Keratin. Molecular graphic showing two molecules of keratin - the protein that gives skin, hair, nails, horns, hooves and beaks their strength. Coloured spheres represent atoms: carbon is grey, hydrogen white, nitrogen blue, oxygen red, sulphur yellow. Keratin is extremely tough, insoluble in water and weak acid, and resistant to proteolytic enzymes. It forms dense mats of microfibrils that are both mechanically strong and able to extend and contract reversibly.