular examples of a hair keratin. Please help improve this section if you can. (October 2022) (Learn how and when to remove this template message)

The human genome encodes 54 functional keratin genes, located in two clusters on chromosomes 12 and 17. This suggests that they originated from a series of gene duplications on these chromosomes.^[13]

The keratins include the following proteins of which

Protein structure

The first sequences of keratins were determined by Israel Hanukoglu and Elaine Fuchs (1982, 1983).^[16][17] These sequences revealed that there are two distinct but homologous keratin families, which were named type I and type II keratins.^[17] By analysis of the primary structures of these keratins and other intermediate filament proteins, Hanukoglu and Fuchs suggested a model in which keratins and intermediate filament proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation.^[17] This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.^[18]

Type 1 and 2 Keratins

The human genome has 54 functional annotated Keratin genes, 28 are in the Keratin type 1 family, and 26 are in the Keratin type 2 family. ^[19]

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.^[20]

The major force that keeps the coiled-coil structure is

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.^[22] 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.

Disulfide bridges

In addition to intra- and intermolecular

It has been proposed that keratins can be divided into 'hard' and 'soft' forms, or 'cytokeratins' and 'other keratins'.^{[clarification needed][dubious – discuss]} That model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account.^[14]

Keratin filaments are

Pairing

Cornification

Cornification is the process of forming an epidermal barrier in stratified squamous epithelial tissue. At the cellular level, cornification is characterised by:

• production of keratin
• production of small proline-rich (SPRR) proteins and transglutaminase which eventually form a cornified cell envelope beneath the plasma membrane
• terminal differentiation
• loss of nuclei and organelles, in the final stages of cornification

Metabolism ceases, and the cells are almost completely filled by keratin. During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton to mechanically stabilize the cell against physical stress. It does this through connections to desmosomes, cell–cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures.

Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis and form protective calluses, which are useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced.

These hard, integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by specialized beds deep within the skin. Hair grows continuously and feathers molt and regenerate. The constituent proteins may be phylogenetically homologous but differ somewhat in chemical structure and supermolecular organization. The evolutionary relationships are complex and only partially known. Multiple genes have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.

Silk

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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

Glues made from partially-hydrolysed keratin include hoof glue and horn glue.

Clinical significance

Abnormal growth of keratin can occur in a variety of conditions including keratosis, hyperkeratosis and keratoderma.

Mutations in keratin gene expression can lead to, among others:

• Alopecia Areata
• Epidermolysis bullosa simplex
• Ichthyosis bullosa of Siemens
• Epidermolytic hyperkeratosis
• Steatocystoma multiplex
• Keratosis pharyngis
• Rhabdoid cell formation in
  Large cell lung carcinoma with rhabdoid phenotype^[31][32]

Keratin is highly resistant to digestive acids if ingested. Cats regularly ingest hair as part of their grooming behavior, leading to the gradual formation of hairballs that may be expelled orally or excreted. In humans, trichophagia may lead to Rapunzel syndrome, an extremely rare but potentially fatal intestinal condition.

Diagnostic use

Keratin expression is helpful in determining epithelial origin in

• Keratin-associated proteins
  (KRTAPs)
• List of cutaneous conditions caused by mutations in keratins
• List of keratins expressed in the human integumentary system
• List of keratins

References

External links

Wikisource has the text of the 1920 Encyclopedia Americana article Keratin.

• Composition and β-sheet structure of silk
• Hair-Science.com's entry on the microscopic elements of hair
• Proteopedia page on keratins