pyruvate acetyl-CoA

The removal of one CO2 for each pyruvate accounts for two of the six carbons originating in the glucose. The NADH2 carries much of the energy of oxidation and will be used to generate ATP in the electron transport system.

Acetyl-CoA is another important metabolic intermediate. It is a point of connection between a number of pathways, including the breakdown of lipids and amino acids for energy. But for purposes of the present discussion, a pair of acetyl-CoAs serve the purpose of carrying four carbons from the original glucose molecule into the Krebs cycle.

Krebs Cycle The Krebs cycle oxidizes the two carbons remaining from each pyruvate to CO2. One ATP is formed as a result, and the remaining four pairs of hydrogens (with their energetic electrons) reduce the cofactors NAD or FAD. The overall reaction is acetyl-CoA + 3NAD + FAD + ADP

Since two such reactions occur for each glucose, all of the original carbon atoms are accounted for. Only two additional ATPs are formed, for a total of four when combined with glycolysis. Thus, the efficiency so far, in terms of standard Gibbs free energy, is four times 7.3 kcal/mol for the ATPs, divided by 686 kcal/mol glucose, or 4.3%. However, there are a total of 10 pairs of energetic hydrogen electrons that have been captured by cofactors.

Some details of the cyclical reactions of the Krebs cycle may help clarify the picture. The cycle starts when the C4 oxaloacetate combines with two carbons of the acetyl-CoA that originated with pyruvate. The products of this first reaction regenerate the CoA and form the C6 tricarboxylic acid citric acid:

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