What is The Krebs cycle? — Biology

Definition 15.6 University Biology — Year 1 · Chapter 15 — Cellular Respiration and Fermentation

The Krebs cycle (citric acid cycle) in the mitochondrial matrix condenses acetyl-CoA (C2) with oxaloacetate (C4) into citrate (C6) and, in eight steps, oxidises the two carbons to $\mathrm{CO_2}$ while regenerating oxaloacetate. Per acetyl group: 3 NADH, 1 $\mathrm{FADH_2}$ , 1 GTP (by substrate-level phosphorylation) and 2 $\mathrm{CO_2}$ . Per glucose (two acetyls): 6 NADH, 2 $\mathrm{FADH_2}$ , 2 GTP, 4 $\mathrm{CO_2}$ — which with the 2 $\mathrm{CO_2}$ of pyruvate dehydrogenase makes the six of the overall equation. No oxygen is consumed in the cycle itself; it runs only as long as the respiratory chain re-oxidises its NADH. It is also amphibolic: its intermediates are drawn off for biosynthesis (Chapter 16) and replenished from amino acids and pyruvate.

The Krebs cycle. An acetyl group enters by condensing with oxaloacetate; two carbons leave as CO_2 at the two decarboxylations; the four oxidations load three NADH and one FADH_2; oxaloacetate is regenerated for the next turn. — The Krebs cycle. An acetyl group enters by condensing with oxaloacetate; two carbons leave as $\mathrm{CO_2}$ at the two decarboxylations; the four oxidations load three NADH and one $\mathrm{FADH_2}$ ; oxaloacetate is regenerated for the next turn.

Hans Krebs (1900–1981), who worked out the cycle in 1937 from the rates at which minced pigeon muscle oxidised its acids. Photograph: Nobel Foundation, public domain.

Examples

Example 15.7 (Following the carbons)

Feed a cell glucose labelled with $\mathrm{^{14}C}$ on carbon 1: the label appears in the methyl carbon of pyruvate, then of acetyl-CoA, enters citrate, and leaves as $\mathrm{CO_2}$ only on the second or third turn of the cycle — the two carbons released in a given turn belong to the oxaloacetate of the previous one. Krebs (1937) deduced the cycle from the observation that catalytic amounts of any of its acids stimulated the oxidation of far more pyruvate than they could account for themselves: they were being regenerated.

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