(A) P, Q

(B) Q, R

(C) Q, S

(D) P, R

Correct Answer

✅ Option (C): Q and S

Explanation

Collagen, α-keratin, and tropomyosin are fibrous proteins, and they share specific structural principles that distinguish them from globular proteins.

Explanation of Each Statement

P) Disulfide bridges to neighboring proteins – ❌ Incorrect

• Disulfide bonds are prominent in α-keratin, especially in hair and nails.

• Collagen and tropomyosin do not primarily rely on inter-protein disulfide bridges.

• Since this feature is not common to all three, this option is incorrect.

Q) Repeating sequences of amino acids – ✅ Correct

• Collagen: Repeating Gly–X–Y sequence (often proline and hydroxyproline).

• α-Keratin: Repetitive heptad sequences important for coiled-coil formation.

• Tropomyosin: Repeating amino acid patterns stabilize its elongated structure.

• ✔️ This feature is common to all three proteins.

R) A high β-sheet content – ❌ Incorrect

• α-keratin is rich in α-helices, not β-sheets.

• Collagen forms a triple helix, not β-sheet structures.

• Tropomyosin is also predominantly α-helical.

• Hence, this statement is false.

S) Superhelical coiling – ✅ Correct

• Collagen: Three polypeptide chains form a right-handed triple helix.

• α-Keratin: Two α-helices twist to form a coiled-coil (superhelix).

• Tropomyosin: Exists as a coiled-coil dimer.

• ✔️ Superhelical coiling is a shared structural feature.

Why Option (C) Is Correct

• Both repeating amino acid sequences (Q) and superhelical coiling (S) are present in collagen, α-keratin, and tropomyosin.

• Other options include features that are not common to all three proteins.