Q.70 The correct sequence of metabolic intermediates in Krebs cycle is
(A) α–ketoglutarate → fumarate → succinate → malate
(B) fumarate → malate → succinate → α–ketoglutarate
(C) α–ketoglutarate → succinate → fumarate → malate
(D) succinate → α–ketoglutarate → malate → fumarate
The correct answer is (C) α-ketoglutarate → succinate → fumarate → malate.
This sequence matches the standard Krebs cycle (TCA cycle) order where α-ketoglutarate undergoes oxidative decarboxylation to succinyl-CoA (then succinate), followed by oxidation to fumarate and hydration to malate.
Option Analysis
Option (A) α-ketoglutarate → fumarate → succinate → malate
Incorrect because fumarate forms after succinate via succinate dehydrogenase oxidation; reversing these skips succinyl-CoA and violates cycle direction.
Option (B) fumarate → malate → succinate → α-ketoglutarate
Incorrect as it runs backward—fumarate hydrates to malate, but succinate precedes fumarate, and α-ketoglutarate precedes succinate, not follows.
Option (C) α-ketoglutarate → succinate → fumarate → malate
Correct sequence: α-ketoglutarate to succinyl-CoA/succinate (α-ketoglutarate dehydrogenase), succinate to fumarate (succinate dehydrogenase, FADH₂), fumarate to malate (fumarase).
Option (D) succinate → α-ketoglutarate → malate → fumarate
Incorrect; succinate follows α-ketoglutarate, and malate precedes fumarate in reverse hydration.
Krebs Cycle Overview
Krebs cycle intermediates follow: citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate.
This aerobic pathway in mitochondria generates NADH, FADH₂, GTP, and CO₂ for ATP production.
The Krebs cycle sequence of metabolic intermediates forms the core of aerobic respiration, crucial for CSIR NET Life Sciences aspirants tackling biochemistry MCQs. This article decodes Q.70 options with precise enzyme reactions.
Krebs Cycle Pathway Steps
Acetyl-CoA condenses with oxaloacetate to citrate, progressing through isocitrate to α-ketoglutarate (isocitrate dehydrogenase, NADH + CO₂).
α-Ketoglutarate converts to succinyl-CoA (α-ketoglutarate dehydrogenase, NADH + CO₂), then succinate (succinyl-CoA synthetase, GTP). Succinate oxidizes to fumarate (succinate dehydrogenase, FADH₂), fumarate hydrates to malate (fumarase), and malate to oxaloacetate (malate dehydrogenase, NADH).
Why Option C is Correct
Option C traces the forward path post-decarboxylation: α-ketoglutarate (5C) loses CO₂ to 4C succinate, then dehydrogenation yields fumarate, hydration gives malate—aligning with energy yield (3 NADH, 1 FADH₂, 1 GTP per acetyl-CoA).
Reverse orders in A, B, D defy thermodynamics and enzyme specificity.
CSIR NET Exam Tips
Focus on intermediate carbon counts (6C citrate to 4C cycle) and cofactors for quick elimination. Practice mnemonics: “Citrate Is Krebs’ Starting Substrate For Making Oxaloacetate” covers full order.
