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 (although 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.
There are two main forms of keratin, alpha and beta-keratin. Alpha-keratin is seen in humans and other mammals, beta-keratin is present in birds and reptiles.