• Nonpolar/hydrophobic
• Polar/hydrophilic
• Acidic (negatively charged)
• Basic (positively charged)

Protein Structure Levels

Primary Structure (1°)

• Sequence of amino acids
• Linked by peptide bonds
• Formed by dehydration synthesis
• Determines all higher structure

Secondary Structure (2°)

Regular folding patterns due to hydrogen bonding:

• α-helix: coiled spring shape
• β-pleated sheet: accordion-like folds

Tertiary Structure (3°)

• 3D shape of entire polypeptide
• Interactions between R groups:
  • Hydrogen bonds
  • Ionic bonds
  • Hydrophobic interactions
  • Disulfide bridges (covalent S-S bonds)

Quaternary Structure (4°)

• Multiple polypeptide subunits
• Example: hemoglobin (4 subunits)

Protein Folding

Denaturation:

• Loss of protein structure and function
• Caused by:
  • High temperature
  • pH changes
  • Chemical denaturants
• Usually irreversible

Chaperone proteins:

• Help proteins fold correctly
• Prevent misfolding

Key Concepts

1. Amino acids are monomers; proteins are polymers
2. Peptide bonds link amino acids (dehydration synthesis)
3. R groups determine amino acid properties
4. Primary structure (sequence) determines final 3D shape
5. Function depends on shape ("structure determines function")
6. Denaturation destroys protein function