Protein Structure & Function

Overview

• Proteins have four levels of structure:
  1. Primary Structure: Sequence of amino acids in a polypeptide chain.
  2. Secondary Structure: Coiling or folding of the polypeptide chain into alpha-helices or beta-pleated sheets, stabilized by hydrogen bonds.
  3. Tertiary Structure: Three-dimensional shape formed by interactions among various side chains (R groups).
  4. Quaternary Structure: Structure formed by multiple polypeptide chains functioning as a single unit.

Primary Structure

• Defined by the sequence of amino acids.
• Determined by the DNA sequence of the gene encoding the protein.
• Critical to the function of the protein; even a single amino acid change can alter its function.

Secondary Structure

• Stabilized by hydrogen bonds between the backbone atoms.
• Two main types:
  • Alpha-helix (α-helix): Right-handed coil.
  • Beta-pleated sheet (β-pleated sheet): Folding of the chain into parallel or anti-parallel strands.

Tertiary Structure

• Involves further folding and coiling of the secondary structure.
• Stabilized by interactions such as:
  • Hydrogen bonds.
  • Ionic bonds between charged R groups.
  • Disulfide bridges between cysteine residues.
  • Hydrophobic interactions among nonpolar R groups.

Quaternary Structure

• Found in proteins with more than one polypeptide chain.
• Example: Hemoglobin, which has four polypeptide subunits.

Bond Types in Protein Structures

• Primary Structure: Covalent peptide bonds.
• Secondary Structure: Hydrogen bonds.
• Tertiary Structure: Hydrogen, ionic, disulfide bonds, hydrophobic interactions.
• Quaternary Structure: Similar interactions as tertiary, but between different polypeptide chains.

Important Points

• Hydrogen bonds are crucial for maintaining secondary and tertiary structures.
• Disulfide bonds contribute to the stability of the tertiary structure.
• Understanding the structure is essential for comprehending the function and potential dysfunction in proteins.

Tips

• Remember how each level of structure contributes to the overall shape and function of the protein.
• Familiarize yourself with the differences in bonds and interactions present at each structural level.

Visual Aids

• Primary Structure: Indicated by the specific sequence of amino acids.
• Secondary Structure: Includes visuals of α-helix and β-sheet configurations.
• Tertiary Structure: Depicts complex folding with various interactions among R groups.
• Quaternary Structure: Shows multiple subunits and their interactions in a functional protein.