14.
Labelling of proteins with iodine is used for tracing. Which amino acid does iodine react
with.
a. Alanine
b. Aspartate
c. Lysine
d. Tyrosine
Iodine Labelling of Proteins: Tyrosine Reactivity in Tracing Techniques
Iodine labelling traces proteins by attaching radioactive isotopes like I-125 to specific amino acids. Tyrosine is the primary target due to its phenolic ring.
Question Analysis
The query asks which amino acid reacts with iodine for protein labelling in tracing: a) Alanine, b) Aspartate, c) Lysine, or d) Tyrosine. Iodination substitutes iodine onto aromatic rings via electrophilic attack, primarily on tyrosine residues in methods like Chloramine-T or lactoperoxidase. This specificity enables high-activity tracers for assays like RIA.
Option Explanations
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Alanine: Non-reactive; lacks aromatic ring or suitable nucleophilic group for electrophilic iodination.
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Aspartate: Carboxylic acid side chain does not undergo iodination under standard protein labelling conditions.
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Lysine: Amino group targets Bolton-Hunter reagent via acylation, not direct iodine substitution.
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Tyrosine: Phenolic ring enables direct iodination at ortho positions, forming mono- or di-iodotyrosine.
Correct answer: d. Tyrosine.
Iodination Mechanism
Oxidative methods generate electrophilic iodine species (e.g., I+) from iodide, which attacks tyrosine’s electron-rich ring. Chloramine-T or IODOGEN oxidizes Na125I for substitution. Proteins use this for tracking in binding assays, with tyrosine accessibility determining efficiency.
In iodine labelling of proteins, researchers attach radioactive iodine (e.g., I-125) to trace biomolecules in assays like radioimmunoassays (RIA). The key question—”Which amino acid does iodine react with?”—targets tyrosine due to its phenolic ring’s reactivity. This technique, vital for CSIR NET Life Sciences, distinguishes tyrosine from alanine, aspartate, and lysine.
Why Tyrosine in Iodine Labelling?
Tyrosine undergoes electrophilic aromatic substitution, where oxidized iodine (I+) adds to its ortho positions, forming stable iodotyrosines. Methods like Chloramine-T expose proteins to Na125I and oxidant for tyrosine-specific labelling. Histidine reacts minimally; others do not.
Evaluating MCQ Options
| Option | Amino Acid | Reactivity with Iodine | Reason |
|---|---|---|---|
| a | Alanine | No | Aliphatic, no aromatic ring |
| b | Aspartate | No | Acidic side chain inert |
| c | Lysine | Indirect (Bolton-Hunter) | ε-amino acylation, not direct iodination |
| d | Tyrosine | Yes | Phenolic ring substitution |
Applications in Research
Used in receptor binding and SPA assays, iodinated proteins track dynamics but degrade post-60-day half-life. CSIR NET aspirants note: Tyrosine’s role mirrors thyroid hormone synthesis.
