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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.

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.

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.

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.

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 inpenetrable. Keratin is also present in sharp structures such as spines and porcupine quills.

see also Bone; chitin.

Judy P. Sheen


Balin, Arthur K., Loretta Pratt Balin, and Marietta Whittlesey. The Life of the Skin: What It Hides, What It Reveals, and How It Communicates. New York: Bantam Books, 1998.

Fraser, R. D. B., T. P. MacRae, and G. E. Rogers. KeratinsTheir Composition, Structure and Biosynthesis. Springfield, IL: Charles C. Thomas, 1972.

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keratin (kĕr´ətĬn), any one of a class of fibrous protein molecules that serve as structural units for various living tissues. The keratins are the major protein components of hair, wool, nails, horn, hoofs, and the quills of feathers. These proteins generally contain large quantities of the sulfur-containing amino acids, particulary cysteine. The helical keratin molecules twist around each other to form elongated strands called intermediate filaments. The formation of a covalent chemical bond called a disulfide bridge between the sulfer atoms on two cysteins on separate polypeptide chains of keratin allows for the cross-linkage of these chains and results in a fairly rigid aggregate. This phenomenon is seen to be consistent with the physiological role of the keratins, which provide a tough, fibrous matrix for the tissues in which they are found. Human hair is approximately 14% cystine (cysteins cross-linked by disulfide bridges).

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ker·a·tin / ˈkerətin/ • n. a fibrous protein forming the main structural constituent of hair, feathers, hoofs, claws, horns, etc. DERIVATIVES: ke·rat·i·nous / kəˈratn-əs/ adj.

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keratin Any of a group of fibrous proteins occurring in hair, feathers, hooves, and horns. Keratins have coiled polypeptide chains that combine to form supercoils of several polypeptides linked by disulphide bonds between adjacent cysteine amino acids. Aggregates of these supercoils form microfibrils, which are embedded in a protein matrix. This produces a strong but elastic structure.

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keratin Fibrous protein present in large amounts in skin cells, where it serves as a protective layer. Hair and fingernails are made up of cells filled with keratin, which is also the basis of claws, horns, and feathers.

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keratin A group of fibrous sclero-proteins that usually contain large amounts of sulphur as cystine and which form the structural bases of hair, wool, nails, and other epidermal structures in animals.

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keratin The insoluble protein of hair, horn, hoofs, feathers, and nails. Not hydrolysed by digestive enzymes, and therefore nutritionally useless. Used as fertilizer, since it is slowly broken down by soil bacteria. Steamed feather meal is used to some extent as a supplement for ruminants.

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keratin (ke-ră-tin) n. one of a family of proteins that are the major constituents of the nails, hair, and the outermost layers of the skin.