Amphibolic Pathway: Definition & Kreb’s Cycle

Amphibolic Pathway is a biochemical route that involves both anabolic and catabolic activities. B. Davis invented the term amphibolic route in 1961. An amphibolic route is a metabolic process that incorporates both catabolism and anabolism. Krebs' cycle is the best explanation for the amphibolic route. The biological process of breaking down glucose to produce energy is known as cellular respiration. Among the carbohydrates, glucose is the most prevalent source of energy. Fats and proteins are two more molecules that are utilised to produce energy. However, because these substrates cannot create energy directly, they must first be transformed into simpler forms.

Key Terms: Respiration, Glycolysis, ATP, Prokaryotes, Eukaryotic Cells, Kreb’s Cycle, Catabolism, Anabolism, Biological Processes

What is Amphibolic Pathway?

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Respiration is the process of breaking down complex chemicals into simpler compounds in order to create the energy molecule- ATP. As a result, the process is referred to as a catabolic process and the pathway as a catabolic pathway. Respiration, in fact, is the product of both creating and breaking. When energy is needed, proteins or fatty acids are broken down to produce acetyl-CoA, which is then used in subsequent respiration activities. This is catabolism in action. When the body wants fatty acids or proteins, the respiratory route is interrupted, and the same acetyl-CoA is used to produce fatty acids. 

Anabolism is the word for this synthesis process. As a result, we may define respiration as the sum of catabolism and anabolism. The Krebs cycle and glycolysis products serve as precursors to the production of lipids, proteins, and other compounds. As an outcome, the respiratory system is referred to as an amphibolic system.

Amphibolic Pathway

Kreb’s Cycle

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The citric acid cycle, also known as the tricarboxylic acid or Kreb's cycle, is the primary motor of cellular respiration. This cycle is the primary source of energy for cells and is an essential component of aerobic respiration. The beginning material is acetyl-CoA, which is sourced from glucose and produced by the oxidation of pyruvate, and when it is present in a series of redox processes, most of its bond energy is captured in the form of NADH, FADH2, and ATP molecules. The reduced electron carriers NADH and FADH2 are formed in the TCA cycle and send their electrons by oxidative phosphorylation into the electron transport chain, where they generate most of the ATP produced in cellular respiration.

In eukaryotes, Kreb's cycle occurs in the mitochondrial matrix, which is comparable to the conversion of pyruvate to acetyl-CoA, but in prokaryotes, all of these activities occur in the cytoplasm. The Kreb's cycle is a closed-loop system in which the molecule utilised in the first step is reformed by the pathway's last step.

• In the first step, acetyl-CoA is coupled with oxaloacetate, a 4-carbon acceptor molecule, to produce citrate, a 6-carbon molecule.
• Then, in a similar set of events, two carbons from citrate, a 6-carbon molecule, are liberated as carbon dioxide molecules, yielding an NADH molecule each time.
• The enzymes that catalyse these processes are the primary regulators of Kreb's cycle. These enzymes speed up or slow down processes depending on the cell's energy requirements.
• The remaining 4 molecules of citrate are then subjected to a series of reactions. In certain organisms, an ATP molecule or a related molecule called GTP is created first. The electron carrier FAD is then converted to FADH2. After then, another NADH is produced.
• The initial molecule oxaloacetate is regenerated in this series of reactions to allow the cycle to be repeated.
• Carbon dioxide molecules are released in a single round of the Kreb's Cycle, yielding three NADH, one FADH2, and one ATP or GTP.
• Because each molecule of glucose that enters cellular respiration includes two pyruvates, the Kreb's cycle cycles around twice, resulting in two acetyls CoAs per glucose.

Kreb’s Cycle

Read More: Difference between Glycolysis and Krebs Cycle

Catabolic Nature of Kreb’s Cycle

The Krebs cycle is the most prevalent oxidative route in cellular respiration. In plants, the breakdown of acetyl CoA produces NADH, FADH, and ATP, while in mammals, it produces GTP. Proteins, lipids, and carbohydrates oxidise to generate acetyl CoA either directly or indirectly through pyruvate, which enters the common oxidation route. The Krebs cycle substrates are not solely obtained from glycolysis, as we know from cellular respiration, but amino acids and lipids also contribute to the substrate. The citric acid cycle's catabolic actions include glucose oxidation, amino acid transamination, and fatty acid oxidation. Glycolysis is defined as the process of converting glucose to pyruvate. Fatty acids on Beta – Acetyl CoA are produced immediately by the oxidation of fatty acids.

Anabolic Nature of Kreb’s Cycle

The Krebs cycle intermediates were responsible for glucose, fatty acids, amino acids, and nucleic acids production. The following are examples of anabolic activities:

• Citrate from mitochondria enters the cytosol and is oxidised to generate acetyl CoA. The production of fatty acids is started because of this.
• α-ketoglutarate is the starting material for amino acid glutamate production.
• Oxaloacetate is used to make the amino acid aspartate, as well as pyrimidines and alkaloids.
• Pyrole compounds such as cytochromes and chlorophylls are formed by succinyl CoA.

Things to Remember

• An amphibolic route is defined as a metabolic pathway that involves both anabolic and catabolic activities. In the process of respiration, complex chemicals are broken down into simple compounds to create the energy molecule ATP.
• The same respiratory mechanism also serves as an anabolic route for the production of many intermediate and secondary metabolic products. 
• As a result, the respiratory route may be classified as an amphibolic pathway since it functions as both a catabolic and anabolic system.
• Because it functions in both the degradation and synthesis processes, the Krebs cycle is a good example of an amphibolic route.
• The TCA cycle, commonly known as the Kreb's cycle or the Citric Acid Cycle, is a tricarboxylic cycle. The matrix of mitochondria is where the second step of cellular respiration takes place. The citric acid cycle's enzymes are all water-soluble.

Read More: Difference Between Catabolism and Anabolism