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Keratin

Keratins are a family of fibrous protein|fibrous structural proteins; tough and insoluble, they form the hard but mineral|nonmineralized structures found in reptiles, birds, amphibians and mammals. They are rivaled as biology|biological materials in toughness only by chitin. There are various types of keratins within a single animal.

Variety of animal uses

Keratins are the main constituent of structures that grow from the skin:
- the ''α-keratins'' in the hair (including wool), horn (anatomy)|horns, nail (anatomy)|nails, claws and Hoof|hooves of mammals
- the harder ''β-keratins'' found in nails and in the scale (zoology)|scales and claws of reptiles, their animal shell|shells (chelonians, such as tortoise, turtle, terrapin), and in the feathers, beaks, claws of birds and quills of porcupines. (These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins.) Arthropods such as crustaceans often have parts of their armour (zoology)|armor or exoskeleton made of keratin, sometimes in combination with chitin. The baleen plates of filter-feeding whales are made of keratin. They can be integrated in the chitinophosphatic material that makes up the sea shell|shell and setae in many brachiopods. Keratins are also found in the gastrointestinal tracts of many animals, including roundworms (which also have an outer layer made of keratin). Although it is now difficult to be certain, the scales, claws, some Thyreophora|protective armour and the beaks of dinosaurs would, almost certainly, have been composed of a type of keratin. In Crossopterygian fish, the outer layer of Scale (zoology)|cosmoid scales was keratin.

Cornification

It used to be thought that keratins were separable into 'hard' and 'soft,' or 'cytokeratins' and 'other keratins', but those designations are now understood to be incorrect. In 2006, a new nomenclature was adopted for describing keratins which takes this into account New consensus nomenclature for mammalian keratins. J Cell Biol. 2006 Jul 17;174(2):169-74.. Keratins 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, the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments. During the process of epithelial cellular differentiation|differentiation, cells become cornification|cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the cell nucleus|nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a apoptosis|programmed death as they become fully keratinized. In many other cells types, such as dermis, keratin functions in the cytoplasm 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. Cell (biology)|Cells in the epidermis (skin)|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 system|integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by sebaceous glands|specialized beds deep within the skin. Hair grows continuously and feathers moult and regenerate. The constituent proteins may be phylogenetics|phylogenetically homology (biology)|homologous but differ somewhat in chemical compound|chemical structure and supermolecule|molecular 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.

Molecular biology and biochemistry

The properties which make structural proteins like keratins useful depend on their supermolecular aggregation. These depend on the properties of the individual peptide|polypeptide strands, which depend in turn on their amino acid composition and sequence. The alpha helix|α-helix and beta sheet|β-sheet motifs, and disulfide bridges, are crucial to the protein structure#Secondary structure elements|conformations of globular protein|globular, functional proteins like enzymes, many of which operate semi-independently, but they take on a completely dominant role in the architecture and aggregation of keratins.

Glycine and alanine

Keratins contain a high proportion of the smallest of the 20 amino acids, glycine, whose "side chain|side group" is a single hydrogen atom; also the next smallest, alanine, with a small and noncharged methyl group. In the case of β-sheets, this allows steric effects|sterically-unhindered hydrogen bonding between the amine|amino and carboxyl groups of peptide bonds on adjacent protein chains, facilitating their close alignment and strong binding. Fibrous keratin molecules can twist around each other to form helix|helical intermediate filaments. 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.http://elmhurst.edu/~chm/vchembook/566secprotein.html A preponderance of amino acids with small, chemical reaction|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 hydrogen bonds, keratins have large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bond|disulfide bridges that confer additional strength and rigidity by permanent, thermally-stable cross-link|crosslinking—a role sulfur bridges also play in vulcanization|vulcanized rubber. Human hair is approximately 14% cysteine. The pungent smells of burning hair and rubber are due to the sulfur compounds formed. Extensive disulfide bonding contributes to the insoluble|solubility of keratins, except in dissociation (chemistry)|dissociating or redox|reducing agents. 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 alpha helix|α-helically-coiled single protein strands (with regular intra-chain hydrogen bond|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

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 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 biology|biologically and commerce|commercially useful properties of spider silk#Properties of spider silk|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 random coil|randomly coiled.Spiders - Silk structure A somewhat analogous situation occurs with chemical synthesis|synthetic polymers such as nylon, developed as a silk substitute. Silk from the hornet Pupa#Cocoon|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 adhesive|glue, as do spiders.

Pairing

Clinical significance

Some infection|infectious fungus|fungi, such as those which cause athlete's foot, ringworm or the ''Batrachochytrium dendrobatidis'' (Chytrid fungus) which is killing amphibians all over the world, feed on keratin. Diseases caused by mutations in the keratin genes include
- Epidermolysis bullosa simplex
- Ichthyosis bullosa of Siemens
- Epidermolytic hyperkeratosis
- Steatocystoma multiplex
- Keratosis pharyngis

Additional images

Image:Keratin.jpg|Keratin (high molecular weight) in bile duct cell and oval cells of mouse liver.

References

External links

- Composition and β-sheet structure of silk
- Spider silk fiber structure
- Hair-Science.com's entry on the miroscopic elements of hair
- keratin antibody review Category:Keratins Category:Cytoskeleton simple:Keratin

Related Images

- Microscopy of keratin filaments inside cells.

Sources: StartLearningNow, Wikipedia | Usage license: GNU FDL

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