Why is the Citric Acid Cycle Called a Cycle

The citric acid cycle begins with the acceptance of acetyl-CoA by oxaloacetate, and at the end of the cycle, oxaloacetate is regenerated. Therefore, the citric acid cycle is considered as a cycle.

The citric acid cycle is part of the chemical reactions involved in the aerobic respiration of organisms. Aerobic respiration is responsible for the breakdown of food, with the use of oxygen for the production of energy in the form of ATP. Further, Acetyl-CoA is the end product of the oxidative decarboxylation of pyruvate produced during the glycolysis.

Key Areas Covered

1. What is Citric Acid Cycle
– Definition, Mechanism
2. Why is the Citric Acid Cycle Called a Cycle
– Regeneration of Oxaloacetate

Key Terms: Acetyl-CoA, Aerobic Respiration, Citric Acid Cycle, Glycolysis, Oxaloacetate

What is Citric Acid Cycle

The citric acid cycle is the second step of the aerobic respiration of living organisms. It is also known as Krebs cycle and tricarboxylic acid cycle (TCA cycle). Pyruvate is the end product of the glycolysis, which is the first step of the aerobic respiration. The pyruvate undergoes oxidative decarboxylation where it converts into acetyl-CoA. The Acetyl-CoA completely breakes down into carbon dioxide and water during the citric acid cycle.

During the citric acid cycle, the acetyl part of the acetyl-CoA combines with an oxaloacetate molecule to form a citrate molecule, which is a six-carbon molecule. Then citrate is oxidized in a series of steps, releasing two carbon dioxide molecules from it. First, the citric acid is converted into isocitrate and oxidized to α-ketoglutarate by reducing an NAD⁺ molecule. The α-ketoglutarate is again oxidized to succinyl-CoA. The succinyl-CoA takes a hydroxyl group from water and forms succinate. The succinate is oxidized to fumarate by FAD. The addition of water molecule to the fumarate produces malate. The malate is then oxidized back to oxaloacetate by NAD⁺. The overall reactions of the citric acid cycle produce six NADH, two FADH₂, and two ATP/GTP molecules per one glucose molecule.

Figure 1: Citric Acid Cycle

Why is the Citric Acid Cycle Called a Cycle

The first reaction of the citric acid cycle is the combining of acetyl-CoA with the oxaloacetate. Acetyl-CoA comes from the oxidative decarboxylation of pyruvate, which is produced by the glycolysis. At the end of the series of reactions of the citric acid cycle, acetyl-CoA is completely broken down into carbon dioxide and water. Oxaloacetate is regenerated. It can then bind to another acetyl-CoA molecule. Since the starting compound is regenerated at the end of the reaction series, the citric acid cycle is considered as a cycle.

Conclusion

The starting compound of the series of chemical reactions in the citric acid cycle is oxaloacetate. It is regenerated at the end of the reaction series. Therefore, the citric acid cycle is considered as a cycle. Oxaloacetate binds to the acetyl-CoA produced at the end of the citric acid cycle. Acetyl-CoA is completely broken down into carbon dioxide and water at the end of the citric acid cycle.