Figure %: Reaction 5.

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.

Figure %: Reaction 6.

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.

Figure %: Reaction 7.

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.

Figure %: Reaction 8.


We have now concluded our discussion of the reactions that compose the citric acid cycle. It is helpful at this point to take a minute to take stock of what the citric acid cycle has generated from one acetyl-CoA molecule.

  • 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.
Keep in mind that a reduction is really a gain of electrons. In other words, NADH and FADH2 molecules act as electron carriers and are used to generate ATP in the next stage of glucose metabolism, oxidative phosphorylation. In the next SparkNote on Oxidative Phosphorylation and the electron transport chain, we will learn what processes take place to ultimately derive the the majority of the ATP we need to fuel our daily activity.