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词条 Alpha-keratin
释义

  1. Structure

  2. Biochemistry

     Synthesis  Properties  Characterization  Type I and type II  Hard and soft 

  3. References

Alpha-keratin, or α-keratin, is a type of keratin found in mammals. This protein is the primary component in hairs, horns, nails and the epidermis layer of the skin. α-keratin is a fibrous structural protein, meaning it is made up of amino acids that form a repeating secondary structure. The secondary structure of α-keratin is very similar to that of a traditional protein α-helix and forms a coiled coil.[1] Due to its tightly wound structure, it can function as one of the strongest biological materials and has various uses in mammals, from predatory claws to hair for warmth. α-keratin is synthesized through protein biosynthesis, utilizing transcription and translation, but as the cell matures and is full of α-keratin, it dies, creating a strong non-vascular unit of keratinized tissue.[2]

Structure

Biochemistry

Synthesis

α-keratin synthesis begins near focal adhesions 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.[2] As this synthesis is occurring, the keratin filament precursors are transported by actin fibers in the cell towards the 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.[7] However, if necessary, instead of continuing to grow, the keratin complex will disassemble into non-filamentous keratin precursors that can 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.[8] These fully matured, or cornified, alpha-keratin cells are the main components of hair, the outer layer of nails and horns, and the epidermis layer of the skin.[9]

Properties

The property of most biological importance of alpha-keratin is its structural stability. When exposed to mechanical stress, α-keratin structures can retain their shape and therefore can protect what they surround.[10] Under high tension, alpha-keratin can even change into beta-keratin, a stronger keratin formation that has a secondary structure of beta-pleated sheets.[11] Alpha-keratin 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.[2]

Characterization

Type I and type II

Alpha-keratins proteins can be one of two types: type I or type II. There are 54 keratin genes in humans, 28 of which code for type I, and 26 for type II.[12] 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.[13] 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.[2] Even within type I and II, there are acidic and basic keratins that are particularly complementary within each organism. For example, in human skin, K5, a type II alpha keratin, pairs primarily with K14, a type I alpha-keratin, to form the alpha-keratin complex of the epidermis layer of cells in the skin.[14]

Hard and soft

Hard alpha-keratins, such as those found in nails, have a higher cysteine content in their primary structure. This causes an increase in 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.[1]

References

1. ^1 {{Cite book|title=Fundamentals of biochemistry : life at the molecular level|last=G.|first=Voet, Judith|last2=W.|first2=Pratt, Charlotte|isbn=9781118918401|oclc=910538334|date = 2016-02-29}}
2. ^1 2 3 4 {{Cite journal|last=Wang|first=Bin|last2=Yang|first2=Wen|last3=McKittrick|first3=Joanna|last4=Meyers|first4=Marc André|date=2016-03-01|title=Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration|url=http://www.sciencedirect.com/science/article/pii/S0079642515000596|journal=Progress in Materials Science|volume=76|pages=229–318|doi=10.1016/j.pmatsci.2015.06.001}}
3. ^{{Cite journal|last=Burkhard|first=Peter|last2=Stetefeld|first2=Jörg|last3=Strelkov|first3=Sergei V|title=Coiled coils: a highly versatile protein folding motif|journal=Trends in Cell Biology|volume=11|issue=2|pages=82–88|doi=10.1016/s0962-8924(00)01898-5|year=2001}}
4. ^{{Cite journal|last=Pace|first=C N|last2=Scholtz|first2=J M|date=1998-07-01|title=A helix propensity scale based on experimental studies of peptides and proteins.|journal=Biophysical Journal|volume=75|issue=1|pages=422–427|issn=0006-3495|pmc=1299714|pmid=9649402|via=|doi=10.1016/S0006-3495(98)77529-0}}
5. ^{{Cite journal|last=Steinert|first=Peter M.|last2=Steven|first2=Alasdair C.|last3=Roop|first3=Dennis R.|title=The molecular biology of intermediate filaments|journal=Cell|volume=42|issue=2|pages=411–419|doi=10.1016/0092-8674(85)90098-4|year=1985}}
6. ^{{Cite journal|last=McKittrick|first=J.|last2=Chen|first2=P.-Y.|last3=Bodde|first3=S. G.|last4=Yang|first4=W.|last5=Novitskaya|first5=E. E.|last6=Meyers|first6=M. A.|date=2012-04-03|title=The Structure, Functions, and Mechanical Properties of Keratin|journal=JOM|language=en|volume=64|issue=4|pages=449–468|doi=10.1007/s11837-012-0302-8|issn=1047-4838}}
7. ^{{Cite journal|last=Windoffer|first=Reinhard|last2=Beil|first2=Michael|last3=Magin|first3=Thomas M.|last4=Leube|first4=Rudolf E.|date=2011-09-05|title=Cytoskeleton in motion: the dynamics of keratin intermediate filaments in epithelia|url=http://jcb.rupress.org/content/194/5/669|journal=The Journal of Cell Biology|language=en|volume=194|issue=5|pages=669–678|doi=10.1083/jcb.201008095|issn=0021-9525|pmid=21893596|via=|pmc=3171125}}
8. ^{{Cite journal|last=Kölsch|first=Anne|last2=Windoffer|first2=Reinhard|last3=Würflinger|first3=Thomas|last4=Aach|first4=Til|last5=Leube|first5=Rudolf E.|date=2010-07-01|title=The keratin-filament cycle of assembly and disassembly|url=http://jcs.biologists.org/content/123/13/2266|journal=J Cell Sci|language=en|volume=123|issue=13|pages=2266–2272|doi=10.1242/jcs.068080|issn=0021-9533|pmid=20554896}}
9. ^{{Cite journal|last=Eckhart|first=Leopold|last2=Lippens|first2=Saskia|last3=Tschachler|first3=Erwin|last4=Declercq|first4=Wim|date=2013-12-01|title=Cell death by cornification|url=http://www.sciencedirect.com/science/article/pii/S0167488913002334|journal=Biochimica et Biophysica Acta (BBA) - Molecular Cell Research|volume=1833|issue=12|pages=3471–3480|doi=10.1016/j.bbamcr.2013.06.010|pmid=23792051}}
10. ^{{Cite journal|last=Pan|first=Xiaoou|last2=Hobbs|first2=Ryan P|last3=Coulombe|first3=Pierre A|year=2013|title=The expanding significance of keratin intermediate filaments in normal and diseased epithelia|journal=Current Opinion in Cell Biology|volume=25|issue=1|pages=47–56|doi=10.1016/j.ceb.2012.10.018|pmid=23270662|pmc=3578078}}
11. ^{{Cite journal|last=Kreplak|first=L.|last2=Doucet|first2=J.|last3=Dumas|first3=P.|last4=Briki|first4=F.|date=2004-07-01|title=New Aspects of the α-Helix to β-Sheet Transition in Stretched Hard α-Keratin Fibers|pmc=1304386|journal=Biophysical Journal|language=en|volume=87|issue=1|pages=640–647|doi=10.1529/biophysj.103.036749|pmid=15240497|via=}}
12. ^{{Cite journal|last=Moll|first=Roland|last2=Divo|first2=Markus|last3=Langbein|first3=Lutz|date=2017-03-07|title=The human keratins: biology and pathology|journal=Histochemistry and Cell Biology|volume=129|issue=6|pages=705–733|doi=10.1007/s00418-008-0435-6|issn=0948-6143|pmc=2386534|pmid=18461349|via=}}
13. ^{{Cite journal|last=Strnad|first=Pavel|last2=Usachov|first2=Valentyn|last3=Debes|first3=Cedric|last4=Gräter|first4=Frauke|last5=Parry|first5=David A. D.|last6=Omary|first6=M. Bishr|date=2011-12-15|title=Unique amino acid signatures that are evolutionarily conserved distinguish simple-type, epidermal and hair keratins|journal=Journal of Cell Science|volume=124|issue=24|pages=4221–4232|doi=10.1242/jcs.089516|issn=0021-9533|pmc=3258107|pmid=22215855|via=}}
14. ^{{Cite journal|last=Lee|first=Chang-Hun|last2=Coulombe|first2=Pierre A.|date=2009-08-10|title=Self-organization of keratin intermediate filaments into cross-linked networks|url=http://jcb.rupress.org/content/186/3/409|journal=The Journal of Cell Biology|language=en|volume=186|issue=3|pages=409–421|doi=10.1083/jcb.200810196|issn=0021-9525|pmc=2728393|pmid=19651890|via=}}

1 : Keratins
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