Q52.Protein purification in 2-dimensional gel electrophoresis is based on:
(1) Molecular weight
(2) Charge
(3) Number of subunits
(4) Both 1 and 2

Protein purification in 2D gel electrophoresis relies on both molecular weight (SDS-PAGE) and charge/isoelectric point (IEF). The correct answer for Q52 is (4) Both 1 and 2.

Option Breakdown

• (1) Molecular weight: Correct (2nd dimension). SDS-PAGE denatures proteins with uniform negative charge via SDS, separating solely by size—smaller proteins migrate faster through polyacrylamide pores.

• (2) Charge: Correct (1st dimension). Isoelectric focusing (IEF) separates by isoelectric point (pI)—proteins migrate in pH gradient until net charge = 0, focusing into sharp bands.

• (3) Number of subunits: Incorrect. 2D-PAGE fully denatures proteins (SDS + reducing agents like DTT); quaternary structure disassembled—no subunit counting.

• (4) Both 1 and 2: Correct. Orthogonal separation: IEF (charge/pI) × SDS-PAGE (MW) resolves >3,000 proteins/gel.

2D Workflow

1. IEF strip: Proteins focus by pI (pH 3-10).

2. Equilibration: SDS coating.

3. SDS-PAGE: MW separation perpendicular to IEF axis.
   Result: Each spot = unique (pI, MW) signature.

Master protein purification in 2-dimensional gel electrophoresis for CSIR NET Life Sciences: Q52 tests core principle—molecular weight (2nd dimension SDS-PAGE) + charge (1st dimension IEF). Resolves thousands of proteins by (pI, MW). Why “subunits” distracts, full workflow, exam strategy.

2D Electrophoresis Principle

First dimension (IEF): Proteins in pH gradient → migrate to pI (net charge=0).
Second dimension (SDS-PAGE): SDS denatures → uniform charge/mass → size-based migration.

Correct Answer: (4) Both 1 and 2

Resolves proteins with identical MW but different pI (post-translational modifications) or vice versa (isoforms).

Why “Subunits” Fails

• Denaturation: SDS + β-mercaptoethanol/urea → monomers only.

• Native PAGE tests subunits (no SDS); 2D uses denaturing conditions.

Exam Applications

• Proteomics: Differential expression (disease vs. control).

• PTMs: Phosphorylation shifts pI.

• CSIR trick: Confuses with 1D SDS-PAGE (MW only).

Resolution Math

• IEF: ΔpI = 0.01 pH units.

• SDS-PAGE: ΔMW = 1-2 kDa.

• Spots: ~10,000 complex samples.