Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
The circular dichroism spectra of three proteins P, Q. and R are given below:
P
Q
R
0
0
-20
-20
-20
Molar ellipticity ( Kdeg cm l dmol* )
200 210
220 230
240 250
Wavelength (nm)
Molar ellipticity ( Kdeg cm' dmol dmol1)
200
210
220
230 240
250
200
210
220 230 240 250
Wavelength (nm)
Wavelength (nm)
Choose the correct combination.
Options:
(A) P: a-helix. Q: ß-sheet, and R: Random coil
(B) P: B-sheet, Q: a-helix, and R: Random coil
(C) P: a-helix, Q: Random coil, and R: B-sheet
(D) P: Random coil, Q: a-helix, and R: B-sheet
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2017 Q.30)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Biophysics / Spectroscopy (Circular Dichroism Signatures of Protein Secondary Structures)
**2. Detailed Step-by-Step Analysis**:
- Far-UV Circular Dichroism (CD) spectra (190–250 nm) exhibit distinctive spectral signatures corresponding to canonical polypeptide secondary structures:
1. **$\alpha$-helix**: Displays two pronounced negative ellipticity minima at **208 nm** and **222 nm**, along with a strong positive band at **192–193 nm**.
2. **$\beta$-sheet**: Characterized by a single broad negative ellipticity minimum around **216–218 nm** and a positive peak near **195–198 nm**.
3. **Random coil (disordered conformation)**: Exhibits a strong negative band near **198–200 nm** and near-zero ellipticity above 210 nm.
- Matching the spectra P, Q, and R in the standard problem:
- Spectrum showing dual minima at 208 and 222 nm corresponds to the predominantly **$\alpha$-helical** protein.
- Spectrum with a single minimum at 217 nm corresponds to the **$\beta$-sheet** protein.
- Spectrum lacking these features corresponds to the disordered/random coil structure.
- The correct mapping is designated by **Option A**.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: Accurately maps the respective CD spectra to $\alpha$-helix (208/222 nm dual minima), $\beta$-sheet (217 nm minimum), and disordered states.
- **Option (B) [INCORRECT]**: Inverts the assignment of $\alpha$-helix and $\beta$-sheet.
- **Option (C) [INCORRECT]**: Misattributes the characteristic 208/222 nm helical doublet.
- **Option (D) [INCORRECT]**: Erroneous spectral assignment.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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