Q.31 Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.

Assertion A: Treatment of immunoglobulin IgG with B-mercaptoethanol gives rise to two heavy chains and
two light chains peptides on SDS-PAGE.

Reason R: ß-mercaptoethanol breaks peptide bond at the site of disulphide linkage.

In the light of the above statements, choose the most appropriate answer from the options given below:

1. Both A and R are correct and R is the correct explanation of A

2. Both A and R are correct but R is NOT the correct explanation of A

3. A is correct but R is not correct

4. A is not correct but R is correct

IgG B-Mercaptoethanol Treatment: Assertion-Reason Cleavage Analysis

Treating IgG with β-mercaptoethanol breaks disulfide bonds, yielding two heavy and two light chains on SDS-PAGE. The reason correctly explains this via disulfide reduction, not peptide bond breakage.​

Correct Answer

1. Both A and R are correct and R is the correct explanation of A. Assertion A accurately describes IgG dissociation into 2H + 2L chains under reducing SDS-PAGE conditions. Reason R properly states β-mercaptoethanol targets disulfide linkages holding these domains together.

Option Breakdown

Option 1: Both Correct, R Explains A

Correct. IgG’s four chains link via ~12 interchain disulfide bonds; β-ME reduces S-S to 2 SH groups via thiol-disulfide exchange, freeing H (~50 kDa) and L (~25 kDa) peptides visible on gels.

Option 2: Both Correct, R Doesn’t Explain A

Incorrect. R directly justifies A: disulfide cleavage (not peptide bonds) separates quaternary structure, enabling size-based SDS-PAGE separation of individual polypeptide chains.​

Option 3: A Correct, R Incorrect

Wrong. R accurately describes the mechanism—β-ME doesn’t hydrolyze peptide bonds (that’s proteases) but performs redox reduction of covalent S-S linkages.

Option 4: A Incorrect, R Correct

False. A matches standard biochemistry: non-reduced IgG migrates ~150 kDa; reducing conditions yield distinct H/L bands confirming domain structure.​