GATE 2024 BT Q.18: solution and answer

GATE 2024 · BTQ.18 · 1 mark · MCQ

2-mercaptoethanol breaks the ________ covalent bond between light and heavy chains of an immunoglobulin molecule.

See the question in the paper

Answer and solution

ANSWER

The answer is (D) S-S.

The light and heavy chains of an immunoglobulin are joined by interchain disulfide bridges between cysteine residues, together with non-covalent forces.
2-Mercaptoethanol ( $HS-CH_{2} CH_{2} OH$ ) is a reducing agent. Its thiol group exchanges with a disulfide, so $R-S-S-R$ is converted into two free thiols, $R-SH$ .
The covalent bond it breaks between the chains is therefore the S-S (disulfide) bond, which is option (D).
(A) C-N is the peptide or amide linkage of the polypeptide backbone. A reducing agent does not cleave it.
(B) N-O is not a bond that links the chains of an immunoglobulin.
(C) S-C is the thioether bond in methionine side chains. It is stable to reduction and is not an interchain bridge.

KEY IDEA

2-Mercaptoethanol is a thiol reducing agent. It cleaves disulfide (S-S) bonds, which is how the heavy and light chains of an antibody are separated.

WATCH OUT FOR

Choosing S-C or C-N because they sound like protein bonds. Only the disulfide bridge between cysteines is reducible and joins the chains.