Keratin's structure primarily consists of polypeptide chains of amino acids that form a backbone with inter- and intramolecular bonding. These bonds contribute to its strength and stability.
Keratin is a fibrous structural protein, known for its toughness and insolubility. It is a key component of skin, hair, and nails (in animals), and its structure is complex and hierarchical, contributing to its unique properties. Here's a breakdown of its structure:
Primary Structure
- Amino Acid Sequence: Keratin is composed of a long chain of amino acids linked together by peptide bonds. The specific sequence of these amino acids determines the unique characteristics of the keratin protein.
Secondary Structure
- Alpha-helix: The polypeptide chains of keratin commonly form an alpha-helix structure. This is a coiled structure stabilized by hydrogen bonds between amino acids in the chain. This is typical for alpha-keratins found in mammalian hair, wool, and nails.
- Beta-pleated Sheet: Beta-keratins, found in avian and reptile sources like feathers and scales, are composed of beta-pleated sheets. These sheets are formed by aligning polypeptide chains side-by-side and connecting them via hydrogen bonds.
Tertiary Structure
- Folding and Interactions: The alpha-helices or beta-sheets further fold and twist into a specific three-dimensional shape. This tertiary structure is stabilized by various interactions between the amino acid side chains (R-groups), including:
- Disulfide Bonds: Strong covalent bonds between cysteine amino acids. These bonds are particularly important in keratin, providing significant strength and rigidity. The more disulfide bonds, the harder and more rigid the keratin structure.
- Hydrogen Bonds: Weaker bonds between polar amino acids.
- Ionic Bonds (Salt Bridges): Attractions between positively and negatively charged amino acids.
- Hydrophobic Interactions: Clustering of nonpolar amino acids away from water.
Quaternary Structure
- Protofilaments, Filaments, and Macrofibrils: Individual keratin molecules assemble into larger, more complex structures. Alpha-keratins, for instance, typically form dimers (two molecules intertwined), which then assemble into protofilaments, filaments (intermediate filaments), and finally macrofibrils. These macrofibrils are what give hair, nails, and other keratin-containing structures their strength and resilience.
Summary Table
| Structure Level | Description | Stabilizing Forces | Example |
|---|---|---|---|
| Primary | Amino acid sequence | Peptide bonds | Specific keratin protein sequence |
| Secondary | Alpha-helix or beta-pleated sheet | Hydrogen bonds | Alpha-keratin (hair), Beta-keratin (feathers) |
| Tertiary | Folding and twisting of the secondary structure into a 3D shape | Disulfide bonds, hydrogen bonds, ionic bonds, hydrophobic interactions | Globular shape of the keratin molecule |
| Quaternary | Assembly of multiple keratin molecules into larger structures (protofilaments, filaments, macrofibrils) | Non-covalent interactions | Macrofibrils in hair |
