Q.56. Group I, Group II and Group III represent enzyme, product of the enzymatic reaction, and
metabolic process, respectively.
Group I
P. Hexokinase
Q. Fumarase
R. PEP Carboxylase
S. Glycolate oxidase
Group II
i. Malate
ii. Glucose 6-P
iii. Hydrogen peroxide
iv. Cinnamic acid
v. Oxaloacetic acid
Group III
1. Glycolysis
2. Photorespiration
3. TCA cycle
4. Photosynthesis
The CORRECT combination for Group I, Group II and Group III is
(A) P-ii-1, Q-iv-3, R-v-2, S-iii-4
(B) P-ii-1, Q-i-3, R-v-4, S-iii-2
(C) P-ii-2, Q-v-3, R-i-4, S-iii-1
(D) P-iii-1, Q-i-3, R-iv-4, S-ii-2

The correct answer is option (B): P-ii-1, Q-i-3, R-v-4, S-iii-2.​

Correct matching overview

• Hexokinase converts glucose to glucose‑6‑phosphate and functions in glycolysis.​

• Fumarase forms malate and is a TCA cycle enzyme.​

• PEP carboxylase produces oxaloacetate and is key in photosynthetic CO₂ fixation.​

• Glycolate oxidase generates hydrogen peroxide and functions in photorespiration.​

So the correct set is:

• P – ii – 1 (Hexokinase – Glucose‑6‑P – Glycolysis)

• Q – i – 3 (Fumarase – Malate – TCA cycle)

• R – v – 4 (PEP carboxylase – Oxaloacetate – Photosynthesis)

• S – iii – 2 (Glycolate oxidase – H₂O₂ – Photorespiration)​

Explanation of the correct option (B)

Hexokinase – Glucose‑6‑phosphate – Glycolysis

• Hexokinase catalyses phosphorylation of glucose to glucose‑6‑phosphate, the first committed step of glycolysis.​

• This reaction traps glucose inside the cell and channels it into the glycolytic pathway, so the correct pairing is P – ii – 1.​

Fumarase – Malate – TCA cycle

• Fumarase (fumarate hydratase) catalyses the reversible hydration of fumarate to malate in the tricarboxylic acid (TCA) cycle.​

• Therefore, Q – i – 3 (Fumarase – Malate – TCA cycle) is correct.

PEP carboxylase – Oxaloacetate – Photosynthesis

• Phosphoenolpyruvate carboxylase (PEP carboxylase) fixes CO₂ into phosphoenolpyruvate to form oxaloacetate, especially in C₄ and CAM plants.​

• This reaction is a primary CO₂ fixation step in photosynthesis in these plants, so R – v – 4 is correct.​

Glycolate oxidase – Hydrogen peroxide – Photorespiration

• Glycolate oxidase oxidizes glycolate to glyoxylate and produces hydrogen peroxide in peroxisomes.​

• This reaction is part of the photorespiratory pathway (photorespiration), so S – iii – 2 is the correct match.​

Why the other options are incorrect

Option (A): P-ii-1, Q-iv-3, R-v-2, S-iii-4

• Q-iv-3: Fumarase does not produce cinnamic acid; it produces malate in the TCA cycle.​

• R-v-2: PEP carboxylase forming oxaloacetate is correct, but assigning this to photorespiration (2) is wrong because this step belongs to photosynthetic CO₂ fixation in C₄/CAM plants, not to photorespiration.​

• S-iii-4: Glycolate oxidase does form hydrogen peroxide, but that belongs to photorespiration, not to photosynthesis directly, so linking to process 4 is incorrect.​

Option (C): P-ii-2, Q-v-3, R-i-4, S-iii-1

• P-ii-2: Hexokinase and glucose‑6‑phosphate are correctly paired, but assigning this to photorespiration (2) is wrong; it is a key step in glycolysis.​

• Q-v-3: Fumarase does not produce oxaloacetate; that is associated with other enzymes (e.g., malate dehydrogenase or PEP carboxylase), so this enzyme–product pair is incorrect.​

• R-i-4: PEP carboxylase does not produce malate directly; it produces oxaloacetate, which can then be reduced to malate by malate dehydrogenase.​

• S-iii-1: Glycolate oxidase–H₂O₂ belongs to photorespiration, not glycolysis, so assigning process 1 is incorrect.​

Option (D): P-iii-1, Q-i-3, R-iv-4, S-ii-2

• P-iii-1: Hexokinase does not produce hydrogen peroxide; it produces glucose‑6‑phosphate and is not an oxidase.​

• R-iv-4: PEP carboxylase does not produce cinnamic acid; cinnamic acid is formed in phenylpropanoid metabolism from phenylalanine via phenylalanine ammonia‑lyase, not by PEP carboxylase.​

• S-ii-2: Glycolate oxidase does not form glucose‑6‑phosphate; it forms glyoxylate and H₂O₂, so this pairing is wrong.​

Quick reference table

This pattern of enzyme–product–pathway matching is frequently tested in plant physiology and biochemistry MCQs and is essential for mastering metabolic integration.​