Keratin is a filamentous protein rich in sulfur, with α-helix domains supercoiled two by two (coiled coil), insoluble in common solvents and polymerizing by self-assembly, give the intermediate filaments specific for each epithelium (all epithelia have keratin, but different and / or in smaller quantities). Numerous sulfate groups (cysteines) are present, which create disulfide bridges between them further stabilizing the structure of the protein; it is very stable and resistant.
It is produced by keratinocytes, and is immersed in their cytosol as intermediate filaments. It is the main constituent of the stratum corneum of the epidermis of tetrapods and especially of amniotes, in which it ensures impermeability.
It is the main constituent of many structures and formations having a protective or coating function, such as wool, hair, spines, the stratum corneum of the epidermis, nails, the coating of horns, the feathers and claws of birds, the scales of fish, etc..
Pure keratin is insoluble in water and most organic solvents; it resists the action of dilute acids and proteolytic enzymes (pepsin, papain). The keratin molecule is rich in sulfur and contains high amounts of the amino acids proline and cysteine. However, its amino acid composition varies with origin.
Based on physical characters, keratins are distinguished into soft and hard. The first ones are translucent, plastic in consistency, easily desquamate in small scales; when exposed to heat they retract while in cold water they hydrate and swell. Hard keratins, on the other hand, are compact, yellowish, not desquamable and very resistant to water and heat.
In the molecular structure of keratins there are two components, one amorphous and one paracrystalline. The latter consists of a protein macromolecule with a molecular weight of over 50,000, poor in sulfur residues, organized into thin filaments with a diameter of 70-80 Å, around which is arranged the amorphous component, which is called “inter-filament matrix”.
Groups of thickened filaments form the so-called “tonofibrils”, visible under an optical microscope and considered in the past as elementary structures of keratins. Keratin is used in the pharmaceutical industry to coat “gastro-resistant” pills and as a raw material for the production of protein hydrolysates. Modern knowledge of the molecular organization of keratins has been put to good use in the cosmetics industry for the preparation of lacquers, lotions, hair dyes, etc. They have also contributed in the textile industry to the refinement of methods to color wool and to make it unshrinkable. Keratin is also used in the preparation of foams for fire extinguishers.
Keratinization is the typical specialization of the multilayered flat tissue, given by the presence of keratins in large quantities in the cells of the outermost layer. Keratinization allows tissues to resist drying as well.
Keratinization abnormalities are a group of skin disorders characterized by qualitative or quantitative alterations, classified as follows: