Krebs cycle reactions

For this idea i want to help the students understand what is going on in the Krebs cycle. I found this explanation below and thought it was very helpful. I would like to use these explanations in the piece as it could be really good for different learners to have the cycle broken down into separate things e.g. Image/ Name/ Explanation. However i do feel that these may be a bit too long for an info graphic. But i will test it out and see.

Reaction 1: Citrate Synthase

The first reaction of the citric acid cycle is catalyzed by the enzyme citrate synthase. In this step, oxaloacetate is joined with acetyl-CoA to form citric acid. Once the two molecules are joined, a water molecule attacks the acetyl leading to the release of coenzyme A from the complex.

Reaction 2: Acontinase

The next reaction of the citric acid cycle is catalyzed by the enzyme acontinase. In this reaction, a water molecule is removed from the citric acid and then put back on in another location. The overall effect of this conversion is that the –OH group is moved from the 3′ to the 4′ position on the molecule. This transformation yields the molecule isocitrate.

Reaction 3: Isocitrate Dehydrogenase

Two events occur in reaction 3 of the citric acid cycle. In the first reaction, we see our first generation of NADH from NAD. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the –OH group at the 4′ position of isocitrate to yield an intermediate which then has a carbon dioxide molecule removed from it to yield alpha-ketoglutarate.

Reaction 4: Alpha-ketoglutarate deydrogenase

In reaction 4 of the citric acid cycle, alpha-ketoglutarate loses a carbon dioxide molecule and coenzyme A is added in its place. The decarboxylation occurs with the help of NAD, which is converted to NADH. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase. The mechanism of this conversion is very similar to what occurs in the first few steps of pyruvate metabolism. The resulting molecule is called succinyl-CoA.

Reaction 5: Succinyl-CoA Synthetase

The enzyme succinyl-CoA synthetase catalyzes the fifth reaction of the citric acid cycle. In this step a molecule of guanosine triphosphate (GTP) is synthesized. GTP is a molecule that is very similar in its structure and energetic properties to ATP and can be used in cells in much the same way. GTP synthesis occurs with the addition of a free phosphate group to a GDP molecule (similar to ATP synthesis from ADP). In this reaction, a free phosphate group first attacks the succinyl-CoA molecule releasing the CoA. After the phosphate is attached to the molecule, it is transferred to the GDP to form GTP. The resulting product is the molecule succinate.

Reaction 6: Succinate Dehydrogenase

The enzyme succinate dehydrogenase catalyzes the removal of two hydrogens from succinate in the sixth reaction of the citric acid cycle. In the reaction, a molecule of FAD, a coenzyme similar to NAD, is reduced to FADH2 as it takes the hydrogens from succinate. The product of this reaction is fumarate. FAD, like NAD, is the oxidized form while FADH2 is the reduced form. Although FAD and NAD perform the same oxidative and reductive roles in reactions, FAD and NAD work on different classes of molecules. FAD oxidizes carbon-carbon double and triple bonds while NAD oxidizes mostly carbon-oxygen bonds.

Reaction 7: Fumarase

In this reaction, the enzyme fumarase catalyzes the addition of a water molecule to the fumarate in the form of an –OH group to yield the molecule L- malate.

Reaction 8: Malate Dehydrogenase

In the final reaction of the citric acid cycle, we regenerate oxaloacetate by oxidizing L–malate with a molecule of NAD to produce NADH.


From one acetyl-CoA molecule the citric acid cycle has generated:

  • The acetyl-CoA, has been oxidized to two molecules of carbon dioxide.
  • Three molecules of NAD were reduced to NADH.
  • One molecule of FAD was reduced to FADH2.
  • One molecule of GTP (the equivalent of ATP) was produced.

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