The correct statement about peptides and proteins is:
- A.Tertiary structure of proteins has two or more polypeptide subunits
- B.Only the proteins having a quaternary structure are biologically active
- C.In β-pleated sheet structures, peptide chains are held together by intermolecular hydrogen bonds✓
- D.In α-helices, the polypeptide chain is twisted into a left-handed screw (helix) through intramolecular hydrogen bonds
Correct Answer
(C) In β-pleated sheet structures, peptide chains are held together by intermolecular hydrogen bonds
Solution & Explanation
Concept — Levels of protein structure and the bonding in secondary structures. Option (C): In the β-pleated sheet, neighbouring stretches of polypeptide chain lie side by side and are held together by hydrogen bonds between the C=O of one chain and the N–H of an adjacent chain. Because these H-bonds are between different segments/chains, they are intermolecular (interchain). This statement is CORRECT. Why the others are wrong: (A) Having two or more polypeptide subunits is the QUATERNARY structure, not the tertiary. Tertiary structure is the overall 3-D folding of a single chain. (B) Many proteins are biologically active with only a tertiary structure (e.g. myoglobin, most enzymes); a quaternary structure is not a requirement for activity. (D) The α-helix is a RIGHT-handed screw (held by intramolecular H-bonds), not left-handed. Hence the correct statement is option C.
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