What is Amphibolic Pathway?

Survival of the organism depends on the nutrient uptake from the environment and complete utilization of nutrients.

This process is vital and forms a dynamic nature of the cell. The conversion of external nutrient to an internal biomolecule is a main function of any biosystem present and is achieved through the process of metabolism.

Metabolism is the process by which nutrient are converted to desirable products for synthesis and breakdown of biochemical components

"A pathway which participates in Catabolism and anabolism is termed as amphibolic pathway"

Metabolism is otherwise termed as Catabolism + Anabolism. Catabolism is a process of breaking down of Complex food molecules to simpler compounds.

Anabolism is mainly concerned with the biosynthesis of Chemical compounds which are essential for storage and other functions from smaller molecules.

Synthesis (Anabolism) and degradation (Catabolism) of any biomolecule takes place in a series of process which are distinct in its own mechanism and have specific pathway.

But there are certain pathways which can act both catabolic an anabolic under appropriate circumstances.

A pathway which participates in Catabolism and anabolism is termed as amphibolic pathway.

Amphibolic Pathway involves many intermediates which forms new compounds (biosynthesis) such as amino acid, fats, glucose etc., or they get oxidized to produce energy.

An ideal example for Amphibolic Pathway is the CITRIC ACID CYCLE. The citric acid cycle is Amphibolic because it oxidizes Carbohydrates, Fats, Proteins to yield energy and it is the common oxidative pathway for any biochemical compound.

Any biochemical degradation either directly yields acetyl Co – A or through pyruvate and other components of citric acid cycle.

Similarly, the intermediates from the citric acid cycle becomes precursor for the synthesis of Glucose, fats, Amino Acid etc.,

The catabolism and anabolism of the intermediates present in the citric cycle depends upon the internal cues of the body.

Internal cues can be the requirement of synthesis or breakdown of a compound and is dependent on many other factors.

Amphibolic Pathway Characteristics

The main characteristics of amphibolic pathway is the involvement of both catabolic and anabolic pathway and Citric Acid Cycle becomes an ideal cycle to understand the Amphibolic Pathway

Catabolic Nature of Citric Acid Cycle

In cellular respiration, citric acid cycle is the common oxidative pathway where the Acetyl Co – A breakdown yielding NADH, FADH and ATP in plants GTP in Animals.

The basic biological compound namely Proteins, Fats and Glucose oxidizes to form Acetyl Co – A directly or through Pyruvate enters the common pathway of oxidation.

From the basic cellular respiration, we know that the substrates of Citric Acid Cycle are not completely derived from glycolysis but the amino acids and lipids also contribute to the substrate.

Catabolic activities of citric acid cycle are,

1. Oxidation of Glucose

2. Transamination of Amino Acids

3. Oxidation of Fatty acids

Glucose is broken down to pyruvate by Glycolysis.

Fatty Acids on Beta – Oxidation provides Acetyl Co – A directly

Amino Acid Catabolism

Essential and non – essential amino acids are present in body for the proper functioning of the living system.

Among all of them each amino acids do not form acetyl Co – A directly.

Few forms other intermediates of the Citric Acid Cycle by the process of Transamination or Aminotransferase. The Aminotransferase reaction involves the transfer of alpha – amino group from the carbon skeleton and transforming the skeleton to Amphibolic intermediate to produce energy.

Few amino acids oxidize to become pyruvate and Acetyl Co -A also.

For example:

I. Glutamine is transformed to Glutamate with enzyme involving Glutaminase. Glutamate then by transamination becomes alpha – ketoglutarate.

II. Similarly, Asparagine becomes Aspartate on transamination’s yields oxaloacetate and Certain Amino acids like alanine, Serine etc., directly oxidizes to produce pyruvate

Anabolic Nature of Citric Acid Cycle

Biosynthesis of glucose, fatty Acids, Amino Acids, Nucleic Acids are produced from the intermediates of the Krebs cycle.

The intermediates form many precursors corresponding to their respective substrates from which the biosynthesis takes place.

Anabolic activities are;

I. Citrate from mitochondria enters cytosol to oxidize to form Acetyl Co – A, this initiates the Biosynthesis of Fatty Acids.

II. α-ketoglutarate is the precursor for Amino Acid Glutamate which on Transamination provides Glutamine, Arginine and Proline and provides Purines.

III. Succinyl Co – A becomes a precursor for Porphyrin and Heme forming oxygen carriers in Blood.

IV. Oxaloacetate is the precursor for Aspartate and they form Asparagine to produce pyrimidines and other Amino acids that acts as an intermediate for Gluconeogenesis