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Keratin is an extremely strong protein that is a major component in skin, hair, nails, hooves, horns, and teeth. The amino acids which 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 dissolve keratin because it contains cysteine disulfide, which means that it is able to form disulfide bridges. These bridges create a helix shape that is extremely strong, as sulfur atoms bond to each other, creating a fibrous matrix that is not readily soluble. Depending on how much cysteine disulfide keratin contains, 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.
Keratin is formed by keratinocytes, living cells that make up a large part of skin, hair, nails, and other parts of the body. The cells slowly push their way upwards, eventually dying and forming a protective layer. Thousands are shed every day, and the process can be accelerated by various medical conditions, such as psoriasis. Damage to the external layer of keratin can cause skin, hair, and nails to look unhealthy or flaky.
Hair and nails on humans especially tend to become dry and brittle, because the dead keratin is being pushed to great lengths. By eating foods like gelatin and keeping hair and nails moist, they can be grown out while still remaining healthy. In general, the thicker the layer of keratin, the healthier the hair or nail is, because the dead cells outside protect the living cells at the core. Keeping the external layer moisturized will also keep it healthy and prevent cracking and splitting, whether it is forming the hooves of a horse of the skin of a human.