hornification

角化

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/10/07 21:55:12」(JST)

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Not to be confused with kerogen, carotene, chitin, or creatine.

Microscopy of keratin filaments inside cells.

Keratin (/ˈkɛrətɪn/) is a family of fibrous structural proteins. Keratin is the key structural material making up the outer layer of human skin. It is the key structural component of hair and nails, and it provides the necessary strength and toughness for masticatory organs, such as the tongue and the hard palate. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized tissues found in reptiles, birds, amphibians, and mammals. The only other biological matter known to approximate the toughness of keratinized tissue is chitin.

Etymology

Keratin derives from Greek κερατίνη from Greek keras (κέρας) (genitive keratos, κέρατος) meaning "horn" originating from the Proto-Indo-European *ḱer- of the same meaning. Its Greek root is composed of "horn like", i.e., kerato (Greek κέρατο) to which the chemical suffix -in is appended.

Examples of uses

Horns such as those of the impala are made up of keratin covering a core of live bone

Keratin filaments are abundant in keratinocytes in the cornified layer of the epidermis; these are cells which have undergone keratinization. In addition, keratin filaments are present in epithelial cells in general. For example, mouse thymic epithelial cells (TECs) are known to react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus.

• the α-keratins in the hair (including wool), horns, nails, claws and hooves of mammals
• the harder β-keratins found in nails and in the scales and claws of reptiles, their shells (Testudines, 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.)

The baleen plates of filter-feeding whales are made of keratin.

Keratins (also described as cytokeratins) are polymers of type III and type IV intermediate filaments, which have only been found in the genomes of chordates (vertebrates, Amphioxus, urochordates). Nematodes and many other non-chordate animals seem to only have type VI intermediate filaments, lamins, which have a long rod domain (vs. a short rod domain for the keratins).

Structural details

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

The first sequences of keratins were determined by Hanukoglu and Fuchs. These sequences revealed that there are two distinct but homologous keratin families which were named as Type I keratin and Type II keratins. By analysis of the primary structures of these keratins and other intermediate filament proteins, Hanukoglu and Fuchs suggested a model that keratins and intermediate filaments proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation. This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.

Fibrous keratin molecules supercoil to form a very stable, left-handed superhelical motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer.

The major force that keeps the coiled-coil structure is hydrophobic interactions between apolar residues along the keratins helical segments.

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. A preponderance of amino acids with small, nonreactive side groups is characteristic for 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 bridges that 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.

Filament formation

It was theorized that keratins are combined into 'hard' and 'soft,' or 'cytokeratins' and 'other keratins'. That model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account.

Keratin filaments 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, if the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments.

Pairing

The neutral-basic keratins are found on chromosome 12 (12q13.13).

While the acidic keratins are found on chromosome 17 (17q21.2).

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

Protein sequence alignment of human Keratin 1, 2A, 3,4, 5, 6A, 7,and 8 (KRT1 - KRT8). Only the first rod domain is shown above. Alignment was created using Clustal Omega (tool available online).

The entries KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36, KRT37, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20.

Cornification

Cornification is the process of forming an epidermal barrier in stratified squamous epithelial tissue. At the cellular level, cornification is characterised by:

• production of keratin
• production of small proline-rich (SPRR) proteins and transglutaminase which eventually form a cornified cell envelope beneath the plasma membrane
• terminal differentiation
• loss of nuclei and organelles, in the final stages of cornification

Metabolism ceases, and the cells are almost completely filled by keratin. During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton 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.

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 thickening of the outer, cornified layer of the epidermis and form protective calluses — useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced.

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 moult 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.

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. 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. 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.

Clinical significance

Some infectious fungi, such as those that cause athlete's foot and ringworm (i.e. the dermatophytes), or Batrachochytrium dendrobatidis (Chytrid fungus), 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
• Rhabdoid cell formation in Large cell lung carcinoma with rhabdoid phenotype

Furthermore, keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas, thymomas, sarcomas and trophoblastic neoplasms. Furthermore, the precise expression pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases. For example, hepatocellular carcinomas typically expresse K8 and K18, and cholangiocarcinomas express K7, K8 and K18, while metastases of colorectal carcinomas express K20, but not K7.

Keratin is highly resistant to digestive acids if it is ingested (Trichophagia). Because of this, cats (which groom themselves with their tongues) regularly ingest hair which will eventually result in the gradual formation of a Hairball that is occasionally vomited when it becomes too big. Rapunzel_syndrome is an extremely rare but potentially fatal intestinal condition in humans that is caused by Tricophagia.

Cosmetic uses

There is no convincing research showing that keratin, intact or hydrolized, can give any benefit to skin or hair.

See also

• List of cutaneous conditions caused by mutations in keratins
• List of keratins expressed in the human integumentary system

References

External links

• Composition and β-sheet structure of silk
• Hair-Science.com's entry on the microscopic elements of hair
• Proteopedia page on keratins

English Journal

• CP/MAS ¹³C NMR study of pulp hornification using nanocrystalline cellulose as a model system.

• Idström A1, Brelid H, Nydén M, Nordstierna L.Author information 1Applied Surface Chemistry, Chalmers University of Technology, Göteborg, Sweden.AbstractThe hornification process of paper pulp was investigated using solid-state (13)C NMR spectroscopy. Nanocrystalline cellulose was used to serve as a model system of the crystalline parts of the fibrils in pulp fibers. Characterization of the nanocrystalline cellulose dimensions was carried out using scanning electron microscopy. The samples were treated by drying and wetting cycles prior to NMR analysis where the hornification phenomenon was recorded by spectral changes of the cellulose C-4 carbon signals. An increase of the crystalline signal and a decrease of the signals corresponding to the accessible amorphous domains were found for both paper pulp and nanocrystalline cellulose. These spectral changes grew stronger with repeating drying and wetting cycles. The results show that cellulose co-crystallization contribute to hornification. Another conclusion is that the surfaces of higher hydrophobicity in cellulose fibrils have an increased preference for aggregation.
• Carbohydrate polymers.Carbohydr Polym.2013 Jan 30;92(1):881-4. doi: 10.1016/j.carbpol.2012.09.097. Epub 2012 Oct 13.
• The hornification process of paper pulp was investigated using solid-state (13)C NMR spectroscopy. Nanocrystalline cellulose was used to serve as a model system of the crystalline parts of the fibrils in pulp fibers. Characterization of the nanocrystalline cellulose dimensions was carried out using
• PMID 23218380

• Effects of drying-induced fiber hornification on enzymatic saccharification of lignocelluloses.

• Luo X1, Zhu JY.Author information 1State Key Lab Pulp and Paper Engineering, South China University of Technology, Guangzhou, China.AbstractThis study investigated the effect of fiber hornification during drying on lignocellulosic substrate enzymatic saccharification. Two chemically pretreated wood substrates and one commercial bleached kraft hardwood pulp were used. Heat drying at 105 and 150°C and air drying at 50% RH and 23.8°C for different durations were applied to produce substrate with various degrees of hornification. It was found that substrate enzymatic digestibilities (SEDs) of hornified substrates made from the same never-dried sample correlate very well to an easily measurable parameter, water retention value (WRV), and can be fitted by a Boltzmann function. The hornification-produced SED reduction at a given degree of hornification as the percentage of the total SED reduction when the substrate is completely hornified depends on two parameters. The first is WRV¯, which is primarily a function of the effective enzyme molecule size, and Δ, which is related to the substrate pore size distribution shape. The low values of SED(CH), SED of a completely hornified substrate, obtained from curve fittings for the three sets of samples studied, suggest that enzyme accessibility to cellulose is mainly through the pores in the cell wall rather than substrate external surface. The SEDs of hornified substrates were found to correlate to Simons' staining measurements well. A new parameter was proposed to better correlate enzyme accessibility to cellulose using the two-color Simons' staining technique.
• Enzyme and microbial technology.Enzyme Microb Technol.2011 Jan 5;48(1):92-9. doi: 10.1016/j.enzmictec.2010.09.014. Epub 2010 Oct 27.
• This study investigated the effect of fiber hornification during drying on lignocellulosic substrate enzymatic saccharification. Two chemically pretreated wood substrates and one commercial bleached kraft hardwood pulp were used. Heat drying at 105 and 150°C and air drying at 50% RH and 23.8°C for
• PMID 22112776

• The effect of chemical composition on microfibrillar cellulose films from wood pulps: mechanical processing and physical properties.

• Spence KL1, Venditti RA, Habibi Y, Rojas OJ, Pawlak JJ.Author information 1Department of Forest Biomaterials, North Carolina State University, Campus Box 8005, Raleigh, NC 27695-8005, USA.AbstractFilms of microfibrillated celluloses (MFCs) from pulps of different yields, containing varying amounts of extractives, lignin, and hemicelluloses, were produced by combining refining and high-pressure homogenization techniques. MFC films were produced using a casting-evaporation technique and the physical and mechanical properties (including density, roughness, fold endurance and tensile properties) were determined. Homogenization of bleached and unbleached Kraft pulps gave rise to highly individualized MFCs, but not for thermo-mechanical pulp (TMP). The resulting MFC films had a roughness equivalent to the surface upon which the films were cast. Interestingly, after homogenization, the presence of lignin significantly increased film toughness, tensile index, and elastic modulus. The hornification of fibers through a drying and rewetting cycle prior to refining and homogenization did not produce any significant effect compared to films from never-dried fibers, indicating that MFC films can potentially be made from low-cost recycled cellulosic materials.
• Bioresource technology.Bioresour Technol.2010 Aug;101(15):5961-8. doi: 10.1016/j.biortech.2010.02.104. Epub 2010 Mar 23.
• Films of microfibrillated celluloses (MFCs) from pulps of different yields, containing varying amounts of extractives, lignin, and hemicelluloses, were produced by combining refining and high-pressure homogenization techniques. MFC films were produced using a casting-evaporation technique and the ph
• PMID 20335025

Japanese Journal

• Three mechanisms affecting the mechanical properties of spruce wood dried at high temperatures

• Borrega Marc,Karenlampi Petri P.
• Journal of wood science 56(2), 87-94, 2010-04-25
• NAID 10026439767

• リサイクルにおける繊維の角質化が繊維交点の潜在結合力に及ぼす影響 : 共焦点型レーザー走査顕微鏡を用いて

• ソムワン カンタヤーヌウオン,江前 敏晴,尾鍋 史彦
• 紙パ技協誌 56(2), 239-245, 2002-02-01
• NAID 10012579755

• リサイクルにおける繊維の角質化が繊維交点の潜在結合力に及ぼす影響--共焦点型レーザー走査顕微鏡を用いて

• Somwang Khantayanuwong,江前 敏晴,尾鍋 史彦
• 紙パ技協誌 56(2), 239-245, 2002-02
• NAID 40000518682

Related Links

• hornification - Definition - Mondofacto

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• Hornification dictionary definition | hornification defined

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Related Pictures

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★リンクテーブル★

リンク元

「角化」

「角化」

　　[★]

英

cornification、keratinization、cornify、keratinize、hornification

同

角質変性

関

角質化、角層、表皮