Alpha-keratin, or α-keratin, is a type of keratin found in mammalian . This protein is the primary component in , horns, , nails and the Epidermis of the skin. α-keratin is a Scleroprotein, meaning it is made up of that form a repeating secondary structure. The secondary structure of α-keratin is very similar to that of a traditional protein Alpha helix and forms a coiled coil.
Structure
α-keratin is a
Peptide, typically high in
alanine,
leucine,
arginine, and
cysteine, that forms a right-handed
Alpha helix.
Two of these polypeptide chains twist together to form a left-handed
Alpha helix structure known as a
coiled coil. These coiled coil dimers, approximately 45 nm long, are bonded together with
Disulfide, utilizing the many
cysteine amino acids found in α-keratins.
The dimers then align, their
C-terminus bonding with the
N-terminus of other dimers, and two of these new chains bond length-wise, all through disulfide bonds, to form a protofilament.
Two protofilaments aggregate to form a protofibril, and four protofibrils
Polymerization 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 be type I, acidic, or type II, basic. The IFs are finally embedded in a keratin matrix that either is high in
cysteine or
glycine,
tyrosine, and
phenylalanine residues. The different types, alignments, and matrices of these IFs account for the large variation in α-keratin structures found in mammals.
Biochemistry
Synthesis
α-keratin synthesis begins near
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 bundle together.
As this synthesis is occurring, the keratin filament precursors are transported by
Actin in the cell towards the
Cell 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.
However, if necessary, instead of continuing to grow, the keratin complex will disassemble into non-filamentous keratin precursors that can
Diffusion 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.
These fully matured, or
Keratin, alpha-keratin cells are the main components of hair, the outer layer of nails and horns, and the
Epidermis of the skin.
Properties
The property of most biological importance of alpha-keratin is its
Structure stability. When exposed to mechanical stress, α-keratin structures can retain their shape and therefore can protect what they surround.
Under high tension, the alpha-helix configuration of alpha-keratin can even change into
Beta sheet.
Not to be confused with
beta-keratin which is a different protein. 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.
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.
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.
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.
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 of cells in the skin.
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 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.
