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Keratin
*** Shopping-Tip: Keratin
Image:KeratinF9.png thumb|right|370px|Microscopy of keratin filaments inside cells.
'''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 and
mammals. The
baleen plates of filter-feeding
whales are made of them. Keratins are also found in the
gastrointestinal tracts of many animals, including
roundworms. They are rivaled in
biology biological toughness only by
chitin, a
cellulose-like
polymer of
glucose glucosamine, the main constituent of the
exoskeletons of
arthropods. There are various types, even within a single
animal. Some
infection infectious fungus fungi, such as those which cause
athlete's foot and
ringworm, feed on keratin. 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.
Keratin in the Kingdom Animalia
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).
In mammals there are soft
epithelial keratins, the
cytokeratins, and harder
hair keratins. As certain skin cells
cellular differentiation differentiate and
cornify, pre-keratin
polypeptides are incorporated into
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.
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 hooves of mammals; also the harder β-keratins in the
scale (zoology) scales and
claws of reptiles, and their
animal shell shells (
tortoises), and in the
feathers,
beaks, and claws of birds. These hard,
Integumentary system integumentary structures are formed by
extracellular 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 super
molecule 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.
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 uncharged
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://www.elmhurst.edu/~chm/vchembook/566secprotein.html] A preponderance of amino acids with small,
chemical reaction unreactive side groups is characteristic of structural proteins, for which H-bonded close packing is more important than
chemical specificity.
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 in
soluble solubility of keratins, except in
dissociation (chemistry) dissociating or
redox reducing agents such as
urea.
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 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.[http://www.amonline.net.au/spiders/toolkit/silk/structure.htm] A somewhat analogous situation occurs with
chemical synthesis synthetic polymers such as
nylon, developed as a silk substitute. Silk from the
hornet cocoon (silk) 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.
See also
*
Acne
*
Keratosis pilaris
*
Intermediate filament
*
Desmosome
References
* World Book Encyclopedia (1998)
External links
-
Composition and β-sheet structure of silk
-
Spider silk fiber structure
Category:KeratinsCategory:Cytoskeleton
bg:Кератин
da:Keratin
de:Keratin
es:Queratina
eo:Keratino
fr:Kératine
he:קרטין
lt:Keratinas
nl:Keratine
ja:ケラ�ン
no:Keratin
pt:Queratina
sv:Keratin
*** Shopping-Tip: Keratin
