Keratins

fibrous proteins whose fibers are a component of the horny layer of the epidermis and of hair, wool, feathers, scales, nails, horns, beaks, and hooves.

Keratins are insoluble in water and organic solvents and resistant to the action of proteolytic enzymes. It has been established by X-ray structural analysis that the polypeptide chains of keratins may be either twisted or elongated. The structure of nonelongate fibers (α-structure) is characteristic of the keratins of mammals; reptiles and birds have the elongate form (β-structure) as well (for example, in feathers). The insolubility of keratins is caused by the presence of transverse disulfide bonds between the polypeptide chains. The principal structural elements of α-keratins are cylindrical microfibrillae (diameter 75Å), which consist of spiraled protofibrillae twisted in pairs. The keratins of mammals are distinguished chiefly by their amino-acid composition, by the manner in which the microfibrillae are packaged, and by the amount of matrix (the high-sulfur protein in which they are embedded).