Overview
This lecture explains the four levels of protein structure—primary, secondary, tertiary, and quaternary—and how each relates to protein function, using hemoglobin as an example.
Protein Structure Overview
- Proteins are made of long chains of amino acids, called polypeptides.
- The function of a protein is determined by its 3D structure, which arises from its structural levels.
Levels of Protein Structure
Primary Structure
- Primary structure is the sequence/order of amino acids in a polypeptide chain.
- This sequence is determined by DNA and built during translation in the ribosome.
Secondary Structure
- Secondary structure results from interactions within the peptide backbone.
- Two common secondary structures are alpha helices and beta-pleated sheets.
- Hydrogen bonds between backbone atoms stabilize secondary structures.
- Beta-pleated sheets can be parallel (chains go in the same direction) or anti-parallel (chains go in opposite directions).
Tertiary Structure
- Tertiary structure arises from interactions between side chains (R groups) of amino acids.
- Types of interactions include hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.
- Tertiary structure defines the overall 3D shape of a single polypeptide.
Quaternary Structure
- Quaternary structure involves the arrangement of multiple polypeptide chains in a protein.
- Example: Hemoglobin has four polypeptide chains (subunits) working together.
Key Terms & Definitions
- Amino Acid — Building block of proteins, containing an amino group, carboxyl group, and unique side chain (R group).
- Polypeptide — A chain of amino acids linked by peptide bonds.
- Primary Structure — The specific linear sequence of amino acids in a protein.
- Secondary Structure — Local folding patterns (like alpha helices and beta-pleated sheets) stabilized by hydrogen bonds.
- Alpha Helix — A spiral-shaped secondary structure formed by hydrogen bonds in the polypeptide backbone.
- Beta-Pleated Sheet — Sheet-like secondary structure with backbone chains aligned next to each other, stabilized by hydrogen bonds.
- Tertiary Structure — The overall 3D shape of a single polypeptide, determined by side chain interactions.
- Quaternary Structure — Structure formed by the assembly of multiple polypeptide chains.
Action Items / Next Steps
- Review the next video: "Tertiary structure of proteins" for more detail on side chain interactions.
- Study examples of alpha helices and beta-pleated sheets in actual protein structures.