Citric Acid Cycle (CAC)

 

Cellular Respiration

Cellular respiration is a fundamental metabolic process through which living cells extract energy from organic molecules, particularly glucose. This process occurs in a series of well-coordinated steps:

1. Glycolysis (in the cytoplasm)
2. Oxidative decarboxylation of pyruvate
3. Citric Acid Cycle (Krebs Cycle)
4. Electron Transport Chain (ETC)

Among these, the Citric Acid Cycle (CAC) is central because it serves as the metabolic hub where carbohydrates, fats, and proteins converge for energy production.

Definition of the Citric Acid Cycle

The Citric Acid Cycle is a cyclic series of enzyme-catalyzed reactions that oxidize acetyl-CoA into carbon dioxide, while generating high-energy molecules such as NADH, FADH₂, and GTP (or ATP).

In simple terms, it is a biochemical pathway that completes the oxidation of food molecules to release energy.

Why It Is Called Krebs Cycle and TCA Cycle?

The cycle is known by multiple names due to its characteristics and history:

Krebs Cycle: Named after Sir Hans Krebs, who discovered the pathway in 1937.

Citric Acid Cycle: Because the first stable compound formed is citric acid (citrate).

TCA Cycle (Tricarboxylic Acid Cycle): Because citric acid contains three carboxyl (-COOH) groups.

Site of Occurrence in Eukaryotic Cells

In eukaryotic cells, the Citric Acid Cycle takes place in the:

Mitochondrial matrix

This location is highly suitable because:

1. It contains all necessary enzymes
2. It is close to the Electron Transport Chain (located in the inner mitochondrial membrane)

Stepwise Explanation of the Citric Acid Cycle

Each turn of the cycle begins with acetyl-CoA (2C) combining with oxaloacetate (4C).

Step 1: Formation of Citrate

Substrates: Acetyl-CoA + Oxaloacetate

Enzyme: Citrate synthase

Product: Citrate (6C)

Step 2: Formation of Isocitrate

Enzyme: Aconitase

Citrate is rearranged to form isocitrate

Step 3: Oxidative Decarboxylation of Isocitrate

Enzyme: Isocitrate dehydrogenase

Products: α-Ketoglutarate (5C), CO₂, NADH

Step 4: Formation of Succinyl-CoA

Enzyme: α-Ketoglutarate dehydrogenase

Products: Succinyl-CoA (4C), CO₂, NADH

Step 5: Formation of Succinate

Enzyme: Succinyl-CoA synthetase

Products: Succinate, GTP (or ATP)

This step shows substrate-level phosphorylation

Step 6: Formation of Fumarate

Enzyme: Succinate dehydrogenase

Product: Fumarate, FADH₂

Step 7: Formation of Malate

Enzyme: Fumarase

Product: Malate

Step 8: Regeneration of Oxaloacetate

Enzyme: Malate dehydrogenase

Product: Oxaloacetate, NADH

The cycle is now ready to begin again.

Net Energy Yield of One Turn of the Cycle

From one molecule of acetyl-CoA, the cycle produces:

1. 3 NADH
2. 1 FADH₂
3. 1 GTP (or ATP)
4. 2 CO₂

Energy Equivalent

1. 3 NADH → 9 ATP
2. 1 FADH₂ → 2 ATP
3. 1 GTP → 1 ATP

Total = 12 ATP (classical calculation)
(Modern estimate ≈ 10 ATP)

Regulation of the Citric Acid Cycle

The cycle is tightly regulated to maintain energy balance.

Key Regulatory Enzymes

1. Citrate synthase
2. Isocitrate dehydrogenase (most important control point)
3. α-Ketoglutarate dehydrogenase

Regulatory Factors

1. Activated by: ADP, NAD⁺
2. Inhibited by: ATP, NADH, succinyl-CoA

High energy levels slow down the cycle.

Biological and Physiological Significance

The Citric Acid Cycle is essential because:

1. It is the final common pathway for oxidation of biomolecules
2. Produces reducing equivalents (NADH, FADH₂) for ATP generation
3. Supplies intermediates for biosynthesis
4. Plays a key role in cellular metabolism and energy balance

Amphibolic Nature of the Cycle

The Citric Acid Cycle is amphibolic, meaning it participates in both:

Catabolism (Breakdown)

1. Oxidation of acetyl-CoA to CO₂
2. Energy release

Anabolism (Synthesis)

Provides intermediates for:

Amino acids

Fatty acids

Glucose (via gluconeogenesis)

Thus, it acts as a metabolic bridge between breakdown and synthesis

Relationship with Carbohydrate, Lipid, and Protein Metabolism

Carbohydrate Metabolism

Glucose → Pyruvate → Acetyl-CoA → Enters CAC

Lipid Metabolism

Fatty acids undergo β-oxidation → Acetyl-CoA → CAC

Protein Metabolism

Amino acids are converted into:

Pyruvate

• 1. Acetyl-CoA
  2. or CAC intermediates (e.g., α-ketoglutarate)

Hence, all major nutrients converge at this cycle.

Conclusion

The Citric Acid Cycle is the central metabolic engine of the cell, integrating and coordinating the breakdown of carbohydrates, fats, and proteins. It not only generates energy-rich molecules but also provides essential intermediates for biosynthetic processes. Understanding this cycle is fundamental for appreciating how living organisms maintain energy homeostasis and metabolic flexibility.

Important Points 

1. Occurs in the mitochondrial matrix
2. Begins with acetyl-CoA + oxaloacetate → citrate
3. Produces 3 NADH, 1 FADH₂, 1 GTP per cycle
4. Releases 2 CO₂ molecules
5. Key enzymes: Citrate synthase, Isocitrate dehydrogenase, α-Ketoglutarate dehydrogenase
6. Regulated by ATP (inhibits) and ADP (activates)
7. It is an amphibolic pathway
8. Links metabolism of carbohydrates, lipids, and proteins
9. Central role in cellular respiration and energy production

References

1. Lehninger Principles of Biochemistry – David L. Nelson & Michael M. Cox
2. Biochemistry – Jeremy M. Berg, John L. Tymoczko & Lubert Stryer
3. Harper’s Illustrated Biochemistry – Rodwell et al.
4. Textbook of Biochemistry for Medical Students – D.M. Vasudevan
5. Fundamentals of Biochemistry – J.L. Jain

Online Resources

1. https://www.ncbi.nlm.nih.gov
2. https://www.khanacademy.org
3. https://www.britannica.com
4. https://www.zoologys.co.in

Multiple Choice Questions (MCQs)

1. The Citric Acid Cycle occurs in:

a) Cytoplasm
b) Nucleus
c) Mitochondrial matrix
d) Ribosome

Answer: c) Mitochondrial matrix

2. The first product formed in the cycle is:

a) Succinate
b) Citrate
c) Malate
d) Oxaloacetate

Answer: b) Citrate

3. Which enzyme catalyzes the formation of citrate?

a) Aconitase
b) Citrate synthase
c) Dehydrogenase
d) Fumarase

Answer: b) Citrate synthase

4. Number of NADH produced per cycle:

a) 1
b) 2
c) 3
d) 4

Answer: c) 3

5. Which step produces FADH₂?

a) Succinate → Fumarate
b) Citrate → Isocitrate
c) Malate → Oxaloacetate
d) α-Ketoglutarate → Succinyl-CoA

Answer: a) Succinate → Fumarate

6. The cycle is also called TCA cycle because:

a) It occurs in mitochondria
b) It produces ATP
c) It involves tricarboxylic acids
d) It uses oxygen

Answer: c) It involves tricarboxylic acids

7. How many CO₂ molecules are released per cycle?

a) 1
b) 2
c) 3
d) 4

Answer: b) 2

8. Which enzyme is a major regulatory point?

a) Fumarase
b) Malate dehydrogenase
c) Isocitrate dehydrogenase
d) Aconitase

Answer: c) Isocitrate dehydrogenase

9. The Citric Acid Cycle is:

a) Only catabolic
b) Only anabolic
c) Amphibolic
d) Synthetic

Answer: c) Amphibolic

10. Acetyl-CoA contains how many carbons?

a) 1
b) 2
c) 3
d) 4

Answer: b) 2

Worksheet for Students

Section A: Fill in the Blanks

1. The Citric Acid Cycle occurs in the __________.
2. The first product formed is __________.
3. __________ enzyme catalyzes the formation of citrate.
4. __________ molecules of NADH are produced per cycle.
5. The cycle releases __________ molecules of CO₂.

Section B: Match the Following

Column A	Column B
Citrate synthase	Formation of citrate
Succinate dehydrogenase	FADH₂ production
Fumarase	Hydration reaction
Malate dehydrogenase	NADH production

Short Answer Questions

1. Explain the significance of the Citric Acid Cycle.
2. Why is it called an amphibolic pathway?
3. Write the net energy yield of the cycle.
4. Describe the role of NADH in cellular respiration.

Long Answer Question

Describe the Citric Acid Cycle with a neat labeled diagram, including all enzymes and intermediates.

Diagram Practice

1. Draw and label the Citric Acid Cycle
2. Indicate: Enzymes, NADH, FADH₂, ATP production, CO₂ release