Amphibolic Pathway: Overview, Diagrams, Examples

What Is an 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.

Why Krebs Cycle Is the Best Example of an Amphibolic Pathway

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

Aerobic respiration provides for the catabolism, i,e., 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.

Catabolic Functions of Krebs Cycle

The catabolic functions of the Krebs cycle include:

Breakdown of Acetyl-CoA

In the citric acid cycle, the chief role seems to be the demolition of acetyl-CoA, derived from carbohydrates, fats, and proteins.

Energy Extraction

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 of Krebs Cycle

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.

Biosynthetic Intermediates Provided by TCA Cycle

• α-Ketoglutarate → amino acids (glutamate family)

• Oxaloacetate → aspartate family of amino acids

• Citrate → fatty acid synthesis, cholesterol

• Succinyl-CoA → heme synthesis

• Malate → gluconeogenesis (glucose formation)

Anaplerotic Reactions (Refilling the Cycle)

• As intermediates are removed to serve as biosynthetic precursors, they are replenished by anaplerotic reactions.

• Under normal conditions, removal and replenishment are in dynamic balance so intermediates are almost constant

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 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.

Why Amphibolic Nature Is Important

Amphibolic nature is important because:

Balances Catabolism & Anabolism

• It allows cells to break down molecules and generate energy when the demand increases.

• When a cell needs to build biomolecules, the amphibolic pathway provides key metabolic intermediates.

Supports Survival Under Stress

• During nutrient scarcity, pathway shifts to catabolic reactions and molecules are broken down to release energy.

• When nutrients are abundant, the pathway supplies building blocks for rapid growth and development.

Maintains Homeostasis

• Amphibolic pathways switch based on what the cell needs. They ensure balance between producing energy and storing nutrients.

Amphibolic Pathway NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Catabolic functions of Krebs Cycle

• Amphibolic Pathways and Energy Production

Practice Questions for NEET

Q1. The most favored carbohydrate which is used as substrate during respiration is

1. Sucrose

2. Cellulose

3. Glucose

4. Fructose

Correct answer: 3) Glucose

Explanation:

Glucose is the preferred substrate for respiration as it serves as a primary energy source for cells. Most carbohydrates are first converted into glucose or glucose derivatives before entering the respiratory pathway. This conversion allows the efficient breakdown of glucose through glycolysis, the Krebs cycle, and oxidative phosphorylation, leading to the production of ATP, the energy currency of the cell.

Hence, the correct answer is option 3) Glucose.

Q2. Fats are broken down into glycerol and fatty acids. Glycerol then enters the respiratory pathway after being converted to

1. Phosphoglycerate

2. Oxaloacetate

3. Malate

4. Phosphoglyceraldehyde

Correct answer: 4) Phosphoglyceraldehyde

Explanation:

The breakdown of fats into glycerol and fatty acids is known as lipolysis. Glycerol and free fatty acids are released into the bloodstream when stored triglycerides are hydrolyzed by hormone-sensitive lipase in adipose tissue. Following conversion to dihydroxyacetone phosphate, the glycerol can proceed through the glycolytic pathway to phosphoglyceraldehyde, while the fatty acids undergo β-oxidation to produce acetyl-CoA.

Hence, the correct answer is option 4) Phosphoglyceraldehyde.

Q3. Which of the following is used for the synthesis of amino acid glutamate.

1. α−ketoglutarate

2. Oxaloacetate

3. Succinyl CoA

4. Citrate

Correct answer: 1) α−ketoglutarate

Explanation:

The intermediates of the Krebs cycle were responsible for the synthesis of glucose, fatty acids, amino acids, and nucleic acids.

Anabolic activities include:

1. Citrate enters the cytosol from the mitochondria and is oxidized to generate acetyl CoA. This stimulates fatty acid biosynthesis.

2. Ketoglutarate is a raw material used in the manufacture of the amino acid glutamate.

3. Oxaloacetate is used in the manufacture of the amino acid aspartate, pyrimidines, and alkaloids.

4. Succinyl CoA synthesizes pyrrole molecules such as cytochromes and chlorophylls.

Hence, the correct answer is option 1) α−ketoglutarate.

Also Read:

• MCQs on Amphibolic Pathway

• Alcoholic Fermentation

• Aerobic and Anaerobic Respiration

Recommended video on "Amphibolic Pathway"