Biology chap. 3 pt 2

Primary Structure

• The primary structure of a protein refers to the linear amino acid sequence.
• It is the lowest level of protein structure and defines the order of amino acids, which is crucial for the final protein shape and function.

Secondary Structure

• The secondary structure involves repeated hydrogen bonds within the amino acid backbone, not between side chains.
• Key features of secondary structure:
  • It is stabilized by hydrogen bonds formed between oxygen and hydrogen in the peptide bonds.
  • Consists of two major motifs:
  • Alpha Helix:
    • Formed when hydrogen bonds stabilize a spiral shape of the amino acid chain.
    • First described by Linus Pauling.
  • Beta Sheet:
    • Formed when parts of the amino acid chain fold back and bond with another part of the chain, producing a flat structure.
• Both motifs can span tens or hundreds of amino acids and occur within the same protein.

Tertiary Structure

• The tertiary structure is the overall 3D shape of a protein, resulting from the interactions between R groups (side chains) of amino acids.
• Interactions can include:
  • Salt bridges (ionic bonds between charged side chains)
  • Hydrogen bonds
  • Covalent bonds (including disulfide bridges formed by thiol groups)
  • Hydrophobic interactions, where nonpolar side chains move to the protein's interior to avoid water.
• This structure is crucial for the function of globular proteins, which often have a hydrophobic core.

Quaternary Structure

• The quaternary structure involves two or more polypeptide chains (subunits) interacting to form a functional protein.
• Examples include:
  • Hemoglobin:
  • Made of two alpha chains and two beta chains, facilitating oxygen transport in the blood.
• Interaction types include hydrogen bonds, salt bridges, and hydrophobic interactions.

Casein Protein Example

• Casein is the primary protein in milk, responsible for nutrient delivery to nursing mammals.
• In yogurt and cheese making, the protein's structure is altered by acid addition:
  • pH lowering affects salt bridges and changes tertiary structure, causing proteins to denature and expose hydrophobic regions.
  • This leads to protein coagulation, forming a solid mass (curds) and influencing the texture and structure of products like cheese.