Keratin
Also searched as: alpha-keratin, hard keratin, trichocyte keratin, wool protein, hair protein
A family of sulfur-rich structural proteins that make up wool, hair and horn; in textile fibers it forms alpha-helical filaments embedded in a protein matrix cross-linked by cystine disulfide bonds.
In detail
Wool and specialty hair fibers are built from trichocyte, or hard, keratins. Type I and type II keratin chains each have a central alpha-helical rod flanked by non-helical head and tail domains. Two helices wind together into a coiled-coil dimer, and dimers assemble through protofilaments into intermediate filaments about 10 nanometers across. Bundles of these filaments are embedded in a matrix of keratin-associated proteins, some very rich in cysteine and others rich in glycine and tyrosine. Disulfide bonds between cysteine residues, which together form cystine, tie filaments and matrix into a network that resists solvents and gives wool its resilience.
The science and numbers
The alpha-helix proposed by Pauling, Corey and Branson in 1951 has about 3.6 amino acid residues per turn and a pitch of about 0.54 nm; in keratin, helices from type I and type II chains wind around one another as coiled coils. Stretching moist wool or hair converts part of this structure toward an extended beta form, a change first recorded by X-ray diffraction in the early 1930s. Hard keratins are cysteine-rich, and the density of disulfide cross-links governs their compactness and insolubility: they dissolve in ordinary protein solvents only at pH extremes or after the disulfides are chemically reduced. Within a wool fiber the paracortex is richer in high-sulfur proteins, while the orthocortex carries more low-sulfur and glycine-rich proteins, a compositional difference linked to crimp.
A practical example
A tailor steams and presses a crease into wool trousers, and the crease lasts through many wearings, while an unpressed wool sleeve slowly relaxes on a humid day. The class uses this to show that keratin's shape is held by bonds that can be loosened and re-formed: moisture and heat free the structure to take a new form, and cooling and drying fix it in place until moisture releases it again.
What to distinguish
Keratin is a protein family, not a single molecule. Different species, and different regions within one fiber, contain different mixtures of keratins and associated proteins. The soft keratins of skin differ from the hard keratins of fibers. Products described as keratin treatments usually contain hydrolyzed protein fragments, which do not rebuild the native filament structure. The word alone says nothing about fineness, softness or animal welfare.
Origins and history
The name derives from the Greek keras, horn. In the early 1930s William Astbury's X-ray diffraction studies of wool and hair identified two distinct states, which he called alpha-keratin and beta-keratin; the alpha and beta labels were later adopted for protein secondary structure generally. The alpha-helix itself was proposed in 1951, and models in which helices twist around each other as compound or coiled coils to explain alpha-keratin followed in the 1950s and 1960s.
Related terms
Sources & further reading
- Nature: Compound helical configurations of polypeptide chains, structure of proteins of the alpha-keratin type
- Journal of Molecular Biology (ScienceDirect): The coiled-coil model of alpha-keratin structure
- PubMed: Trichocyte keratin-associated proteins (KAPs)
- History of Information: William Astbury conducts the first studies of proteins by X-ray analysis
Technical references reviewed 2026-09-22. Examples are illustrative. Industry organizations and manufacturers describe their own fields; their references are not independent product endorsements. Figures are approximate and depend on the stated test conditions.