Q.14 In the above mentioned Ramachandran Plot, A and B represent the following respectively
(1) Right-handed α-helix and left-handed α-helix
(2) Right-handed α-helix and β-pleated sheet
(3) β-pleated sheet and left-handed α-helix
(4) β-pleated sheet and right-handed α-helix
Ramachandran Plot MCQ: What Do A and B Represent?
The Ramachandran plot is a fundamental concept in structural biology and biochemistry. It shows the allowed and disallowed regions of backbone dihedral angles:
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Φ (phi) – rotation around N–Cα bond
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Ψ (psi) – rotation around Cα–C bond
In the given Ramachandran plot:
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Region A lies in the upper left quadrant (Φ negative, Ψ positive)
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Region B lies in the lower left quadrant (Φ negative, Ψ negative)
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Region C lies in the upper right quadrant
The question asks:
In the above Ramachandran Plot, A and B represent respectively:
Options:
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Right-handed α-helix and left-handed α-helix
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Right-handed α-helix and β-pleated sheet
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β-pleated sheet and left-handed α-helix
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β-pleated sheet and right-handed α-helix
✅ Correct Answer: (4) β-pleated sheet and right-handed α-helix
Explanation of Regions in Ramachandran Plot
🔹 Region A (Upper Left Quadrant)
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Φ ≈ –120°
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Ψ ≈ +120°
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Characteristic of β-pleated sheets
Beta sheets have extended conformations, which place them in the upper-left allowed region of the plot.
🔹 Region B (Lower Left Quadrant)
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Φ ≈ –60°
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Ψ ≈ –40°
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Characteristic of right-handed α-helix
Most naturally occurring α-helices in proteins are right-handed, and they cluster in this region.
🔹 Region C (Upper Right Quadrant)
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Represents left-handed α-helix
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Rare in natural proteins due to steric hindrance
Why Other Options Are Incorrect
❌ Option (1): Right-handed α-helix and left-handed α-helix
Incorrect because:
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Region A is not right-handed α-helix.
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Region B is not left-handed α-helix.
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Right-handed α-helix appears in lower-left quadrant, not upper-left.
❌ Option (2): Right-handed α-helix and β-pleated sheet
Incorrect because:
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The order is reversed.
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Region A corresponds to β-sheet, not α-helix.
❌ Option (3): β-pleated sheet and left-handed α-helix
Incorrect because:
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Region B represents right-handed α-helix, not left-handed.
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Left-handed α-helix appears in upper-right quadrant (Region C).
Summary Table
| Region | Phi (Φ) | Psi (Ψ) | Secondary Structure |
|---|---|---|---|
| A | Negative | Positive | β-pleated sheet |
| B | Negative | Negative | Right-handed α-helix |
| C | Positive | Positive | Left-handed α-helix |
Understanding Ramachandran plots is essential for:
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Protein structure prediction
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X-ray crystallography interpretation
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Structural bioinformatics
Final Answer:
✔ Option (4) – β-pleated sheet and right-handed α-helix
