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)
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.
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.