Keratin's structure primarily consists of peptide chains arranged in approximately equal amounts of antiparallel and parallel pleated sheets, linked by hydrogen bonds.
Keratin is a fibrous structural protein that forms the key structural material in the outer layer of human skin and is the main component of hair, nails, feathers, horns, claws, hooves, and the outer layer of teeth. Its structure is complex and hierarchical, contributing to its strength and resilience. Here's a breakdown of its structure:
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Primary Structure: This refers to the amino acid sequence of the polypeptide chain. Keratin is rich in cysteine residues, which are crucial for forming disulfide bonds.
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Secondary Structure: Keratin primarily exists in two forms:
- Alpha-keratin (α-keratin): Found in mammals (hair, skin, nails). It has an alpha-helical structure. Two alpha-helices wind around each other to form a coiled coil. These coiled coils further associate to form protofilaments, which then assemble into protofibrils and finally into macrofibrils.
- Beta-keratin (β-keratin): Found in birds and reptiles (feathers, scales). It exhibits a beta-sheet structure. The beta-sheets are typically arranged in a pleated fashion and are linked by hydrogen bonds between the carbonyl and imino groups.
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Tertiary Structure: This involves the folding of the secondary structures into a specific three-dimensional shape. Disulfide bridges between cysteine residues play a significant role in stabilizing the tertiary structure, contributing to keratin's strength and insolubility. The more disulfide bonds, the harder and more rigid the keratin (e.g., horns and nails have more than hair).
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Quaternary Structure: Keratin proteins assemble into larger, more complex structures. For example, in hair, alpha-keratin molecules associate to form intermediate filaments, which provide structural support. Multiple intermediate filaments combine to form macrofibrils, which are visible under a microscope.
Key Structural Features Summarized:
| Feature | Description |
|---|---|
| Primary | Amino acid sequence, rich in cysteine. |
| Secondary | Alpha-helix (α-keratin) or beta-sheet (β-keratin). |
| Tertiary | 3D folding stabilized by disulfide bonds. |
| Quaternary | Assembly into intermediate filaments and macrofibrils. |
| Bonding | Hydrogen bonds between carbonyl and imino groups (especially in beta-keratin); disulfide bonds (cysteine residues). |
