Krebs Cycle: Cellular Energy Production Pathway

The Krebs Cycle

The Krebs cycle, also known as the Citric Acid Cycle, is an amphibolic pathway because:

• It facilitates the degradation of Acetyl-CoA.
• Its intermediate compounds serve as precursors for other synthetic processes.

Stages of the Krebs Cycle

The cycle is divided into the following stages:

1. Formation of Citric Acid

   Citric acid is formed from oxaloacetic acid and Acetyl-CoA.

   • Modifications: Oxaloacetic acid + Acetyl-CoA → Citric acid
   • Inputs/Outputs: Enters H₂O and Acetyl-CoA

2. Isomerization of Citric Acid to Isocitric Acid

   • Modifications: Citric acid → Isocitric acid
   • Inputs/Outputs: None

3. Oxidation of Isocitric Acid

   Isocitric acid is oxidized, reducing NAD⁺ to NADH, forming alpha-ketoglutarate, and releasing CO₂.

   • Modifications: Isocitric acid → Alpha-ketoglutarate
   • Inputs/Outputs: Enters NAD⁺ (forms NADH + H⁺) and releases CO₂

4. Oxidation of Alpha-Ketoglutarate

   Alpha-ketoglutarate is oxidized, reducing NAD⁺ to NADH, forming Succinyl-CoA, and releasing CO₂.

   • Modifications: Alpha-ketoglutarate → Succinyl-CoA
   • Inputs/Outputs: Enters CoA and NAD⁺ (forms NADH + H⁺) and releases CO₂

5. Cleavage of Succinyl-CoA

   The bond within Succinyl-CoA is broken, releasing energy that phosphorylates GDP to form GTP. GTP can then synthesize ATP from ADP.

   • Modifications: Succinyl-CoA → Succinic acid
   • Inputs/Outputs: Enters H₂O and GDP, releases CoA

6. Oxidation of Succinic Acid

   Succinic acid is oxidized, reducing FAD to FADH₂, forming fumaric acid.

   • Modifications: Succinic acid → Fumaric acid
   • Inputs/Outputs: Enters FAD (forms FADH₂)

7. Hydration of Fumaric Acid

   Fumaric acid is hydrated to form malic acid.

   • Modifications: Fumaric acid → Malic acid
   • Inputs/Outputs: Enters H₂O

8. Oxidation of Malic Acid

   Malic acid is oxidized, reducing NAD⁺ to NADH, regenerating oxaloacetic acid.

   • Modifications: Malic acid → Oxaloacetic acid
   • Inputs/Outputs: Enters NAD⁺ (forms NADH + H⁺)

Energy Yield from Glucose

• For every Glucose molecule, 2 molecules of Pyruvate are produced.
• For every Pyruvate molecule, 1 molecule of Acetyl-CoA is produced (thus, 2 Acetyl-CoA per glucose).
• For every Acetyl-CoA molecule entering the Krebs cycle, the following are produced:
  • 3 CO₂
  • 3 NADH
  • 1 FADH₂
  • 1 ATP (or GTP equivalent)

  (Note: These values are doubled for each glucose molecule, as two Acetyl-CoA molecules are generated per glucose.)