Keratin is a protein that forms a variety of epidermal outgrowths in vertebrates: hair, nails and claws, as well as horns.
Do not confuse the protein Keratin with the pigment Carotene
Keratinocytes are the main type of cell in the epidermis, the outermost layer of the skin, and keratin contributes to the physical protection this provides against the external environment, as well as maintaining a state of hydration in the outer layers of cells.
There are several forms of keratin, built up from parallel polypeptide chains, with a high proportion of (residues of) the amino acids alanine
, and especially cysteine
, which uniquely forms covalent disulphide bonds with cysteine on neighbouring polypeptides. These polypeptides have a fairly typical but tightly coiled 'right-handed' alpha helix structure.
Initially pairs of polypeptide chains form into 'coiled coils' and these 'dimers' join lengthways with others as well as well as sideways to form fairly tough fibres.
The example displayed shows 127-residue sections ('2B domains') of a KRT5 filament (type II) as well as a KRT14 filament (type I).
Type II characteristically have more basic side-chains (e.g lysine)
whereas type I have more acidic side-chains (e.g. aspartic acid, glutamic acid )
Ball & Stick
Colour by Chains
Show Backbone H bonds
Sidechain H bonds
H bonds off