Proteins have four levels of structure, each contributing to their shape and function.
1. Primary structure:
- Definition: The sequence of amino acids in a polypeptide chain.
- Bond type: Peptide bonds between amino acids.
- Importance: Determines all higher levels of protein structure.
- Example: A chain of amino acids like Ala-Gly-Ser-Leu…
2. Secondary structure:
- Definition: Local folding of the polypeptide chain into regular patterns.
- Bond type: Hydrogen bonds between backbone atoms.
- Common types:
- α-helix: Spiral shape
- β-pleated sheet: Folded sheet-like structure
- Importance: Provides stability and initial folding.
3. Tertiary structure:
- Definition: The overall 3D shape of a single polypeptide chain.
- Bond types: Hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions.
- Importance: Determines the functional shape of the protein.
- Example: Globular proteins like enzymes.
4. Quaternary structure:
- Definition: Assembly of multiple polypeptide chains (subunits) into one protein.
- Bond types: Same as tertiary structure (hydrogen bonds, ionic bonds, etc.)
- Importance: Enables proteins to perform complex functions.
- Example: Hemoglobin (4 subunits working together).
In short:
- Primary: Amino acid sequence
- Secondary: Local folding (α-helix, β-sheet)
- Tertiary: 3D shape of a single chain
- Quaternary: Combination of multiple chains
These structures together determine the shape and function of a protein.