9. You are studying an unusual protein which has a pKa of 5. You wish to purify this
protein. For this you can:
a. Use anion-exchange chromatography, and a low pH buffer for elution
b. Use anion-exchange chromatography, and a high pH buffer for elution
c. Use cation-exchange chromatography, and a low pH buffer for elution
d. Use cation-exchange chromatography, and a high pH buffer for elution
Protein pKa 5 Purification: Correct Answer c. Cation-Exchange Chromatography with Low pH Buffer
A protein with pKa 5 carries a net positive charge below pH 5 and negative charge above pH 5, making cation-exchange chromatography ideal for binding at low pH followed by elution.
Option Analysis
a. Anion-exchange chromatography, low pH buffer for elution
At low pH (<5), the protein is positively charged and repelled by the positively charged anion-exchange resin, preventing binding. Elution fails as no initial retention occurs.
b. Anion-exchange chromatography, high pH buffer for elution
High pH (>5) makes the protein negatively charged for anion-resin binding, but elution typically uses salt gradients, not just high pH which strengthens binding. Inefficient for purification.
c. Cation-exchange chromatography, low pH buffer for elution
Low pH (<5) protonates the protein (net + charge), binding it to negatively charged cation-resin. Elution with salt or higher pH displaces it selectively. Correct strategy.
d. Cation-exchange chromatography, high pH buffer for elution
High pH (>5) deprotonates the protein (net – charge), causing repulsion from cation-resin and no binding. Purification impossible.
Protein pKa 5 purification relies on ion exchange chromatography principles where protein charge at specific pH determines resin binding and elution. For CSIR NET Life Sciences aspirants, understanding how a protein’s pKa 5 shifts net charge enables targeted separation using cation-exchange chromatography at low pH.
Charge Basis in Ion Exchange
Proteins below pKa/pI carry positive charge (protonated), binding cation exchangers (negative resin); above pKa, negative charge suits anion exchangers. Low pH buffers (<5) ensure + charge for cation binding, eluted by salt competition.
| Technique | pH Condition | Protein Charge | Binding? | Elution Method |
|---|---|---|---|---|
| Cation-Exchange | Low (<5) | Positive | Yes | Salt/high pH |
| Anion-Exchange | High (>5) | Negative | Yes | Salt/low pH |
This table highlights optimal conditions for protein pKa 5 purification ion exchange chromatography, key for exam success.
