Amphibolic Pathway: Overview, Diagrams, Examples

What Is Amphibolic Pathway?

An amphibolic pathway is part of the metabolic pathways with both catabolic and anabolic roles: it serves in the degradation of molecules to release energy and in the synthesis of new molecules. Cellular metabolism with these dual capabilities confers on such pathways, like the Krebs cycle, a dual function in degrading nutrients for energy production and contributing to the synthesis of important biomolecules.

For instance, during the degradation of glucose, fatty acids, and amino acids, the Krebs cycle generates not only ATP and electron carriers but also furnishes many of the important intermediates used in the biosynthetic pathways for making amino acids, nucleotides, and other essential compounds. This kind of versatility is extremely useful for maintaining metabolic homeostasis, adapting to changes in energy demands, and supporting the dynamic nature of cellular processes.

The Amphibolic Nature Of The Citric Acid Cycle (Krebs Cycle)

Another major pathway of intermediary metabolism, the citric acid cycle, is localised within the mitochondrial matrix

Overview Of The Citric Acid Cycle

Aerobic respiration provides for the oxidation of acetyl-CoA to carbon dioxide and water with the production of energy-rich reduced compounds.

Condensation of acetyl-CoA with oxaloacetate to form citrate initiates the cycle, which then continues through a series of enzyme-catalyzed transformations ending with the regeneration of oxaloacetate for recycling through another turn of the cycle.

Dual Role In Metabolism

The functions include:

Catabolic Functions

In the citric acid cycle, the chief role seems to be the demolition of acetyl-CoA, derived from carbohydrates, fats, and proteins. In this degradation, large amounts of energy are released in high-energy electron carriers, NADH and FADH2, and GTP—or, in some steps, ATP—and carbon dioxide is formed as the waste product.

Anabolic Functions

Apart from the catabolic role, the Krebs cycle also displays anabolic functions through some of the cycle's intermediates being used as precursors for different biosynthetic pathways. To give examples, alpha-ketoglutarate and oxaloacetate are precursors in the synthesis of amino acids, and citrate participates in the synthesis of fatty acids and sterols.

Amphibolic Pathway And Energy Production

This includes the following:

ATP Generation

The citric acid cycle indirectly contributes to the generation of ATP due to the production of high-energy electron carriers, NADH and FADH2, which donate electrons to the electron transport chain. At each turn of the cycle, there is produced one GTP, easily convertible into ATP, and two molecules of carbon dioxide.

NADH And FADH2 Production

Now, during one turn of this cycle, there are three NADH molecules and one FADH2 generated. This will be very important later on in cellular respiration because NADH and FADH2 play their roles in carrying electrons to the electron transport chain, which in turn uses them to drive ATP synthesis.

Electron Transport Chain

The electrons NADH and FADH2 are used by the electron transport chain to set up a proton gradient across the inner mitochondrial membrane. These gradients are then coupled through the action of the enzyme ATP synthase in generating ATP through the process of oxidative phosphorylation. The ETC is the final step in aerobic respiration, where most of the ATPs are produced.

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