Q.63 Identify the CORRECT combination(s) between the enzymes in Group 1 and the reactions in Group 2 they catalyze.
| Group 1 | Group 2 | ||
|---|---|---|---|
| (P) | Cinnamate-4-hydroxylase | (i) | L-phenylalanine → Cinnamic acid |
| (Q) | Glycerate kinase | (ii) | Glyceraldehyde 3-phosphate → dihydroxyacetone phosphate |
| (R) | PEP carboxylase | (iii) | Glycolate + O2 → Glyoxylate + H2O2 |
| (S) | Nitrate reductase | (iv) | NO3– + NAD(P)H + H+ → NO2– + NAD(P)+ + H2O |
Options:
- S – iv
- Q – ii
- P – i
- R – iii
The correct matching is only S–iv; options Q–ii, P–i and R–iii are incorrect.
Question overview
The question asks to match enzymes in Group 1 with the reactions they catalyse in Group 2:
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Group 1:
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(P) Cinnamate‑4‑hydroxylase
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(Q) Glycerate kinase
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(R) PEP carboxylase
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(S) Nitrate reductase
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Group 2:
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(i) L‑phenylalanine → Cinnamic acid
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(ii) Glyceraldehyde‑3‑phosphate → Dihydroxyacetone phosphate
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(iii) Glycolate + O₂ → Glyoxylate + H₂O₂
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(iv) NO₃⁻ + NAD(P)H + H⁺ → NO₂⁻ + NAD(P)⁺ + H₂O
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The task is to identify which enzyme truly catalyses which reaction and then see which options (A–D) are correct.
Option A: S–iv (Correct)
Nitrate reductase is the enzyme that reduces nitrate (NO₃⁻) to nitrite (NO₂⁻) using NADH or NADPH as an electron donor.
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The overall reaction is:
NO3−+NAD(P)H+H+→NO2−+NAD(P)++H2O. -
This matches exactly with reaction (iv) in Group 2, so S–iv is a correct pairing.
Therefore, statement A (S–iv) is correct.
Option B: Q–ii (Incorrect)
Glycerate kinase is not involved with triose phosphates; it phosphorylates glycerate to glycerate‑3‑phosphate or glycerate‑2‑phosphate using ATP.
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Typical reaction: glycerate + ATP → glycerate‑3‑phosphate (or 2‑phosphate) + ADP.
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Reaction (ii) in the question is Glyceraldehyde‑3‑phosphate ↔ Dihydroxyacetone phosphate, which is the reversible interconversion of two triose phosphates. This reaction is catalysed by triose phosphate isomerase (phosphotriose isomerase), not by glycerate kinase.
Thus, Q–ii is an incorrect match, so option B is wrong.
Option C: P–i (Incorrect)
Cinnamate‑4‑hydroxylase (C4H) is a cytochrome P450 monooxygenase of the phenylpropanoid pathway that hydroxylates trans‑cinnamic acid at the 4‑position to form p‑coumaric acid.
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Reaction: trans‑cinnamic acid + NADPH + O₂ → p‑coumaric acid + NADP⁺ + H₂O.
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Reaction (i) in the table is L‑phenylalanine → Cinnamic acid, which is catalysed by phenylalanine ammonia‑lyase (PAL), not by cinnamate‑4‑hydroxylase.
So P–i is not a correct combination, and therefore option C is incorrect.
Option D: R–iii (Incorrect)
PEP carboxylase (phosphoenolpyruvate carboxylase) catalyses the carboxylation of phosphoenolpyruvate (PEP) to oxaloacetate, using bicarbonate (HCO₃⁻) as the carbon source.
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Reaction: PEP + HCO₃⁻ → Oxaloacetate + Pi.
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Reaction (iii) in the question is Glycolate + O₂ → Glyoxylate + H₂O₂, which is catalysed by glycolate oxidase (also called glycolate oxidase/glycolate dehydrogenase) in peroxisomes during photorespiration.
Since PEP carboxylase does not act on glycolate, R–iii is a wrong match and option D is incorrect.
Correct enzyme–reaction mapping (for clarity)
Enzyme (Group 1) Correct reaction Correct enzyme for Group‑2 reaction given Cinnamate‑4‑hydroxylase (P) Trans‑cinnamic acid → p‑coumaric acid L‑phenylalanine → Cinnamic acid is by PAL, not C4H Glycerate kinase (Q) Glycerate + ATP → Glycerate‑3‑phosphate + ADP G3P ↔ DHAP is by triose phosphate isomerase PEP carboxylase (R) PEP + HCO₃⁻ → Oxaloacetate + Pi Glycolate + O₂ → Glyoxylate + H₂O₂ is by glycolate oxidase Nitrate reductase (S) NO₃⁻ + NAD(P)H + H⁺ → NO₂⁻ + NAD(P)⁺ + H₂O Matches reaction (iv) exactly Only S–iv from the options matches correctly, so the single correct answer is Option (A).
SEO‑oriented introduction
Understanding the correct combinations between enzymes in Group 1 and reactions in Group 2 they catalyze is essential for solving CSIR‑NET and GATE life‑science questions on plant metabolism and enzymology. This detailed explanation walks through each enzyme—cinnamate‑4‑hydroxylase, glycerate kinase, PEP carboxylase and nitrate reductase—and clarifies why only S–iv is the correct match for the given reaction set.
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