39. The following observations are made on a 30-residue polypeptide
    (a) Unordered structure is observed in water but a helical conformation is observed in medium of low dielectric constant.
    (b)The peptide is resistant to degradation by proteases.
    (c) Red blood cells are lysed by the peptide.
    (d) β-mercaptoethanol has no effect on peptide structure.
    Which of the following statements can be correctly attributed to the above observations?
    (1)The peptide is entirely composed of D-amino acids and is amphipathic.
    (2)The peptide is entirely composed of L-amino acids and is not amphipathic.
    (3)The peptide is rich in disulphide bonds between D-cysteines.
    (4)The peptide is entirely composed of L –aromatic amino acids.

Structural and Functional Analysis of a 30-Residue Polypeptide

Understanding polypeptide structure in different environments is crucial for predicting function and stability. The observations given provide insights into the nature of the peptide, its resistance to proteases, and its interaction with cell membranes.

Correct Answer:

The correct option is (1) The peptide is entirely composed of D-amino acids and is amphipathic.

Explanation:

(a) Unordered Structure in Water but Helical Conformation in a Low Dielectric Medium ✅

• A peptide adopting a random coil in water but forming a helix in a hydrophobic environment suggests it is amphipathic.
• Amphipathic helices have polar residues on one side and non-polar residues on the other, allowing them to interact with membranes.
• This is characteristic of antimicrobial peptides.

(b) Peptide is Resistant to Degradation by Proteases ✅

• D-amino acids are not recognized by most proteases, making them highly resistant to enzymatic degradation.
• This strongly suggests the peptide is composed of D-amino acids rather than L-amino acids.

(c) Red Blood Cells Are Lysed by the Peptide ✅

• Peptides that disrupt membranes and cause hemolysis are often amphipathic.
• Amphipathic helices can insert into the lipid bilayer and destabilize the membrane.

(d) β-Mercaptoethanol Has No Effect on Peptide Structure ✅

• β-Mercaptoethanol (BME) breaks disulfide bonds.
• If BME has no effect, it means the peptide does not contain disulfide bonds.

Why the Other Options Are Incorrect:

• (2) The peptide is entirely composed of L-amino acids and is not amphipathic ❌
  • The peptide behaves like an amphipathic helix, which contradicts this statement.
  • L-amino acids are typically recognized by proteases, but this peptide is resistant.
• (3) The peptide is rich in disulfide bonds between D-cysteines ❌
  • β-Mercaptoethanol has no effect, ruling out disulfide bonds.
• (4) The peptide is entirely composed of L-aromatic amino acids ❌
  • Aromatic residues alone do not explain protease resistance or membrane disruption.

Thus, the correct answer is (1) The peptide is entirely composed of D-amino acids and is amphipathic.

Nearby Topics for Better Understanding

1. Role of D-Amino Acids in Peptide Stability

• D-amino acids are rare in natural proteins but are used in antimicrobial peptides (AMPs) to resist degradation.
• Examples: Gramicidin, Daptomycin.

2. Amphipathic Helices and Membrane Interaction

• Amphipathic helices have a hydrophobic side (interacts with lipids) and a hydrophilic side (interacts with water).
• Found in antimicrobial peptides, membrane-active toxins, and viral fusion proteins.

3. β-Mercaptoethanol and Disulfide Bonds

• β-Mercaptoethanol reduces disulfide bonds by breaking cysteine-cysteine (Cys-Cys) linkages.
• If a peptide remains unchanged in BME, it lacks disulfide bonds.

4. Hemolysis by Amphipathic Peptides

• Red blood cell lysis suggests the peptide can insert into and disrupt lipid membranes.
• Example: Melittin (bee venom toxin), Hemolysins.

Conclusion

This polypeptide is likely an amphipathic helical peptide composed of D-amino acids, making it resistant to proteases and capable of disrupting membranes. Such peptides have important applications in antimicrobial therapy and drug design.