Keratinis an extremely strong protein that is a major component in skin, hair, nails, hooves, horns, and teeth. Theamino acidswhich combine to form it have several unique properties and, depending on the levels of the various amino acids, it can be inflexible and hard, like hooves, or soft, as is the case with skin. Most people interact with this tissue after it is actually dead; hair, skin, and nails are all formed from dead cells that the body sheds as new cells push up from underneath. If the dead cells are kept in good condition, they will serve as an insulating layer to protect the delicate new tissue below them.
It is difficult to dissolvekeratinbecause it containscysteinedisulfide, which means that it is able to form disulfide bridges. These bridges create ahelixshape that is extremely strong, as sulfur atomsbondto each other, creating a fibrous matrix that is not readily soluble. Depending on how much cysteine disulfidekeratincontains, the bond can be extremely strong to make hard cells like those found in hooves, or it can be softer to make flexible tissue like hair and skin. Because of the high levels of sulfur in this protein, it emits a distinct sulfurous odor when burned that some people find distasteful.
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