Keratin is a tough, insoluble protein found in the outer layer of the skin of human beings and many other animals. This outer layer of skin is called the epidermis. The outermost layer of cells of the epidermis contains keratin. The keratin in these cells makes the skin tough and almost completely waterproof. In places where the skin is exposed to much rubbing and pressure such as parts of the hands and feet the number of cells containing keratin increases and a callus develops. Cells that contain keratin are constantly being shed and replaced by new ones. The condition known as dandruff results when the scalp sheds such cells.
Keratin molecules are helical and fibrous, twisting around each other to form strands called intermediate filaments. These proteins contain a high percentage of sulfur-containing amino acids, largely cysteine, which form disulfide bridges between the individual molecules, resulting in a fairly rigid structure. Human hair is approximately 14% cysteine.
There are two main forms of keratin, alpha-keratin and beta-keratin. Alpha-keratin is seen in humans and other mammals, beta-keratin is present in birds and reptiles. Beta-keratin is harder than alpha-keratin. Structurally alpha-keratin have alpha-helical coiled coil structure while beta-keratin have twisted beta sheet structure.
In humans, the keratin family of proteins is divided into the soft epithelial keratins or cytokeratins and the hard hair keratins. These can be divided into type I (acidic) and type II (basic to neutral) subfamilies. Keratin is also what fungi that infect us (such as Athlete's Foot and Ringworm) feed on.
Keratin is also a part of certain structures that grow from the skin. The nails and hair of human beings contain keratin. It can be thought of as nature's all-purpose plastic, forming such various growths as horns, hoofs, claws, beaks, feathers and scales, all of which consist mainly of keratin. It helps make these structures stronger and better-suited to protect the body from the environment.