Keratin is a fibrous structural protein found in animal cells and used to form specialized tissues. Specifically, the proteins are only produced by chordates (vertebrates, Amphioxus, and urochordates), which includes mammals, birds, fish, reptiles, and amphibians. The tough protein protects epithelial cells and strengthens certain organs.

There are different forms of keratin, such as α-keratins and harder β-keratins. Keratins are considered examples of scleroproteins or albuminoids. The protein is rich in sulfur and insoluble in water. The high sulfur content is attributed to the richness of the amino acid cysteine. Disulfide bridges add strength to the protein and contribute to insolubility. Keratin is not typically digested in the gastrointestinal tract.

Silk and Keratin

Some scientists classify the silk fibroins that are produced by spiders and insects as keratins, although there are differences between the phylogeny of the materials, even if their molecular structure is comparable.

Keratin and Disease

While animal digestive systems aren’t equipped to deal with keratin, certain infectious fungi feed on the protein. Examples include the ringworm and athlete’s foot fungus.

Mutations in the keratin gene can produce diseases, including epidermolytic hyperkeratosis and keratosis pharyngis.

Because keratin is not dissolved by digestive acids, ingesting it causes problems in people who eat hair (tricophagia) and results in vomiting of hairballs in cats, once enough hair has accumulated from grooming. Unlike felines, humans don’t vomit hairballs, so a large accumulation of hair in the human digestive tract can cause the rare but fatal intestinal blockage called Rapunzel syndrome.