Essentially, cells in the epidermis contain a structural matrix of keratin.
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.
Keratins contain a high proportion of the smallest of the 20 amino acids, glycine, whose "side group" is a single hydrogen atom.
Multiple genes have been identified for the ?-keratins in feathers, and this is probably characteristic of all keratins.
Keratin's characteristic toughness and resilience depend on its amino acid composition and sequence and the particular protein folding that results.
Keratins are the chief constituent of structures that grow from the skin of vertebrates.
Some beta sheets are also found in ?-keratins (Kreplak et al.
Extensive disulfide bonding contributes to the insolubility of keratins, except in dissociating or reducing agents.
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.
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).
Keratinized epidermal cells are constantly shed and replaced (such as dandruff).
Keratins are rivaled as biological materials in toughness only by chitin.
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.
Keratins also are found in the gastrointestinal tracts of many animals, including roundworms (who also have an outer layer made of keratin).
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.
Keratins also can be integrated in the chitinophosphatic material that makes up the shell and setae (bristles) in many brachiopods.
The ?-keratins are formed primarily as helical fibers, while the ?-keratins are formed primarily in beta sheets.
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.
Fibrous keratin molecules can twist around each other to form double-wound helical intermediate filaments.
Hair and other ?-keratins consist of ?-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 ?-helix and ?-sheet motifs, and the disulfide bridges, are central to the architecture and aggregation of keratins.
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.
The properties that make structural proteins like keratins useful depend on their supermolecular aggregation, i.e., their pattern of protein (polypeptide strand) folding.
The ?-keratins of reptiles and birds have ?-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.