新規更新July 23, 2018 at 09:29PM
【外部リンク】
Alfa-queratina
Furado: Creación del artículo con la traducción de en:Alpha-keratin.
La '''alfa-queratina''' ('''α-queratina''', '''queratina alfa''' o '''queratina-α''') es un tipo de [[queratina]] que se encuentra en los [[Mammalia|mamíferos]]. Esta [[proteína]] es el principal componente de los [[pelo]]s, [[cuerno]]s, [[uña]]s y la capa [[Epidermis|epidérmica]] de la [[piel]]. Es una [[Proteína fibrosa|proteína estructural fibrosa]], esto es, que sus [[aminoácido]]s forman una [[Estructura secundaria de las proteínas|estructura secundaria]] repetida. La estructura secundaria de la α-queratina es muy parecida a la de las típicas proteínas en [[hélice alfa]] y forma una [[hélice superenrollada]].<ref name="fundamentals"></ref> Debido a su estructura tan firmemente enrollada, puede funcionar como uno de los materiales biológicos más fuertes y tiene diversos usos en los mamíferos, como [[garra]]s en los predadores o pelos para abrigarse. Se sintetiza mediante [[biosíntesis proteica]], utilizando [[Transcripción genética|transcripción]] y [[Traducción (genética)|traducción genética]], pero cuando la célula madura y está llena de queratina-α, muere, creando una sólida unidad no [[Vaso sanguíneo|vascularizada]] de tejido queratinizado.<ref name="wang"></ref>
== Estructura ==
[[Archivo:Keratin creation red.svg|thumb|308x308px|Estructura molecular de la alfa-queratina.]]
[[Archivo:Disulfide bonds.svg|thumb|139x139px|[[Enlace disulfuro|Enlaces disulfuro]] entre dos [[Hélice alfa|hélices alfa]] de [[queratina]].]]
La queratina alfa es una [[Péptido|cadena polipeptídica]] rica en [[alanina]], [[leucina]], [[arginina]] y [[cisteína]], que forma una [[hélice alfa]] [[drae:dextrógiro|destrógira]].<ref></ref><ref></ref> Dos de estas cadenas polipeptídicas se enrollan entre sí formando una estructura helicoidal [[drae:levógiro|levógira]] llamada [[hélice superenrollada]]; estos [[Dímero (química)|dímeros]] de hélices superenrolladas, de aproximadamente 45 [[Nanómetro|nm]] de longitud, están unidos por medio de [[Enlace disulfuro|Enlaces disulfuro]], utilizando los numerosos [[aminoácido]]s de [[cisteína]] que se encuentran en las alfa-queratinas.<ref name="wang" /> Entonces los dímeros se alinean y sus extremos [[C-terminal]] se enlazan con los extremos [[N-terminal]] de otros dímeros y dos de estas nuevas cadenas se unen en sentido longitudinal, siempre por medio de enlaces disulfuro, para formar un protofilamento.<ref></ref> Dos protofilamentos se juntan para formar una protofibrila y cuatro protofibrilas se [[Polimerización|polimerizan]] para formar los filamentos intermedios, que son la subunidad básica de las queratinas alfa. Estos filamentos intermedios pueden condensarse en una formación en hélice superenrollada de unos 7 nm de diámetro, y puede ser de tipo I, ácida, o de tipo II, básica. Finalmente, los filamentos intermedios están insertados en una matriz de queratina que o bien tiene un alto contenido de residuos de cisteína o bien de [[glicina]], [[tirosina]] y [[fenilalanina]]. Los diferentes tipos, alineamientos y matrices de estos filamentos intermedios explican la gran variedad de estructuras de queratina alfa, cada una con distintas propiedades, que se encuentran en los mamíferos.<ref></ref>
== Bioquímica ==
=== Síntesis ===
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α-keratin synthesis begins near [[focal adhesion]]s on the [[cell membrane]]. There, the keratin filament precursors go through a process known as [[nucleation]], where the keratin precursors of dimers and filaments elongate, fuse, and bundles together.<ref name="wang" /> As this synthesis is occurring, the keratin filament precursors are transported by [[Actin|actin fibers]] in the cell towards the [[Cell nucleus|nucleus]]. There, the alpha-keratin intermediate filaments will collect and form networks of structure dictated by the use of the keratin cell as the nucleus simultaneously degrades.<ref>Liquid error: wrong number of arguments (1 for 2)</ref> However, if necessary, instead of continuing to grow, the keratin complex will disassemble into non-filamentous keratin precursors that can [[Diffusion|diffuse]] throughout the cell [[cytoplasm]]. These keratin filaments will be able to be used in future keratin synthesis, either to re-organize the final structure or create a different keratin complex. When the cell has been filled with the correct keratin and structured correctly, it undergoes keratin stabilization and dies, a form of [[programmed cell death]]. This results in a fully matured, non-vascular keratin cell.<ref>Liquid error: wrong number of arguments (1 for 2)</ref> These fully matured, or [[Keratin|cornified]], alpha-keratin cells are the main components of hair, the outer layer of nails and horns, and the [[Epidermis|epidermis layer]] of the skin.<ref>Liquid error: wrong number of arguments (1 for 2)</ref>
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=== Propiedades ===
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The property of most biological importance of alpha-keratin is its [[Structure|structural]] stability. When exposed to [[Stress (mechanics)|mechanical stress]], α-keratin structures can retain their shape and therefore can protect what they surround.<ref></ref> Under high tension, alpha-keratin can even change into [[beta-keratin]], a stronger keratin formation that has a secondary structure of [[Beta sheet|beta-pleated sheets]].<ref>Liquid error: wrong number of arguments (1 for 2)</ref> Alpha-keratin [[Tissue (biology)|tissues]] also show signs of [[viscoelasticity]], allowing them to both be able to stretch and absorb impact to a degree, though they are not impervious to [[fracture]]. Alpha-keratin strength is also affected by [[water]] content in the intermediate filament matrix; higher water content decreases the strength and stiffness of the keratin cell due to their effect on the various hydrogen bonds in the alpha-keratin network.<ref name="wang" />
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=== Caracterización ===
==== Tipo I y tipo II ====
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Alpha-keratins proteins can be one of two types: [[Type I keratin|type I]] or [[Type II keratin|type II]]. There are 54 keratin genes in humans, 28 of which code for type I, and 26 for type II.<ref name=":2">Liquid error: wrong number of arguments (1 for 2)</ref> Type I proteins are acidic, meaning they contain more acidic amino acids, such as [[aspartic acid]], while type II proteins are basic, meaning they contain more basic amino acids, such as [[lysine]].<ref>Liquid error: wrong number of arguments (1 for 2)</ref> This differentiation is especially important in alpha-keratins because in the synthesis of its sub-unit dimer, the [[coiled coil]], one protein coil must be type I, while the other must be type II.<ref name="wang" /> Even within type I and II, there are acidic and basic keratins that are particularly complementary within each organism. For example, in human skin, [[Keratin 5|K5]], a type II alpha keratin, pairs primarily with [[Keratin 14|K14]], a type I alpha-keratin, to form the alpha-keratin complex of the [[Epidermis|epidermis layer]] of cells in the skin.<ref>Liquid error: wrong number of arguments (1 for 2)</ref>
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==== Dura y blanda ====
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Hard alpha-keratins, such as those found in nails, have a higher [[cysteine]] content in their [[Protein primary structure|primary structure]]. This causes an increase in [[Disulfide|disulfide bonds]] that are able to stabilize the keratin structure, allowing it to resist a higher level of [[force]] before fracture. On the other hand, soft alpha-keratins, such as ones found in the skin, contain a comparatively smaller amount of disulfide bonds, making their structure more flexible.<ref name="fundamentals" />
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== Referencias ==
[[Categoría:Proteínas]]
https://ift.tt/2JMUY4I
