Difference between revisions of "Keratin" - New World Encyclopedia

From New World Encyclopedia
Line 1: Line 1:
 
{{Images OK}}
 
{{Images OK}}
[[Image:KeratinF9.png|thumb|right|300px|Microscopy of keratin filaments inside cells.]]
+
[[Image:KeratinF9.png|thumb|right|240px|Microscopy of keratin filaments inside cells.]]
 
'''Keratin''' is any one of the family of tough and insoluble [[fibrous protein|fibrous structural proteins]] that form the chief, hard, [[mineral|nonmineralized]] structural component of [[hair]], [[wool]], [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claw]]s, [[Hoof|hooves]], and other [[vertebrate]] tissues, as well as part of various [[invertebrate]] structures. Keratins are rivaled as [[biology|biological]] materials in toughness only by [[chitin]].
 
'''Keratin''' is any one of the family of tough and insoluble [[fibrous protein|fibrous structural proteins]] that form the chief, hard, [[mineral|nonmineralized]] structural component of [[hair]], [[wool]], [[horn (anatomy)|horns]], [[nail (anatomy)|nails]], [[claw]]s, [[Hoof|hooves]], and other [[vertebrate]] tissues, as well as part of various [[invertebrate]] structures. Keratins are rivaled as [[biology|biological]] materials in toughness only by [[chitin]].
  

Revision as of 03:23, 24 May 2008

Microscopy of keratin filaments inside cells.

Keratin is any one of the family of tough and insoluble fibrous structural proteins that form the chief, hard, nonmineralized structural component of hair, wool, horns, nails, claws, hooves, and other vertebrate tissues, as well as part of various invertebrate structures. Keratins are rivaled as biological materials in toughness only by chitin.

The properties that make keratin so useful depend on the amino acid composition and sequence and the particular protein folding that results. In particular, the prevalence of the sulfur-containing amino acid cysteine, with its ability to form strong covalent chemical bonds between sulfur atoms (disulfide bridge), helps confer strength and rigidity. The complexity involved in just this one type of protein is remarkable, and yet it is formed by a vast diversity of living organisms.

There are various types of keratins within a single animal. Keratin is nutritionally useless to humans, since it is not hydrolyzed by digestive enzymes, but it can be used as fertilizer, being slowly broken down by bacteria (Bender and Bender 2005).

Uses in animals

Keratins are the chief constituent of structures that grow from the skin of vertebrates. These include:

  • the α-keratins in the hair (including wool), horns, nails, claws, and hooves of mammals;
  • the harder β-keratins in the scales and claws of reptiles, their shells (chelonians, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds. These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins (Kreplak et al. 2004).

For example, hair, a filamentous outgrowth from the skin that is found only on mammals, involves fibers that comprise nonliving cells whose primary component is long chains (polymers) of amino acids forming the protein keratin. The keratinized cells arise from cell division in the hair matrix at the base of a hair follicle and are tightly packed together.

Keratin (high molecular weight) in bile duct cell and oval cells of mouse liver

Keratins also are a principle part of the cells in the tooth enamel of mammals, and the baleen plates of filter-feeding whales are made of keratin. Although it is now difficult to be certain, the scales, claws, some 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 cosmoid scales was keratin.

In invertebrates, arthropods such as crustaceans often have parts of their armor or exoskeleton made of keratin, sometimes in combination with chitin. Chitin is a hard, semitransparent polysaccharide, and is the main component of the shells of crustaceans, such as crabs, lobsters, and shrimp. In arthropods, however, it is frequently modified by being embedded in a hardened proteinaceous matrix of keratin, giving a more rigid exoskeleton, than seen in the use of chitin in the soft, more pliable body wall of a caterpillar.

Keratins also can be integrated in the chitinophosphatic material that makes up the shell and setae in many brachiopods. Keratins also are found in the gastrointestinal tracts of many animals, including roundworms (who also have an outer layer made of keratin).

Molecular biology and biochemistry

The properties that make structural proteins like keratins useful depend on their supermolecular aggregation. These depend on the properties of the individual polypeptide strands, which depend in turn on their amino acid composition and sequence. The α-helix and β-sheet motifs, and disulfide bridges, are crucial to the conformations of 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.

Disulfide bridges

Cystine, showing disulfide bond joining two cysteine residues

Keratins have large amounts of the sulfur-containing amino acid cysteine. Cysteine is characterized by the presence of a thiol group (or sulfhydryl group), which is a functional group composed of a sulfur atom and a hydrogen atom (-SH). Since thiol groups can undergo reduction (redox) reactions, cysteine can undergo redox reactions. Oxidation of cysteine can produce a disulfide bond with another thiol. A disulfide bond, also called a SS-bond or disulfide bridge, is a single covalent bond derived from the coupling of thiol groups; the overall connectivity is C-S-S-C. That is, when cysteine is oxidized it can form cystine, which is two cysteine residues joined by a disulfide bond (cys-S-S-cys) between the -SH group.

Disulfide bridges confer additional strength and rigidity by permanent, thermally-stable crosslinking—a role sulfur bridges also play in 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 insolubility of keratins, except in dissociating or 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 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.

Glycine and alanine

Keratins contain a high proportion of the smallest of the 20 amino acids, glycine, whose "side group" is a single hydrogen atom. It also contains a high proportion of the next smallest, alanine, with a small and noncharged methyl group. In the case of β-sheets, this allows sterically-unhindered hydrogen bonding between the 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 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 (Ophardt 2003). 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.

Cornification

In mammals, there are soft epithelial keratins, the cytokeratins, and harder hair keratins. As certain skin cells differentiate and become cornified, pre-keratin polypeptides are incorporated into intermediate filaments. Eventually, the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized.

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 molt 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.

Another example of keratinzed cells are nails. According to Levit and Boissy (2001), the nail plate is composed of "closely packed, fully keratinized, multilayered lamellae of cornified cells" (Levit and Boissy 2001). Essentially, cells in the epidermis contain a structural matrix of keratin. The nail matrix cells differentiate and create the nail plate by flattening, broadening, and by nuclear fragmentation, with an accumulation of cytoplasmic microfibrils (Levit and Boissy 2001). As skin cells become cornified, and the nucleus and cytoplasmic organelles disappear and metabolism ceases, the cells become fully keratinized. Hard structures are formed by intercellular cementing of fibers formed from the dead, cornified cells.The keratins in the nail plate are believed to be held in place by surrounding globular matrix proteins with a high concentration of disulfide bonds between cystine (rather than by means of calcium, as in bones), creating the rigid structure (Levit and Boissy 2001).

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 micrometers (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 (AMO 2002).

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.

Pairing

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

Medical significance

Some infectious fungi, such as those that cause athlete's foot, ringworm, and the amphibian disease chytridiomycosis (caused by the chytrid fungus, Batrachochytrium dendrobatidis), feed on keratin.

Diseases caused by mutations in the keratin genes include:

  • Epidermolysis bullosa simplex
  • Ichthyosis bullosa of Siemens
  • Epidermolytic hyperkeratosis
  • Steatocystoma multiplex

Although keratin is insoluble and is not easily hydrolysed by digestive enzymes (Bender and Bender 2005), it can be used for coating pills designed to be dissolved when in the intestine. A supplement for ruminants also is made from steamed feather meal (Bender and Bender 2005).

References
ISBN links support NWE through referral fees

  • Bender, D. A., and A. E. Bender. 2005. A Dictionary of Food and Nutrition. New York: Oxford University Press. ISBN 0198609612.
  • Levit, E. K., and R. E. Boissy, R. E. 2001. Chapter 6. Basic science of the nail unit. In R. K. Freinkel, and D. T. Woodley. The Biology of the Skin. New York: Parthenon Pub. Group. ISBN 1850700060.

Credits

New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:

The history of this article since it was imported to New World Encyclopedia:

Note: Some restrictions may apply to use of individual images which are separately licensed.