Discussion
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 |
Answer: A
The correct combination is S-iv.
Nitrate reductase reduces nitrate to nitrite using NAD(P)H, the first step of nitrate assimilation in plants:
\[ \begin{aligned} &\mathrm{NO_3^- + NAD(P)H + H^+} \\ &\rightarrow \mathrm{NO_2^- + NAD(P)^+ + H_2O} \end{aligned} \]
The other pairs are wrong. Phenylalanine to cinnamic acid is catalysed by phenylalanine ammonia-lyase; cinnamate-4-hydroxylase converts cinnamate to p-coumarate. Glyceraldehyde 3-phosphate to dihydroxyacetone phosphate is triose phosphate isomerase, not glycerate kinase. Glycolate oxidation to glyoxylate and H2O2 is glycolate oxidase in photorespiration, not PEP carboxylase. Only one pair is correct, the whole key of this multiple-select question.
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