Keratins are hard and tough structures which give rise to furs, nails, hair, horns and hooves of animals. The fibrous proteins that make up keratins are usually designated as α-keratins since they are mostly composed of α-helical polypeptide chains. The keratin fibers are composed of bundles of fibrils, each fibril being made up of three polypeptide chains. A single unit of keratin is right handed α-helix which may consist of as many as seven protofibrils, three of which form a left-handed coiled . The protofibrils have certain regions where the side chains of some constituent amino acids form an α-helix. Likewise there are regions rich in proline, glycine and serine residues which do not favor helical shape. Thus the basic unit has two regions, one where an α-helix is formed, and the other where α-helix formation does not take place. The non-helical regions presumably bring the fibrils in contact with each other and with the matrix.

The keratin fibers are embedded in the cell matrix made of polypeptides rich in cysteine residues. The fibrils are stabilized through covalent cross-links that exist within fibrils or between them. The stabilization of fibrils is achieved by cross-linking disulphide bridges.