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:
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-keratin and beta-keratin. Alpha-keratin is seen in humans and other mammals, beta-keratin is present in birds and reptiles. Beta-keratin is harder than alpha-keratin. Structurally alpha-keratin have alpha-helical coiled coil structure while beta-keratin have twisted beta sheet structure.
In humans, the keratin family of proteins is divided into the soft epithelial keratins or cytokeratins and the hard hair keratins. These can be divided into type I (acidic) and type II (basic to neutral) subfamilies.
