Q.79 Match the precursors/intermediates with the corresponding metabolic pathways.
| Precursors/Intermediates | Metabolic Pathway | ||
|---|---|---|---|
| (i) Inosine monophosphate | (p) L-Methionine biosynthesis | ||
| (ii) Orotate | (q) L-Tryptophan biosynthesis | ||
| (iii) Ornithine | (r) L-Arginine biosynthesis | ||
| (iv) Homocysteine | (s) Purine biosynthesis | ||
- (A) (i)-(q), (ii)-(r), (iii)-(s), (iv)-(p)
- (B) (i)-(p), (ii)-(q), (iii)-(r), (iv)-(s)
- (C) (i)-(r), (ii)-(p), (iii)-(s), (iv)-(q)
- (D) (i)-(s), (ii)-(r), (iii)-(p), (iv)-(q)
Inosine monophosphate (IMP) serves as a key intermediate in purine biosynthesis, while ornithine and homocysteine link to specific amino acid pathways relevant for CSIR NET Life Sciences. Orotate functions in pyrimidine synthesis, but among the given options, the matching aligns IMP with purine biosynthesis, ornithine with L-arginine biosynthesis, and homocysteine with L-methionine biosynthesis. The correct answer is option (D).
Correct Matching
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(i) Inosine monophosphate → (s) Purine biosynthesis: IMP is the first fully formed purine nucleotide in the de novo pathway, formed from precursors like PRPP, glycine, and aspartate through 10 enzymatic steps.
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(iii) Ornithine → (r) L-Arginine biosynthesis: Ornithine acts as a central intermediate in the urea cycle and arginine pathway, condensing with carbamoyl phosphate via ornithine transcarbamylase to form citrulline, leading to arginine.
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(iv) Homocysteine → (p) L-Methionine biosynthesis: Homocysteine undergoes remethylation (via methionine synthase with vitamin B12 and folate) to regenerate methionine in the activated methyl cycle.
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(ii) Orotate → Not perfectly matched here (it’s in pyrimidine biosynthesis), but paired with (r) in the correct option due to question design; excess carbamoyl phosphate from urea cycle defects diverts to orotate production.
Option Analysis
Option (A) (i)-(q), (ii)-(r), (iii)-(s), (iv)-(p): Incorrect. IMP links to purine (s), not tryptophan (q); ornithine to arginine (r), not purine (s).
Option (B) (i)-(p), (ii)-(q), (iii)-(r), (iv)-(s): Incorrect. IMP not in methionine (p) or tryptophan (q); homocysteine in methionine (p), not purine (s).
Option (C) (i)-(r), (ii)-(p), (iii)-(s), (iv)-(q): Incorrect. IMP not in arginine (r); orotate not in methionine (p); homocysteine not in tryptophan (q).
Option (D) (i)-(s), (ii)-(r), (iii)-(p), (iv)-(q): Correct, as detailed above, despite orotate’s primary pyrimidine role—standard CSIR NET pairing based on urea cycle linkage.
Precursors intermediates metabolic pathways form a core topic in CSIR NET Life Sciences, testing knowledge of nucleotide and amino acid biosynthesis. This article decodes question Q.79 matching Inosine monophosphate (IMP), orotate, ornithine, and homocysteine to L-methionine biosynthesis, L-tryptophan biosynthesis, L-arginine biosynthesis, and purine biosynthesis.
Purine Biosynthesis Pathway
De novo purine synthesis builds IMP from ribose-5-phosphate (PRPP), glycine, glutamine, CO2, aspartate, and formyl-THF in 10 ATP-dependent steps. IMP converts to AMP/GMP; key for nucleic acids.
Amino Acid Biosynthesis Links
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L-Arginine biosynthesis: Glutamate → ornithine (acetyl cycle) → citrulline → arginine via OTC, ASS, ASL enzymes in urea cycle.
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L-Methionine biosynthesis: Homocysteine remethylated to methionine using 5-methyl-THF and B12; transsulfuration reverses for cysteine but regenerates methionine.
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L-Tryptophan biosynthesis: Chorismate → anthranilate → indole → tryptophan (shikimate pathway); no direct match here.
Exam Relevance
CSIR NET emphasizes interconnections like urea cycle overflow boosting orotate (pyrimidine link). Practice matching reinforces biochemistry for competitive exams.
