Keratin is a type of protein that makes up the skin, hair nails, horns, hooves and teeth. Keratin is composed of certain amino acids that combine together and gives keratin certain characteristics. Depending on the levels of amino acids, keratin can range from being flexible to hard. It can become as hard as hooves or as soft as skin. The keratin that people see and touch is dead. The hair, skin and nails are actually made from dead cells that are shed by the body as new cells grow. The dead cells, if maintained in good condition, will act as an insulation that will help in protecting the new keratin that continue grows below them.
Keratin is not easily dissolved because one of its components is cysteine disulfide, which helps in the formation of disulfide bridges. These bridges actually build a helix shape that is known for its superior strength. The shape is formed by the sulfur atoms bonding to each other from right across the helix. What results is a strong fibrous matrix that cannot be easily dissolved. Based on how much cysteine disulfide is contained in the keratin, the resulting bond can be very strong just like horns, or it can be soft and flexible like hair and skin. Since keratin also contains high amounts of sulfur, burning it will result in the emission of a sulfurous smell.
Keratin is made by keratinocytes, which are cells that comprise the skin, hair, nails and other parts of the body that have keratin in it. The keratinocytes will slowly push itself upwards, where it usually dies. The dead cells form a protective layer.