Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
The hexapeptide P has an isoelectric point (pI) of 6.9. Hexapeptide Q is a variant of P that
contains valine instead of glutamate at position 3. The two peptides are analyzed by
polyacrylamide gel electrophoresis at pH 8.0. Which one of the following statements is
Options:
(A) P will migrate faster than Q towards the anode
(B) P will migrate faster than Q towards the cathode
(C) Both P and Q will migrate together
(D) Q will migrate faster than P towards the anode
Official Answer Key:
A
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.43)**
**1. Governing Concept**:
- **Topic**: Biochemistry & Peptide Isoelectric Point (pI)
- The isoelectric point (pI) of a peptide is the pH at which its net electric charge is strictly zero.
- For a hexapeptide containing ionizable side-chains (Glu/Asp acidic groups, Lys/Arg/His basic groups, along with N-terminal $\alpha$-amino and C-terminal $\alpha$-carboxyl):
- At $\text{pH} < \text{pI}$, the peptide carries a net positive charge.
- At $\text{pH} > \text{pI}$, the peptide carries a net negative charge.
- Electrophoresis or chromatography at specific buffer pH values determines migration direction based on $\text{pH} - \text{pI}$.
**2. Option Analysis**:
- **Option (A) is CORRECT**: Accurately computes peptide net charge and electrophoretic mobility at the specified pH.
- **Option (B) is INCORRECT**: Sign error in charge calculation.
- **Option (C) is INCORRECT**: Miscalculates ionization state of side-chains.
- **Option (D) is INCORRECT**: Ignores terminal ionizable groups.
**3. Final Answer**:
- **Correct Choice**: **A**
Practice This in Real CBT Exam Interface
Test yourself with the official TCS-iON palette, virtual calculator, and immediate performance analytics.