Question

Explain primary, secondary, tertiary and quaternary structures of protein in detail.

Hint:

Remember: The primary structure is about amino acid sequence, secondary is local folding (α-helix, β-sheet), tertiary is overall 3D folding, and quaternary is association of multiple chains.

Answer 1

Proteins are large biomolecules made of amino acids linked by peptide bonds.
The function and properties of proteins depend on the arrangement of amino acids and their folding into higher structures.
Protein structure can be described at four different levels:
1. Primary Structure:
- The primary structure refers to the linear sequence of amino acids in a polypeptide chain.
- This sequence is determined by the genetic code (DNA).
- Even a small change in the sequence (mutation) can drastically alter the protein's function.
2. Secondary Structure:
- The secondary structure is the regular folding or coiling of the polypeptide chain due to hydrogen bonding between the carbonyl oxygen and the amide hydrogen of the peptide backbone.
- The two most common patterns are:
(a) $\alpha$-helix (coiled structure)
(b) $\beta$-pleated sheet (zig-zag structure).
3. Tertiary Structure:
- The tertiary structure refers to the overall three-dimensional folding of a polypeptide chain.
- This folding occurs due to interactions among side chains (R-groups), such as:
- Hydrogen bonds
- Ionic interactions
- Hydrophobic interactions
- Disulfide bonds (covalent).
- The tertiary structure determines the biological activity of the protein (e.g., enzyme active site).
4. Quaternary Structure:
- The quaternary structure arises when two or more polypeptide chains (subunits) associate together to form a functional protein.
- Examples include hemoglobin (composed of four polypeptide subunits).
- This structure is stabilized by the same interactions as tertiary structure but involves multiple polypeptide chains.
Thus, proteins exhibit a hierarchical structure: primary → secondary → tertiary → quaternary, each level being essential for their stability and biological function.