Q.105 Which of the following amino acid change (mutation) would MOST adversely affect the structure of an α-helix?
(A) A valine residue changed to an isoleucine residue
(B) A methionine residue changed to a proline residue
(C) An aspartic acid residue changed to a glutamic acid residue
(D) A histidine residue changed to an arginine residue
Proline substitution most severely disrupts α-helix structure due to its rigid ring structure that breaks hydrogen bonding and forces backbone kinks.
Correct Answer
(B) A methionine residue changed to a proline residue
Detailed Option Analysis
(A) Valine → Isoleucine
Minimal disruption. Both are branched hydrophobic amino acids with similar β-carbon branching. Val (shorter isopropyl) to Ile (longer sec-butyl) maintains helix hydrophobicity and H-bonding capacity. Slight steric increase rarely destabilizes helices significantly.
(B) Methionine → Proline MOST ADVERSE
Severe disruption. Proline’s pyrrolidine ring:
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Lacks N-H amide proton → cannot form i to i+4 H-bond critical for helix stability
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Rigid φ angle (-60°) → forces ~30° kink, distorting regular 3.6 residue/turn geometry
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Steric clash in helix core
Methionine (flexible, H-bond capable) to proline creates “helix breaker.”
(C) Aspartic Acid → Glutamic Acid
Negligible effect. Both acidic, negatively charged. Asp (shorter side chain) to Glu (one CH₂ longer) maintains charge position for salt bridges/i to i+4 H-bonds. Common conservative substitution in helices.
(D) Histidine → Arginine
Minor effect. Both basic. His (imidazole ring) to Arg (longer guanidinium) changes pKa but preserves positive charge for stabilizing interactions. Helix propensity similar (both moderate helix formers).
| Substitution | Helix Disruption | Mechanism |
|---|---|---|
| Val→Ile | None | Similar hydrophobicity |
| Met→Pro | Severe | No H-bond, rigid kink |
| Asp→Glu | None | Conservative length change |
| His→Arg | Minimal | Charge conserved |
Structural Biochemistry Details
α-helix stability requires:
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i to i+4 H-bonds (N-H…O=C) → Proline lacks amide H
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Regular φ=-57°, ψ=-47° → Proline φ fixed at -65°
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Core packing → Proline ring clashes
Proline effect quantified: ΔΔG unfolding +1.5-2.5 kcal/mol per Pro, versus <0.5 kcal/mol for other substitutions.
In protein biochemistry, proline substitution represents the amino acid change mutation that most adversely affects alpha helix structure due to its unique ring structure eliminating critical hydrogen bonding.
Why Proline Destroys Helices
Normal helix: ...-N-H...O=C-... (i to i+4 H-bond)
Proline: ...-N<ring...O=C-... (NO H-bond donor)
Result: 30° kink, destabilization ΔG = +2 kcal/mol.
Exam-Relevant Comparisons
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Conservative (Asp→Glu, His→Arg): Helix preserved
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Hydrophobic (Val→Ile): Packing maintained
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Proline: Helix breaker – CSIR-NET favorite!
Biotech students: Memorize “Proline = helix terminator” for structural biology questions.
