CoachPyruvate enters the mitochondrion and is oxidised by the pyruvate dehydrogenase complex to acetyl-CoA — a two-carbon acetyl group carried by coenzyme A (Chapter 11) on a high-energy thioester bond — releasing one and one NADH. Acetyl-CoA is the common entry of all fuels into the cycle: carbohydrate through pyruvate, fats through -oxidation, many amino acids directly.
Examples
Example 15.7 (Following the carbons)
Feed a cell glucose labelled with on carbon 1: the label appears in the methyl carbon of pyruvate, then of acetyl-CoA, enters citrate, and leaves as 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.