Q.50 Arrange the enzymes in sequence on the basis of their involvement in citrie acid cycle.
A. Aconitase
B. Isocitrate dehydrogenase
C. Fumarase
D. Malate-dehydrogenase
E. Succinate dehydrogenase
Choose the correct answer from the options given below:
1. A. B, C, D, E
2. E, D, C, A, B
3. A, B, E, C, D
4. D. C, B. A, E
The correct sequence of enzymes in the citric acid cycle matches option 3, reflecting their precise order from citrate to oxaloacetate.
Correct Answer
Option 3: A, B, E, C, D
Cycle Sequence Overview
Citric acid cycle (TCA/Krebs cycle) enzymes follow: Aconitase (A) converts citrate to isocitrate; Isocitrate dehydrogenase (B) forms α-ketoglutarate; Succinate dehydrogenase (E) oxidizes succinate to fumarate; Fumarase (C) hydrates fumarate to malate; Malate dehydrogenase (D) regenerates oxaloacetate.
Enzyme Roles Detailed
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Aconitase (A): Step 2, isomerizes citrate to isocitrate via cis-aconitate.
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Isocitrate dehydrogenase (B): Step 3, oxidative decarboxylation to α-ketoglutarate + NADH + CO₂.
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Succinate dehydrogenase (E): Step 6, forms fumarate + FADH₂.
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Fumarase (C): Step 7, adds water to yield malate.
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Malate dehydrogenase (D): Step 8, oxidizes malate to oxaloacetate + NADH.
Options Breakdown
| Option | Sequence | Why Correct/Incorrect |
|---|---|---|
| 1. A, B, C, D, E | Starts right but places C/D/E wrong (C after B skips succinate step) | Wrong |
| 2. E, D, C, A, B | Reverse order ignores cycle flow from aconitase early | Wrong |
| 3. A, B, E, C, D | Matches exact pathway: citrate → isocitrate → succinate → fumarate → malate → OAA | Correct |
| 4. D, C, B, A, E | Jumbled; starts at end (D), skips intermediates | Wrong |
