Question

Why is the citrate cycle considered an amphibolic pathway? Give an example of an anaplerotic reaction that provides substrates to the citrate cycle.

Short answer

Answer: An amphibolic pathway is a metabolic pathway that can function both in anabolic (building) and catabolic (breaking down) processes. The citrate cycle is an example of an amphibolic pathway as it plays a crucial role in both the breakdown of carbohydrates, fats, and proteins (catabolism) and in the synthesis of biomolecules (anabolism). An example of an anaplerotic reaction that supplies substrates to the citrate cycle is the carboxylation of pyruvate to form oxaloacetate, which replenishes the intermediates of the citrate cycle and allows it to continue functioning.

Step-by-step solution

Understand the terms amphibolic pathway and anaplerotic reaction

An "amphibolic pathway" refers to a metabolic pathway that can function both in anabolic (building) and catabolic (breaking down) processes. An "anaplerotic reaction" is a reaction that replenishes the intermediates of a metabolic cycle when they are depleted.

Describe the citrate cycle

The citrate cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a central metabolic pathway in the cell's mitochondria. It plays a crucial role in both the breakdown of carbohydrates, fats, and proteins (catabolism) and in the synthesis of biomolecules (anabolism). The citrate cycle breaks down molecules of acetyl-CoA (derived from carbohydrates, fats, and proteins) into carbon dioxide and generates ATP, NADH, and FADH2, which are used for energy production.

Explain why the citrate cycle is considered an amphibolic pathway

The citrate cycle serves as an amphibolic pathway because it has both catabolic and anabolic functions. Its catabolic function is to break down acetyl-CoA into carbon dioxide and generate ATP, NADH, and FADH2 for energy production. Meanwhile, its anabolic function is to provide intermediates for the synthesis of biomolecules such as amino acids, nucleotides, and lipids.

Provide an example of an anaplerotic reaction

An example of an anaplerotic reaction that provides substrates to the citrate cycle is the carboxylation of pyruvate to form oxaloacetate. This reaction is catalyzed by the enzyme pyruvate carboxylase and requires the coenzyme biotin. The newly formed oxaloacetate can then enter the citrate cycle, replenishing its intermediates and allowing the cycle to continue functioning.