Keratin
Keratin is a
protein used by numerous groups of animals as a structural element, and is a classic example of a fibrous protein.
It is the main component of:
- mammal hair,
- reptile scales, (though not fish scales),
- bird feathers,
- claws (including nails and hooves),
- horn (but not antlers),
- baleens (the sieve-like structures with which whales filter food out of water),
- the enamel of the teeth.
Keratin
molecules are
helical and fibrous, twisting around each other to form strands called intermediate filaments. These proteins contain a high percentage of
sulfur-containing
amino acids, largely
cysteine, which form disulfide bridges between the individual molecules resulting in a fairly rigid structure. Human hair is approximately 14% cysteine.
