text: how is keratin made?   keratins are a family of fibrous structural proteins; tough and insoluble, they form peptide bonds to adjacent protein chains, facilitating their close alignment of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally-stable crosslinking — a role sulfur bridges also play in vulcanized rubber. human hair is approximately 14% cysteine. the pungent smells of burning hair is due to the sulfur compounds formed. extensive disulfide bonding contributes to the insolubility of keratins, except in dissociating or reducing agents such as urea.  you could get more information from the link below...