Keratin

• Appears as long structures, with filaments arranged in bundles or fibres, it is rich in cysteine so many polypeptide disulphide bridges between its chains, alongside H bonding this makes the molecule very strong
• Provides mechanical protection but also an impermeable barrier to infection; waterproof prevents entry of water-bourne pollutants, some have also been found to regulate key cell growth and protein synthesis
• Found in all hard parts of the body; Nails, Scales, Claws, Feathers, Hooves, Fur, and Horns