Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): The citric acid cycle is amphibolic in nature rather than only catabolic.
Reason (R): Several biosynthetic pathways utilize citric acid cycle intermediates as precursors for the synthesis of important products.
In light of the above statements, choose the correct answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

To easily identify if a metabolic pathway is amphibolic, evaluate if its intermediates serve as key biosynthetic starting points.
The siphoning of Krebs cycle intermediates (like succinyl-CoA for heme and oxaloacetate for aspartate) is a frequent topic in biochemistry exams.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Metabolic pathways are broadly classified into catabolic pathways, which break down larger molecules to harvest energy, and anabolic pathways, which utilize energy to synthesize essential cellular components.
An amphibolic pathway is a dual-purpose biochemical pathway that integrates both catabolic and anabolic processes.
The citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) is the primary example of an amphibolic pathway in aerobic organisms, operating in the mitochondrial matrix of eukaryotes and the cytosol of prokaryotes.

Step 2: Detailed Explanation:
The dual functionality of the citric acid cycle is described through its metabolic integration:
1. Catabolic Functions: The cycle receives acetyl-CoA derived from glycolysis, fatty acid beta-oxidation, and amino acid degradation.
Through a series of eight enzyme-catalyzed steps, it completely oxidizes the acetyl group of acetyl-CoA into two molecules of carbon dioxide.
This oxidation is coupled with the reduction of electron carriers, producing three molecules of \( \text{NADH} \), one molecule of \( \text{FADH}_2 \), and one molecule of \( \text{GTP} \) (or \( \text{ATP} \)) per turn of the cycle.
2. Anabolic Functions: Rather than functioning strictly as a closed loop for energy generation, several intermediates of the citric acid cycle are continuously siphoned off as crucial carbon precursors for anabolic pathways:
- \( \alpha \)-Ketoglutarate is transaminated to produce the amino acid glutamate, which serves as a precursor for other amino acids (proline, arginine) and purine nucleotides.
- Oxaloacetate is transaminated to yield aspartate, a critical starting block for pyrimidine nucleotide synthesis.
- Succinyl-CoA is a vital intermediate for the biosynthesis of porphyrins, heme groups in hemoglobin, and chlorophyll in phototrophic tissues.
- Citrate can be exported out of the mitochondria into the cytoplasm, where ATP-citrate lyase cleaves it back to acetyl-CoA for fatty acid and cholesterol biosynthesis.
- Malate is utilized in gluconeogenesis to synthesize glucose during periods of low carbohydrate availability.
Because these intermediates are continuously drained for biosynthesis, they must be replenished through anaplerotic reactions (such as the carboxylation of pyruvate to oxaloacetate by pyruvate carboxylase) to maintain the catalytic capacity of the cycle.
This extensive biosynthetic drainage of intermediates is the precise biological reason why the cycle is classified as amphibolic.

Step 3: Final Answer:
Both Assertion (A) and Reason (R) are true, and Reason (R) provides the correct explanation for Assertion (A).