Keratin: The Structural Protein of Skin, Hair, and Nails

Keratin is a family of fibrous structural proteins (scleroproteins) that form the key structural material of the epidermis, hair, nails, and analogous structures (horns, feathers, claws).

Types

• Alpha-keratin (α-keratin): Mammals. Coiled-coil dimers → intermediate filaments (IFs). Soft (skin) to hard (hair, nails) depending on cysteine cross-linking (disulfide bonds).
• Beta-keratin (β-keratin): Birds/reptiles. β-sheet structure; harder (feathers, scales, beaks, claws).

Properties

• High mechanical strength, toughness, insolubility.
• Water-resistant (lipid-rich matrix in stratum corneum).
• Elasticity (low in hard keratin due to cross-links).
• Biocompatible, biodegradable.

Biological Roles

• Epidermis (stratum corneum): Keratinocytes terminally differentiate → cornified envelope (keratin IFs + cross-linked proteins + lipids) = barrier against water loss, pathogens, UV, chemicals.
• Hair shaft: Cortex = aligned keratin filaments + matrix (high cysteine) → strength, texture. Cuticle = overlapping scales.
• Nail plate: Hard α-keratin (low sulfur) → rigid protection for distal digits.

Clinical Relevance

• Ichthyosis, epidermolysis bullosa: keratin gene mutations (KRT1, KRT10, KRT14, KRT5).
• Hair disorders: monilethrix (KRT81, KRT83, KRT86), trichorrhexis nodosa.
• Nail dystrophies: pachyonychia congenita (KRT6a, KRT6b, KRT16, KRT17).
• Keratin in tumors: immunohistochemical marker for epithelial origin (carcinomas).