A native protein is a protein in its natural, functional, and properly folded form (the form found in living cells).

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Structure of native protein

A native protein usually has its secondary, tertiary, and sometimes quaternary structure intact.

That’s what gives it its specific shape and biological activity.

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Summary

Term	Meaning
Primary structure	Sequence of amino acids
Secondary structure	α-helix or β-pleated sheet (H-bonds)
Tertiary structure	3D folding → active shape
Native protein	Protein in its functional 3D (tertiary or quaternary) form

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Secondary Structure Proteins

• Meaning: Local folding of polypeptide chains into α-helix or β-pleated sheets (held by hydrogen bonds).
• Examples:
  • Keratin → mostly α-helix (found in hair, nails, wool).
  • Fibroin → mostly β-pleated sheet (found in silk).

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Tertiary Structure Proteins

• Meaning: 3D folding of a single polypeptide chain → forms a globular protein.
• Forces involved: H-bonds, disulfide bridges, ionic bonds, hydrophobic interactions.
• Examples:
  • Myoglobin → stores oxygen in muscles.
  • Lysozyme → enzyme that breaks bacterial cell walls.
  • Ribonuclease → enzyme for RNA breakdown.

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Quaternary Structure Proteins

• Meaning: Association of two or more polypeptide chains (subunits) in a functional complex.
• Examples:
  • Hemoglobin → 4 subunits (2 α + 2 β chains).
  • Insulin → made of multiple polypeptide chains (A and B chains linked by disulfide bonds).
  • DNA polymerase → multi-subunit enzyme.