Metabolism is the process through which the degradation of substances takes place to generate energy. Metabolism follows two pathways- catabolism and anabolism. Anabolism means where large molecules are formed with the help of small molecules and energy. Catabolism means where larger molecules are degraded to produce energy and smaller molecules. The dual metabolic pathway which involves both anabolism and catabolism is the amphibolic pathway.

The Krebs cycle is a typical example of an amphibolic pathway. Kreb’s cycle works both in the degradation and synthesis processes. In this article, we will provide detailed information on the amphibolic pathway. Continue reading to learn more!

Define Amphibolic Pathway

The biochemical pathway in which both catabolism and anabolism occur simultaneously is known as the amphibolic pathway. B. Davis first coined the term in 1961.

What is Amphibolic Pathway?

Organic substances like carbohydrates, fats, proteins, etc., are broken down to release energy by cellular respiration. The process is known as the catabolic pathway or catabolism. The breakdown of respiratory substrates provides a carbon skeleton to synthesise a large number of other essential plant products, such as polysaccharides, proteins, fats, nucleic acids, pigments, cytochromes, etc.

Thus, the same respiratory process acts as a catabolic pathway for respiratory substrates and acts as an anabolic pathway to synthesise various intermediary metabolic products and secondary metabolites. The respiratory pathway, therefore, acts as both catabolic as well as an anabolic pathway and can be called an amphibolic pathway.

Amphibolic Pathway Diagram

Below we have provided the diagram for the amphibolic pathway:

Fig: Amphibolic Pathway

Examples of Amphibolic Pathway

The pathways associated with respiration are Glycolysis, the Krebs cycle and the electron transport chain; all are examples of the amphibolic pathway.

Why is Respiration Called an Amphibolic Pathway?

1. The citric acid cycle or Krebs cycle is the central metabolic pathway in which complete oxidative degradation of carbohydrates, fats and amino acids occur. Generally, glucose is the primary substrate for respiration for most organisms.
2. All carbohydrates are first broken down into glucose which enters the glycolysis cycle and gets converted into pyruvate.
3. Pyruvates are converted into acetyl CoA, and this acetyl CoA will enter into the Krebs cycle. In the Krebs cycle, \(36\) ATPs are formed from \(1\) molecule of glucose.
4. Different substrates like proteins, fats can also be respired, but they do not enter the respiratory pathway at the very first step.
5. Fats are mainly used as respiratory substrates after the exhaustion of carbohydrate reserves. Fats are first broken down into glycerol and fatty acids first.
6. If fatty acids are used as respiratory substrates, they would first be degraded to acetyl CoA, which enters the Krebs cycle.
7. Glycerol can be converted into \(3-\)Phosphoglyceraldehyde (PGAL) before entering the pathway.
8. Proteins are used as respiratory substrates only when carbohydrates and fat reserves are exhausted.
9. The proteins are degraded by proteases, and the individual amino acids (after deamination), depending on their structure, would enter the pathway at some stage within the Krebs’ cycle or even as pyruvate or acetyl CoA.
10. The catabolism and anabolism of the intermediates present in the Krebs cycle depend upon the internal cues of the body, i.e. requirement of synthesis or breakdown of a compound and is dependent on many other factors for their synthesis. Thus it is clear that respiration involves the breakdown of organic compounds. So the respiratory pathway is called a catabolic pathway.
11. At the same time, certain intermediates of the respiratory pathway are also withdrawn to synthesise other compounds like fatty acids that would be broken down into acetyl CoA. Still, when organisms need to synthesise the fatty acids, the same acetyl CoA will be withdrawn from the respiratory pathway.
12. Hence, the respiratory pathway is involved during both degradation and synthesis.
13. The degradation process in the living organism is catabolism, and synthesis is anabolism. Thus the respiratory pathway is better considered as an amphibolic pathway rather than as a catabolic pathway.

Why is Krebs Cycle Called an Amphibolic Pathway?

The Krebs cycle is a typical example of an amphibolic pathway because it works both in the degradation and synthesis processes. The citric acid cycle or Krebs cycle plays a vital role in cellular respiration. The Krebs cycle takes place on the cytosol of bacteria and within the mitochondria of eukaryotic cells.

Catabolic Nature of Krebs Cycle

1. In cellular respiration, the Krebs cycle is the common oxidative pathway. The breakdown of acetyl CoA yields NADH, FADH and ATP in plants and GTP in animals.

2. The biological compounds like proteins, fats and carbohydrates oxidise to form acetyl CoA directly or through pyruvate, which enters the common pathway of oxidation.

3. Through cellular respiration, we know that the substrates of the Krebs cycle are not entirely derived from glycolysis, but the amino acids and lipids also contribute to the substrate.

4. The catabolic activities of the citric acid cycle are: Oxidation of glucose, transamination of amino acids and oxidation of fatty acids.

5. Glucose is broken down to pyruvate by glycolysis. Fatty acids on Beta – Oxidation directly provides acetyl CoA.

Amino Acid Catabolism

1. In the body, both essential and non–essential amino acids are present for the proper functioning of the living system.
2. Acetyl CoA does not form directly among each of the amino acids.
3. The process of transamination or aminotransferase forms a few forms of other intermediates of the Krebs cycle.
4. The aminotransferase reaction involves the transferring of the alpha-amino group from the carbon skeleton. This transforms the carbon skeleton into an intermediate of an amphibolic pathway.
5. Few of the amino acids also oxidise to become pyruvate and acetyl CoA.
6. For example:
a) Glutamine is transformed to glutamate with the enzyme Glutaminase. Glutamate then, by the transamination process, becomes alpha-ketoglutarate.
b) Asparagine is transferred to aspartate on transaminations which yields oxaloacetate and certain amino acids like alanine, serine etc. These are directly oxidised to produce pyruvate.

Anabolic Nature of Krebs Cycle

The Krebs cycle intermediates produced the biosynthesis of glucose, fatty acids, amino acids and nucleic acids.
Anabolic activities are;

1. Citrate from mitochondria enters the cytosol to oxidise to form acetyl CoA. This initiates the biosynthesis of fatty acids.
2. \(α-\)ketoglutarate is the raw material for the synthesis of amino acid glutamate.
3. Oxaloacetate is the raw material for the synthesis of amino acid aspartate, pyrimidines and alkaloids.
4. Succinyl CoA forms pyrrole compounds like cytochromes and chlorophylls.

Summary

The biochemical pathway in which both catabolism and anabolism occur simultaneously is known as the amphibolic pathway. Respiration is the result of both degradation and synthesis, so it is called an amphibolic pathway. The Krebs cycle or citric acid cycle is a typical example of an amphibolic pathway. In the TCA cycle, all the food compounds like carbohydrates, protein and fat release energy by oxidation and also synthesis takes place from the same molecules, which are part of the respiratory pathways. Hence, TCA is called a common metabolic pathway.

FAQs on Amphibolic Pathway

Q.1. What is an Amphibolic Pathway?
Ans: The biochemical pathway in which both catabolism and anabolism occur simultaneously is known as the amphibolic pathway.

Q.2. What is an example of an Amphibolic Pathway?
Ans: The Krebs cycle or the citric acid cycle is an example of an amphibolic pathway.

Q.3. Why is respiration called an Amphibolic Pathway?
Ans: Respiration is the result of both degradation and synthesis, so it is called an amphibolic pathway.

Q.4. Is Glycolysis an Amphibolic Pathway?
Ans: Yes, Glycolysis is an amphibolic pathway because they provide ATP via a catabolic pathway and chemical intermediates to build new cell material through an anabolic pathway.

Q.5. Why is the TCA cycle called the Common Metabolic Pathway?
Ans: In the TCA cycle, all the food compounds like carbohydrates, protein and fat release energy by oxidation and also synthesis takes place from the same molecules which are part of the respiratory pathways. Hence, TCA is called a common metabolic pathway.