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:
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.