Question

The bonds responsible for maintaining the quaternary structure of proteins is/are:

• A. Disulfide bond
• B. Peptide bond
• C. Covalent bond
• D. Hydrogen bonds, hydrophobic interactions, van der Waals interaction and ionic bonds

Hint:

Remember that primary structure relies on covalent peptide bonds, secondary relies on backbone hydrogen bonds, tertiary is stabilized by side-chain interactions, and quaternary is stabilized by inter-subunit non-covalent interactions.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Proteins are organized into four distinct structural levels: primary, secondary, tertiary, and quaternary.
Quaternary structure refers to the spatial arrangement and assembly of two or more polypeptide chains (subunits) into a multi-subunit functional complex.
Unlike the primary structure which is held by strong covalent peptide bonds, the association of separate subunits in quaternary structures is primarily stabilized by weak, non-covalent forces.

Step 2: Detailed Explanation:
The quaternary structure of multi-subunit proteins, such as hemoglobin, is held together by several types of non-covalent interactions:
Hydrophobic Interactions: Non-polar side chains of amino acids aggregate away from the aqueous environment and are buried at the subunit interfaces.
This hydrophobic collapse is the primary thermodynamic force driving the association of separate subunits.
Hydrogen Bonds: Hydrogen atoms shared between polar groups at the interfaces of adjacent polypeptide chains stabilize their relative alignment.
Ionic Bonds (Salt Bridges): Electrostatic attractions between positively charged amino acid side chains (e.g., lysine, arginine) and negatively charged side chains (e.g., aspartate, glutamate) occur at the contact areas of different subunits.
Van der Waals Interactions: Weak, short-range attractive forces between transient dipoles of closely packed atoms also help secure the interface.
While some proteins may contain inter-subunit disulfide bonds, the primary and universal stabilizing force for typical quaternary assemblages consists of the collection of non-covalent interactions described in Option D.

Step 3: Final Answer:
The quaternary structure of proteins is primarily maintained by a combination of hydrogen bonds, hydrophobic interactions, van der Waals forces, and ionic bonds.