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Here is a complete breakdown of the Electron Transport Chain (ETC) in relation to the Krebs Cycle, drawn from Harper's Illustrated Biochemistry and Guyton & Hall.
The Krebs Cycle and the ETC - How They Connect
The Krebs cycle (citric acid cycle / TCA cycle) and the ETC are the two central stages of aerobic respiration. The Krebs cycle's primary job is not to make ATP directly - it is to generate reducing equivalents (NADH and FADH2) that feed into the ETC to drive massive ATP production.
Step-by-Step: The Krebs Cycle
The cycle takes place in the mitochondrial matrix. It begins when acetyl-CoA (a 2-carbon unit) condenses with oxaloacetate (4C) to form citrate (6C).
The 8 reactions per turn:
| Step | Substrate → Product | Enzyme | Products |
|---|---|---|---|
| 1 | Oxaloacetate + Acetyl-CoA → Citrate | Citrate synthase | CoA released |
| 2 | Citrate → Isocitrate | Aconitase | via cis-aconitate (2 steps) |
| 3 | Isocitrate → α-Ketoglutarate | Isocitrate dehydrogenase | NADH, CO₂ |
| 4 | α-Ketoglutarate → Succinyl-CoA | α-Ketoglutarate dehydrogenase | NADH, CO₂ |
| 5 | Succinyl-CoA → Succinate | Succinyl-CoA synthetase | GTP (= ATP) - substrate-level phosphorylation |
| 6 | Succinate → Fumarate | Succinate dehydrogenase | FADH₂ |
| 7 | Fumarate → Malate | Fumarase | H₂O added |
| 8 | Malate → Oxaloacetate | Malate dehydrogenase | NADH |
Per turn of the cycle:
- 3 NADH
- 1 FADH₂
- 1 GTP/ATP (substrate-level)
- 2 CO₂ released
The ETC - Where the Real ATP Is Made
The NADH and FADH₂ produced by the Krebs cycle are passed to the respiratory chain (ETC) on the inner mitochondrial membrane.
The ETC consists of 5 protein complexes:
| Complex | Name | Accepts | Passes electrons to |
|---|---|---|---|
| I | NADH-Q oxidoreductase | NADH | Coenzyme Q (ubiquinone) |
| II | Succinate-Q dehydrogenase | FADH₂ (from succinate) | Coenzyme Q |
| III | Q-cytochrome c oxidoreductase | Coenzyme Q | Cytochrome c |
| IV | Cytochrome c oxidase | Cytochrome c | O₂ → H₂O |
| V | ATP synthase | Proton gradient (H⁺) | ADP + Pi → ATP |
FADH₂ enters at Complex II - it bypasses Complex I, so it yields fewer ATP than NADH.
ATP Yield Per Turn of the Krebs Cycle
| Source | ATP yield |
|---|---|
| 3 NADH × ~2.5 ATP | ~7.5 ATP |
| 1 FADH₂ × ~1.5 ATP | ~1.5 ATP |
| 1 GTP (substrate-level) | 1 ATP |
| Total per turn | ~10 ATP |
Since 2 acetyl-CoA enter per glucose molecule, the Krebs cycle + ETC yields ~20 ATP from one glucose (not counting glycolysis or pyruvate dehydrogenase).
Key Points
- The ETC requires oxygen as the final electron acceptor (Complex IV). Without O₂, NADH/FADH₂ cannot be reoxidized, the Krebs cycle stalls, and only glycolysis continues (anaerobic).
- 4 B-vitamins are essential cofactors in the Krebs cycle: riboflavin (FAD), niacin (NAD⁺), thiamine (B1, for α-ketoglutarate dehydrogenase), and pantothenic acid (as CoA).
- The Krebs cycle is amphibolic - it functions in both catabolism AND anabolism (supplying intermediates for gluconeogenesis, amino acid synthesis, and fatty acid synthesis).
- The mechanism linking the ETC to ATP synthesis is the chemiosmotic mechanism - protons are pumped out across the inner mitochondrial membrane by Complexes I, III, and IV, creating a gradient that drives ATP synthase (Complex V).
Sources: Harper's Illustrated Biochemistry, 32nd Ed., Ch. 16; Guyton & Hall Textbook of Medical Physiology, Ch. 68
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