Fibrous structural protein of hair, nails, hooves, wool, feathers, and skin. A quarter of the amino acids in keratin are cystine, whose ability to form strong bridging (disulfide) bonds with other cystine units accounts for keratin's great stability. Keratin does not dissolve in cold or hot water and does not easily undergo proteolysis. Its fibres are 10–12percnt longer at maximum water content (about 16percnt) than when dry. The sulfurous smell of burning keratin is distinctive.
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Keratins are a family of fibrous structural proteins; tough and insoluble, they form the hard but nonmineralized structures found in reptiles, birds, amphibians and mammals. They are rivaled as biological materials in toughness only by chitin.
There are various types of keratins within a single animal.
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 keratin to proliferate with the formation of protective calluses — useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced (see dandruff).
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 moult 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.
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. 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.
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. Hair and other α-keratins consist of α-helically-coiled single protein strands (with regular intra-chain H-bonding), which are then further twisted into superhelical ropes that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.
Silk found in insect pupae, and in spider webs 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 glands, provide remarkable control of fast extrusion. Spider silk is typically about 1 to 2 micrometres (µm) thick, compared with about 60 µm for human hair, and more for some mammals. (Hair, or fur, occurs only in mammals.) The biologically and commercially useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, crystalline regions of varying size, alternating with flexible, amorphous regions where the chains are randomly coiled. A somewhat analogous situation occurs with synthetic polymers such as nylon, developed as a silk substitute. Silk from the hornet cocoon 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 glue, as do spiders.
|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|
Diseases caused by mutations in the keratin genes include
Molecular evolution of the keratin associated protein gene family in mammals, role in the evolution of mammalian hair.(Research article)
Aug 23, 2008; Authors: Dong-Dong Wu [1,3]; David M Irwin [4,5]; Ya-Ping Zhang (corresponding author) [1,2] Background The availability of the...
Mutations in the rod domains of keratins 1 and 10 in epidermolytic hyperkeratosis. (bullous congenital ichyosiform erythroderma)
Aug 21, 1992; Epidermolytic hyperkeratosis (EHK) (bullous congenital ichyosiform erythroderma) is clinically distinct from epidennolysis...