Q.48 Malate, Fumarate and Succinate precede the formation of oxaloacetate in the citric acid cycle.
Find the option which shows correct order of their formation in the citric acid cycle.
A. Malate
B. Fumarate
C. Oxaloacetate
D. Succinate
Choose the correct answer from the options given below:
- D, B, A, C
- D, B, C, A
- B, D, C, A
- B, D, A, C
Correct Answer: D, B, A, C
The correct order of formation for these intermediates preceding oxaloacetate in the citric acid cycle (TCA cycle or Krebs cycle) is Succinate (D), then Fumarate (B), then Malate (A), and finally Oxaloacetate (C).
Step-by-Step Cycle Sequence
Succinate forms from succinyl-CoA via succinyl-CoA synthetase.
Next, succinate dehydrogenase oxidizes succinate to fumarate, producing FADH₂.
Fumarase then adds water to fumarate, yielding L-malate.
Finally, malate dehydrogenase oxidizes malate to oxaloacetate, producing NADH.Option Analysis
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D, B, A, C (Succinate → Fumarate → Malate → Oxaloacetate): Matches the exact sequence above. Correct.
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D, B, C, A (Succinate → Fumarate → Oxaloacetate → Malate): Wrong; oxaloacetate follows malate, not precedes it.
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B, D, C, A (Fumarate → Succinate → Oxaloacetate → Malate): Wrong; reverses succinate-to-fumarate direction.
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B, D, A, C (Fumarate → Succinate → Malate → Oxaloacetate): Wrong; fumarate forms after succinate.
Citric Acid Cycle Order: Malate, Fumarate, Succinate Before Oxaloacetate
The citric acid cycle order—also called the TCA cycle or Krebs cycle—crucially involves intermediates like succinate, fumarate, malate, and oxaloacetate. These four-carbon compounds drive ATP production in mitochondria, vital for cellular respiration in exams like GATE Life Sciences. This guide breaks down their precise order of formation before oxaloacetate regenerates to restart the cycle.
Citric Acid Cycle Pathway Overview
The cycle processes acetyl-CoA from glycolysis and pyruvate, yielding NADH, FADH₂, GTP, and CO₂. Key late steps focus on your query’s intermediates:
Step Intermediate Reaction Enzyme 6 Succinate Succinyl-CoA → Succinate + GTP/CoA Succinyl-CoA synthetase 7 Fumarate Succinate + Q → Fumarate + FADH₂ Succinate dehydrogenase 8 Malate Fumarate + H₂O → Malate Fumarase 9 Oxaloacetate Malate + NAD⁺ → Oxaloacetate + NADH Malate dehydrogenase This confirms succinate precedes fumarate, malate precedes oxaloacetate—core to MCQs on TCA cycle order.
Why This Sequence Matters
Succinate oxidation links to the electron transport chain via FADH₂. Fumarate hydration is reversible, ensuring malate buildup. Malate’s oxidation closes the cycle, enabling acetyl-CoA condensation anew. Disruptions here affect energy yield: full glucose oxidation produces ~30-32 ATP partly via these steps.
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