Keratin

 ( Keratins ) 

Fibrous keratin molecules supercoil to form a very stable, left-handed superhelix motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer.

Keratin filaments are intermediate filaments. 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, if the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments.

• 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

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 found in insect , and in 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 , 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. The biology and commerce useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, regions of varying size, alternating with flexible, amorphous regions where the chains are random coil. A somewhat analogous situation occurs with synthetic such as nylon, developed as a silk substitute. Silk from the hornet Silkworm cocoons 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, as do spiders.

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

Several diseases, such as athlete's foot and ringworm, are caused by dermatophytes that feed on keratin.

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

• 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
• Keratinase

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