What tough fibrous protein makes up hair and nails?

Keratin is the tough fibrous protein that makes up much of human hair and nails. It is a structural protein whose tightly organized fibers help these tissues resist everyday pulling, bending, and abrasion.

Hair and nails are not living tissues along their visible lengths. Specialized cells produce keratin in hair follicles and nail-producing regions; as those cells mature, they become densely packed and die, forming the hard keratinized material that grows outward. Hair remains more flexible than nails because its keratin structure contains fewer disulfide cross-links.

Keratin is rich in the sulfur-containing amino acid cysteine. Bonds between cysteine residues help stabilize the protein network, contributing to the strength of hair, nails, claws, horns, hooves, feathers, and other biological structures. The smell of burning hair comes partly from sulfur-containing compounds released as keratin breaks down.

Keratin is often confused with collagen or elastin. Collagen is a major connective-tissue protein, while elastin gives certain tissues stretch and recoil. Melanin, another tempting choice, is a pigment responsible for much of the color in hair and skin, not their primary structural material.