Download 1 keratin, fibrous structural protein of hair, nails, horn, hoofs, wool

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keratin, fibrous structural protein of hair, nails, horn,
hoofs, wool, feathers, and of the epithelial cells in the
outermost layers of the skin. The polypeptide chains of
keratin are arranged in parallel sheets held together
by hydrogen bonding.
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Of the amino acids in keratin, cystine may account
for as much as 24 percent. The numerous disulfide bonds
formed by cystine are responsible for the great stability of
keratin: it is completely insoluble in hot or cold water and
is not attacked by proteolytic enzymes (the enzymes that
cleave protein molecules).
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The length of keratin fibres depends on their water
content: complete hydration (approximately 16 percent
water) increases their length by 10 to 12 percent.
Keratin
Animal Sciences | 2002 | Sheen, Judy P. |
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Keratin is a highly durable protein that provides
structure to several types of living tissues. It is a major
component of mammalian hair and hooves, mammalian
and reptilian nails and horns, reptile and fish scales, bird
feathers, bird beaks, and the outermost layer of skin in
most animals. Keratin provides a tough, fibrous matrix to
these tissues. An important quality of keratin is its ability to
flex in multiple directions without tearing.
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Keratin's microscopic structure is the key to its
durability and flexibility. The molecules of this protein twist
into coils called alpha helices and contain many
disulfide bonds (bonds between pairs of sulfur ions).
Disulphide bonds are particularly stable and can resist
the action of proteolytic enzymes, which specialize in
breaking apart proteins. Keratin is also insoluble in water.
When human hair is straightened or curled in a beauty
salon, special chemicals must be used to break the
disulfide bonds. The breaking and subsequent
reconfiguration of these bonds allows the hair to change
shape. The final shape depends on the relative positions
of the sulfur ions in the new bonds.
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The protective structures containing keratin form
through a process called keratinization. In keratinization,
precursor cells to the specific tissue types first migrate
from the germinal layer to their target location. Then
fibers of keratin gradually invade the precursor cells,
displacing cell organelles such as the nucleus and
mitochondria . These organelles are resorbed and are
not present in the mature tissue type. The differentiated,
mature keratinized tissue is nonliving and incapable of
sensory perception. Keratinized structures grow through
the additional migration of differentiating germ cells , not
through the division of the existing tissue cells.
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Keratinized tissues can form onto a base of skin or
bone. Keratinized structures such as hairs and fingernails
are embedded in the skin. Calluses on hands or feet are
mounds of keratin which have been created in response
to repeated stress on a particular region of skin. Other
structures, such as the horns of a bull, are rooted onto a
bony core.
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Keratinized structures take on a wide range of
characteristics depending on the thickness of the protein
layers. Hair is thin and flexible, whereas scales are often
tough and impenetrable. Keratin is also present in sharp
structures such as spines and porcupine quills.