Citric Acid Cycle - Creating Energy

The Reactions of the Citric Acid Cycle

The Citric Acid Cycle begins with the reaction between acetyl-CoA and the four-carbon oxaloacetate to form six-carbon citric acid. In the following steps, two of the six carbons of the citric acid leave as carbon dioxide to yield the four carbon product, oxaloacetate, which is used again in the first step of the next cycle. During the eight reactions that take place, every molecule of acetyl-CoA, the cycle produces three NADH and one flavin adenine dinucleotide (FAD/FADH2), and one molecule of ATP.

Reaction 1: Citrate Synthase

The first reaction 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 is catalyzed by the enzyme acontinase. A water molecule is removed from the citric acid and then put back on in another location. The overall effect of this conversion is 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. First we see the 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 has a carbon dioxide molecule removed to yield alpha-ketoglutarate.

Reaction 4: Alpha-ketoglutarate deydrogenase

Alpha-ketoglutarate loses a carbon dioxide molecule, then 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 is similar to the occurance in the first 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. A molecule of guanosine triphosphate (GTP) is synthesized. GTP, a molecule very similar in structure and energy properties to ATP and can be used in 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. A molecule of FAD, a coenzyme similar to NAD, is reduced to FADH2 as it takes the hydrogens from succinate producing 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

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, we regenerate oxaloacetate by oxidizing L–malate with a molecule of NAD to produce NADH.

Summary

The reactions of the citric acid cycle generates from one acetyl-CoA molecule.

The acetyl-CoA, is oxidized to two molecules of carbon dioxide.

Three molecules of NAD are reduced to NADH.

One molecule of FAD is reduced to FADH2.

One molecule of GTP (the equivalent of ATP) is produced.

Note: A reduction is actually a gain of electrons. NADH and FADH2 molecules act as electron carriers and used to generate ATP in the next stage of glucose metabolism, oxidative phosphorylation. Info originally found on SparkNotes.