Q.58 Given below are two statements :
Statement I : The peptide bond Ca-C-N-Ca present in proteins is rigid and planar.
Statement II : Proteins are highly dynamic and undergo large conformational changes.
In the light of the above statements, choose the correct answer from the options given below :
1. Both Statement I and Statement II are true
2. Both Statement I and Statement II are false
3. Statement I is true but Statement II is false
4. Statement I is false but Statement II is true
Peptide Bonds and Protein Dynamics Overview
Peptide bonds link amino acids in proteins, forming the backbone critical for Ramachandran plots and folding. Exams test their planarity against overall protein flexibility—a classic both-true scenario.
The Two Statements
Statement I: The peptide bond Ca-C-N-Ca present in proteins is rigid and planar.
Statement II: Proteins are highly dynamic and undergo large conformational changes.
Options:
-
Both Statement I and Statement II are true
-
Both Statement I and Statement II are false
-
Statement I is true but Statement II is false
-
Statement I is false but Statement II is true
Correct Answer: Option 1 (Both Statements True)
Peptide bonds exhibit partial double-bond character from resonance (C=O ↔ C⁺-O⁻, C-N ↔ C=N⁺), restricting rotation around Cα-C-N-Cα, making them rigid and planar (trans configuration dominant). Proteins remain dynamic via flexible single bonds (phi/psi angles at Cα), enabling motions like allostery, folding, and enzyme catalysis.
Peptide Bond vs Protein Flexibility Table
| Aspect | Rigid/Planar Feature | Dynamic Feature |
|---|---|---|
| Peptide Bond | Resonance stabilization; no C-N rotation | N/A (fixed trans/cis) |
| Backbone Overall | Cα single bonds allow phi/psi rotation | Conformational changes (α-helix ↔ β-sheet) |
| Examples | Stabilizes secondary structure | Myoglobin breathing motions |
Detailed Option Analysis
-
Option 1: Both true
Correct. I reflects resonance (Pauling/Corey discovery); II matches reality—proteins fluctuate (e.g., chaperones, NMR data show ms-μs dynamics). -
Option 2: Both false
Incorrect. I is textbook fact (Lehninger); II confirmed by crystallography/X-ray. -
Option 3: Statement I true, II false
Incorrect. II holds—proteins aren’t static statues despite rigid bonds. -
Option 4: Statement I false, II true
Incorrect. I defines peptide geometry enabling H-bonding in helices/sheets.
Why Both Coexist: Resonance and Rotations
Visualize: Resonance delocalizes electrons (C-N bond ~1.33 Å, partial double), fixing planarity. Yet, Cα-N and C-Cα bonds rotate freely, allowing global changes (e.g., hemoglobin O₂ binding). Exam trap: Local rigidity ≠ global stiffness.
Tip for Ramachandran Plots: Planar bonds map phi/psi to allowed regions; dynamics sample them. Practice: Proline (cis possible) vs Ala (trans).
This builds on prior α-carbon questions—link to protein folding mastery.
