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

Step 1: Recall the levels of protein structure.
Proteins have primary, secondary, tertiary and quaternary structure. The two common secondary structures are the \(\alpha\)-helix and the \(\beta\)-pleated sheet, both held by hydrogen bonds between the C=O and N-H of peptide links.
Step 2: Test option 1 (the alpha-helix).
The \(\alpha\)-helix is a right-handed coil, not left-handed. So option 1 is wrong.
Step 3: Test option 2 (tertiary structure).
Tertiary structure is the overall folding of a single polypeptide chain. Having two or more subunits is the quaternary structure, not tertiary. So option 2 is wrong.
Step 4: Test option 3 (quaternary and activity).
Many proteins with only tertiary structure (for example several enzymes) are fully active. So it is false that only proteins with quaternary structure are active. Option 3 is wrong.
Step 5: Test option 4 (beta-pleated sheet).
In a \(\beta\)-pleated sheet, separate peptide chains lie side by side and are held together by hydrogen bonds between different chains, that is intermolecular hydrogen bonds. This statement is correct.
Step 6: Choose the right option.
The only correct statement is option 4.
\[ \boxed{\text{In } \beta\text{-pleated sheets, chains are held by intermolecular hydrogen bonds}} \]