Keratin: Difference between revisions

===Disulfide bridges===

In addition to intra- and intermolecular [[hydrogen bond]]s, the distinguishing feature of keratins is the presence of large amounts of the [[sulfur]]-containing amino acid [[cysteine]], required for the [[disulfide bond|disulfide bridges]] that confer additional strength and rigidity by permanent, thermally stable [[cross-link|crosslinking]]<ref>{{cite web|title=What is Keratin?|url=http://www.wisegeek.org/what-is-keratin.htm|publisher=WiseGEEK|access-date=11 May 2014|archive-date=13 May 2014|archive-url=https://web.archive.org/web/20140513010609/http://www.wisegeek.org/what-is-keratin.htm|url-status=live}}</ref>—in much the same way that non-protein sulfur bridges stabilize [[vulcanization|vulcanized]] [[rubber]]. Human hair is approximately 14% cysteine. The [[pungency|pungent]] smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the [[soluble|insolubility]] of keratins, except in a small number of solvents such as [[dissociation (chemistry)|dissociating]] or [[redox|reducing]] agents.

The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in [[mammalian]] [[fingernail]]s, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes.<ref>{{cite journal |last1=H Bragulla |first1=Hermann |last2=G Homberger |first2=Dominique |title=Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia |journal=Journal of Anatomy |year=2009 |volume=214 |issue=4 |pages=516–559 |doi=10.1111/j.1469-7580.2009.01066.x |pmid=19422428 |pmc=2736122 }}</ref> Hair and other α-keratins consist of [[alpha helix|α-helically]] coiled single protein strands (with regular intra-chain [[hydrogen bond|H-bonding]]), which are then further twisted into superhelical [[rope]]s that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.