GATE BT 2018 • Q40
MCQ • 2 Marks
Section 2: General Biotechnology
Biomolecules, enzyme kinetics
Subtopic: Fundamentals of Biomolecules, enzyme kinetics
Question Statement:
Which one of the following amino acids is catalyzed by activated macrophages to produce
reactive nitrogen species?
Options:
-
(A) Arginine
-
(B) Asparagine
-
(C) Cysteine
-
(D) Histidine
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.40)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Immunology & Enzymology (Nitric Oxide Synthase & Reactive Nitrogen Species)
**2. Detailed Step-by-Step Analysis**:
- Activated macrophages (stimulated by IFN-$\gamma$ and bacterial LPS) express **inducible Nitric Oxide Synthase (iNOS or NOS2)**.
- iNOS catalyzes the 5-electron oxidation of the basic amino acid **L-Arginine** to produce **L-Citrulline** and **Nitric Oxide ($\cdot ext{NO}$)** in the presence of $O_2$ and NADPH:
$ ext{L-Arginine} + 2O_2 + 1.5 ext{ NADPH} + 1.5 ext{ H}^+ \xrightarrow{ ext{iNOS}} ext{L-Citrulline} + \cdot ext{NO} + 1.5 ext{ NADP}^+ + 2 ext{H}_2 ext{O}$
- Nitric oxide reacts with superoxide ($ ext{O}_2^{\cdot-}$) to form peroxynitrite ($ ext{ONOO}^-$), a lethal **Reactive Nitrogen Species (RNS)** that exerts cytotoxic antimicrobial and antitumor actions.
- Therefore, **Arginine** is the specific amino acid substrate.
- **Option-by-Option Evaluation**:
- **Option (A) [CORRECT]**: L-Arginine is the physiological substrate catalyzed by iNOS in activated macrophages to produce nitric oxide (RNS).
- **Option (B) [INCORRECT]**: Asparagine is not a precursor for reactive nitrogen species.
- **Option (C) [INCORRECT]**: Cysteine provides sulfhydryl groups for glutathione synthesis and redox buffering, not the substrate for iNOS.
- **Option (D) [INCORRECT]**: Histidine is decarboxylated by histidine decarboxylase to form histamine.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **A**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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