Alpha-keratin, or α-keratin, is a type ofkeratin found in mammalianvertebrates. Thisprotein is the primary component inhairs,horns,claws,nails and theepidermis layer of theskin. α-keratin is afibrous structural protein, meaning it is made up ofamino acids that form a repeatingsecondary structure. The secondary structure of α-keratin is very similar to that of a traditional proteinα-helix and forms acoiled coil.[1] Due to its tightly wound structure, it can function as one of the strongest biological materials and has various functions in mammals, frompredatoryclaws to hair for warmth. α-keratin is synthesized throughprotein biosynthesis, utilizingtranscription andtranslation, but as the cell matures and is full of α-keratin, it dies, creating a strong non-vascular unit ofkeratinized tissue.[2]
α-keratin is apolypeptide chain, typically high inalanine,leucine,arginine, andcysteine, that forms a right-handedα-helix.[3][4] Two of these polypeptide chains twist together to form a left-handedhelical structure known as acoiled coil. These coiled coildimers, approximately 45 nm long, are bonded together withdisulfide bonds, utilizing the manycysteine amino acids found in α-keratins.[2] The dimers then align, theirtermini bonding with thetermini of other dimers, and two of these new chains bond length-wise, all through disulfide bonds, to form a protofilament.[5] Two protofilaments aggregate to form a protofibril, and four protofibrilspolymerize to form the intermediate filament (IF). The IF is the basic subunit of α-keratins. These IFs are able to condense into a super-coil formation of about 7 nm in diameter, and can betype I, acidic, ortype II, basic. The IFs are finally embedded in a keratinmatrix that either is high incysteine orglycine,tyrosine, andphenylalanine residues. The different types, alignments, and matrices of these IFs account for the large variation in α-keratin structures found in mammals.[6]
α-keratin synthesis begins nearfocal adhesions on thecell membrane. There, the keratin filament precursors go through a process known asnucleation, where the keratin precursors of dimers and filaments elongate, fuse, and bundle together.[2] As this synthesis is occurring, the keratin filament precursors are transported byactin fibers in the cell towards thenucleus. 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 candiffuse throughout the cellcytoplasm. 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 ofprogrammed cell death. This results in a fully matured, non-vascular keratin cell.[8] These fully matured, orcornified, alpha-keratin cells are the main components of hair, the outer layer of nails and horns, and theepidermis layer of the skin.[9]
The property of most biological importance of alpha-keratin is itsstructural stability. When exposed tomechanical stress, α-keratin structures can retain their shape and therefore can protect what they surround.[10] Under high tension, the alpha-helix configuration of alpha-keratin can even change intobeta-pleated sheets.[11] Not to be confused withbeta-keratin which is a different protein. Alpha-keratintissues also show signs ofviscoelasticity, allowing them to both be able to stretch and absorb impact to a degree, though they are not impervious tofracture. Alpha-keratin strength is also affected bywater 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]
Alpha-keratins proteins can be one of two types:type I ortype 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 asaspartic acid, while type II proteins are basic, meaning they contain more basic amino acids, such aslysine.[13] This differentiation is especially important in alpha-keratins because in the synthesis of its sub-unit dimer, thecoiled 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 withK14, a type I alpha-keratin, to form the alpha-keratin complex of theepidermis layer of cells in the skin.[14]
Hard alpha-keratins, such as those found in nails, have a highercysteine content in theirprimary structure. This causes an increase indisulfide bonds that are able to stabilize the keratin structure, allowing it to resist a higher level offorce 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]
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