Question

The correct statement about peptides and proteins is

• A. In \(\alpha\)-helices, the polypeptide chain is twisted into a left-handed screw (helix) through intramolecular hydrogen bonds.
• B. Tertiary structure of proteins has two or more polypeptide subunits.
• C. Only the proteins having a quaternary structure are biologically active.
• D. In \(\beta\)-pleated sheet structures, peptide chains are held together by intermolecular hydrogen bonds.

Hint:

To avoid confusion with protein structural properties, remember: - \(\alpha\)-Helix \(\rightarrow\) Right-handed screw stabilized by intramolecular H-bonds. - \(\beta\)-Sheet \(\rightarrow\) Extended sheets stabilized by intermolecular H-bonds. - Tertiary = Single chain 3D shape; Quaternary = Multiple subunits/chains.

Answer 1

The Correct Option is D

Concept: Proteins display four levels of structural organization: primary, secondary, tertiary, and quaternary structures. Secondary structure refers to the local spatial arrangement of the polypeptide backbone, stabilized primarily by hydrogen bonding interactions between the carbonyl oxygen (\(-\text{C}=\text{O}\)) and amide hydrogen (\(-\text{N}-\text{H}\)) groups of the peptide linkage. The two most common ordered secondary structures are:

• \(\alpha\)-Helix: A right-handed coiled conformation where intramolecular hydrogen bonds form within the single, continuous polypeptide chain.

• \(\beta\)-Pleated Sheet: A structure composed of extended adjacent polypeptide strands lying side-by-side, which are locked together via intermolecular hydrogen bonding networks.

Step 1: Evaluation of Statement (1)
Statement (1) posits that the polypeptide chain in an \(\alpha\)-helix forms a left-handed screw. In nature, virtually all naturally occurring \(\alpha\)-helices found in proteins are

right-handed due to the steric constraints imposed by L-amino acid side chains. While the statement accurately mentions intramolecular hydrogen bonds, the assertion of a left-handed orientation makes this statement scientifically incorrect.

Step 2: Evaluation of Statement (2)
Statement (2) asserts that the tertiary structure contains two or more polypeptide subunits. This is a definition mismatch. The arrangement of multiple polypeptide chains (subunits) describes the

quaternary structure of a protein. The tertiary structure represents the comprehensive three-dimensional folding of a single, individual polypeptide chain. Thus, this statement is incorrect.

Step 3: Evaluation of Statement (3)
Statement (3) claims that only proteins with quaternary structure exhibit biological activity. This is completely false. Numerous monomeric proteins consisting of just one polypeptide chain (possessing up to tertiary structure only), such as myoglobin and lysozyme, are entirely functional and biologically active. Quaternary organization is not a prerequisite for biological activity across all proteins.

Step 4: Evaluation of Statement (4)
Statement (4) describes the architecture of \(\beta\)-pleated sheets. In a \(\beta\)-pleated sheet structure, individual segments of polypeptide chains (either from different strands or distant segments of the same strand) align parallel or anti-parallel to each other. The structure is stabilized by extensive

intermolecular hydrogen bonds running between adjacent strands. This description matches the structural criteria precisely and is correct.