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Protein Structure - an Overview of the Classes
4 classes of protein structure
  • Primary
  • Secondary
  • Tertiary
  • Quaternary
KEY PROTEIN FUNCTIONS
  • Accelerate reactions
  • Provide structural support
  • Send communication signals
  • Aid in cellular movement
  • Provide immune defense
  • Storage of nutrients (nitrogen)
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Protein Structure - an Overview of the Classes

CLASSES OF PROTEIN STRUCTURE 1.
Primary protein structure
  • Together, peptide bonds and amino acids form a polypeptide chain (aka a protein).
  • The primary structure of a protein determines its secondary and tertiary structures.
As a clinical correlate, in sickle cell anemia, a single substitution of an amino acid in the primary structure of the protein results in a structural defect in hemoglobin.
2.
Secondary protein structure
  • Two basic forms: alpha-helices and beta-sheets
  • Hydrogen bond interactions within the alpha-helix and beta-sheet provide the stability of secondary structure of proteins.
Alpha-helices
  • Low energy conformations that enable higher-order packing of proteins.
  • Large or charged amino acid groups (such as proline) can disable the alpha helix conformation by manually disrupting the hydrogen bond interactions.
Beta-pleated sheets
  • More structurally diverse than alpha helices and thus facilitate more diverse protein functions.
  • Create stable, diverse structures within a protein to allow higher order functions.
  • There are two notable ways in which the beta-pleated sheet can exist:
    • Parallel — the peptide chain advances in a single direction.
    • Anti-parallel — the peptide chain advances in two opposite directions.
As a clinical correlate, prions are pathogenic, transmissible agents, which cause conversion from an alpha-helical form to a beta-sheet-rich conformer. Prions accumulate in the brain and cause a variety of spongiform encephalopathies, such as “mad cow disease”.
3.
Tertiary protein structure
  • Is the protein’s three-dimensional shape (its "native conformation") and the function of a protein is dependent on this three-dimensional globular structure.
  • Primarily comprises alpha helices and beta sheets.
4.
Quaternary protein structure
  • Forms through the interaction of 2 or more separate proteins.
  • Comprises multiple polypeptide chains and occurs in certain protein types, called functional multimeric proteins.
Tertiary & Quaternary protein bonding
  • The most significant stabilizer of tertiary and quaternary protein structures are hydrophobic interactions.
  • The following additional forces stabilize these structures:
    • Hydrophilic interactions.
    • Electrostatic interactions.
    • Hydrogen bonds between side chains.
    • Strong disulfide bonds.