Q.9 Identify the con•ect sequence of the given enzymes involed in TCA cycle
(A). Aconitase
(B). Fumarase
(C). Maltate dehydrogenase
(D). a-ketoglutarate dehydrogenase
Answer: (D) → (A) → (B) → (C) (α-ketoglutarate dehydrogenase → Aconitase → Fumarase → Malate dehydrogenase).
TCA cycle enzyme sequence follows: α-KG dehydrogenase (step 4, α-KG→succinyl CoA) → Aconitase (step 2, citrate→isocitrate) → Fumarase (step 7, fumarate→malate) → Malate dehydrogenase (step 8, malate→oxaloacetate).
Option Analysis
Correct TCA Cycle Positions:
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(A) Aconitase: Step 2. Citrate ⇌ isocitrate via cis-aconitate intermediate.
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(B) Fumarase: Step 7. Fumarate + H₂O → L-malate.
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(C) Malate dehydrogenase: Step 8. Malate + NAD⁺ → oxaloacetate + NADH (regenerates OAA).
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(D) α-ketoglutarate dehydrogenase: Step 4. α-KG + CoA + NAD⁺ → succinyl CoA + CO₂ + NADH (irreversible, like PDH complex).
Sequence Logic: D (early oxidative decarboxylation) precedes A (early isomerization), then late hydration/oxidation steps B→C.
TCA cycle enzymes sequence is critical for GATE Life Sciences biochemistry. Among given enzymes, correct order: α-ketoglutarate dehydrogenase (D) → Aconitase (A) → Fumarase (B) → Malate dehydrogenase (C).
Complete TCA Cycle 8 Steps with Enzymes
| Step | Enzyme | Substrate → Product | Cofactors |
|---|---|---|---|
| 1 | Citrate synthase | Acetyl CoA + OAA → Citrate | |
| 2 | Aconitase (A) | Citrate → Isocitrate | |
| 3 | Isocitrate dehydrogenase | Isocitrate → α-KG | NAD⁺, CO₂ |
| 4 | α-KG dehydrogenase (D) | α-KG → Succinyl CoA | NAD⁺, CoA, CO₂ |
| 5 | Succinyl CoA synthetase | Succinyl CoA → Succinate | GTP |
| 6 | Succinate dehydrogenase | Succinate → Fumarate | FAD |
| 7 | Fumarase (B) | Fumarate → Malate | H₂O |
| 8 | Malate dehydrogenase (C) | Malate → OAA | NAD⁺ |
GATE Strategy
Memory trick: “D-A-B-C” = Dehydrogenase (early), Aconitase (citrate processing), B (hydration), C (cycle closure). α-KG dehydrogenase produces first succinyl CoA; malate dehydrogenase regenerates oxaloacetate.
TCA cycle enzymes sequence mastery = guaranteed Krebs cycle scores in GATE Life Sciences PYQ.
