During one turn of the Krebs cycle, how many molecules of NADH are produced per acetyl-CoA that enters the cycle?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The Krebs cycle (citric acid cycle) operates within the mitochondrial matrix, where it systematically dismantles the two-carbon acetyl group from acetyl-CoA into carbon dioxide while capturing liberated electrons in reduced coenzymes. Three distinct oxidation-reduction reactions within a single turn generate NADH, each catalyzed by a specific dehydrogenase enzyme that facilitates electron transfer from a carbon-based substrate to the dinucleotide electron carrier NAD⁺.

Why Other Options Are Wrong

The molecular mechanism underlying each NAD⁺ reduction involves hydride ion (H⁻) transfer. NAD⁺ possesses a nicotinamide ring that accepts one hydrogen atom as a hydride ion (carrying two electrons) at the C-4 position, while a proton (H⁺) is released into the surrounding matrix solution. This energetically favorable reduction occurs only when coupled to an oxidation reaction yielding a sufficiently negative change in free energy (ΔG). The three enzymes orchestrating these transfers—-isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, and malate dehydrogenase—-each bind their respective organic acid substrates in orientations that lower the activation energy barrier for hydride extraction. For instance, isocitrate dehydrogenase employs a manganese or magnesium cofactor to stabilize the enol-intermediate formed when isocitrate undergoes oxidative decarboxylation, releasing CO₂ and transferring electrons to NAD⁺. Similarly, α-ketoglutarate dehydrogenase utilizes thiamine pyrophosphate and lipoamide cofactors in a multi-step mechanism analogous to pyruvate dehydrogenase, cleaving a carbon-carbon bond while channeling electrons to NAD⁺. Malate dehydrogenase, operating near thermodynamic equilibrium, oxidizes the secondary alcohol group of malate to the ketone of oxaloacetate, again reducing NAD⁺ to NADH.

PILLAR 2 — STEP-BY-STEP LOGIC

Tracing one complete revolution of the cycle from the moment acetyl-CoA (two carbons) condenses with oxaloacetate (four carbons) to form citrate (six carbons), we can enumerate every NADH-generating event:

First, citrate is isomerized to isocitrate via aconitase (no redox change). Isocitrate dehydrogenase then catalyzes the oxidative decarboxylation of isocitrate, producing one molecule of NADH and releasing one molecule of CO₂ while yielding α-ketoglutarate (five carbons). This constitutes the first NADH.

Second, the α-ketoglutarate dehydrogenase complex drives oxidative decarboxylation of α-ketoglutarate to succinyl-CoA (four carbons), generating a second molecule of NADH and a second molecule of CO₂. The cofactor architecture—thiamine pyrophosphate, lipoamide, CoA, FAD, and NAD⁺—ensures directional electron flow toward NAD⁺ reduction.

Third, succinyl-CoA is converted to succinate (substrate-level phosphorylation yielding GTP or ATP), then succinate is oxidized to fumarate by succinate dehydrogenase (which reduces FAD to FADH₂, not NAD⁺). Fumarate is hydrated to malate by fumarase. Finally, malate dehydrogenase oxidizes malate back to oxaloacetate, reducing a third NAD⁺ to NADH. This third NADH regenerates the oxaloacetate acceptor, closing the cycle.

Thus, exactly three molecules of NADH are synthesized per acetyl-CoA oxidized in one full turn of the Krebs cycle. The correct answer is C.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (1 NADH) traps students who confuse the Krebs cycle's NADH yield with the single FADH₂ produced at the succinate dehydrogenase step. These students conflate the flavin-based electron transfer (which uses FAD covalently tethered to Complex II of the electron transport chain) with the nicotinamide-based transfers, failing to distinguish between the three NAD⁺-dependent dehydrogenases and the one FAD-dependent enzyme.

Option B (2 NADH) reflects a partial-counting error. Students selecting this option likely recall only the two oxidative decarboxylation events (isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl-CoA) while forgetting the final oxidation of malate to oxaloacetate. This reveals an incomplete mental map of the cycle, where the regeneration phase (succinate → fumarate → malate → oxaloacetate) is underemphasized.

Option D (4 NADH) ensnares students who sum all reduced coenzymes (three NADH + one FADH₂ = four total electron carriers) but fail to distinguish NADH from FADH₂. This error reflects a broader misunderstanding of electron carrier diversity; although both NADH and FADH₂ deliver electrons to the electron transport chain, they enter at different complexes (I vs. II) and generate different ATP yields via oxidative phosphorylation (approximately 2.5 ATP per NADH vs. 1.5 ATP per FADH₂). Conflating these carriers obscures critical differences in proton-pumping efficiency and chemiosmotic potential.