Tricarboxylic acid cycle KREBS

1. Introduction to the Krebs Cycle

• Definition: The final oxidative pathway where carbohydrates, fats, and proteins converge, converting carbon skeletons into CO₂ and generating energy.

• Key Features:

  • Occurs in the mitochondria.

  • Produces NADH and FADH₂ for oxidative phosphorylation.

  • An aerobic process, as it requires oxygen indirectly for ETC.

• Overall Reaction:
  Acetyl-CoA + 3 NAD⁺ + FAD + GDP + Pi → 2 CO₂ + 3 NADH + FADH₂ + GTP + CoA

2. Entry Point: Oxidative Decarboxylation of Pyruvate to Acetyl CoA

• Enzyme: Pyruvate dehydrogenase complex (PDH).

  (aggregate of 3 enzymes)

  1. Pyruvate dehydrogenase (PDH or E1, also called a decarboxylase)

  2. Dihydrolipoyl transacetylase (E2)

  3. Dihydrolipoyl dehydrogenase (E3)

     

     
     

• Key Reaction: Pyruvate (3C) → Acetyl-CoA (2C) + NADH + CO₂

• Coenzymes ( acts as carriers or oxidants) : Thiamine pyrophosphate (TPP) for E1 , lipoic acid, CoA for E2 , NAD⁺, FAD for E3.

Deficiencies of thiamine (vitamin B1) can cause serious CNS problems → brain cells are unable to produce sufficient ATP (via TCA cycle) if PDH complex is inactive.

Regulation of PDH Complex :

alternately activate & inactivate E1 of PDH complex

1. Pyruvate dehydrogenase (PDH) kinase

Phosphorylates → Inhibits PDH complex

2. Pyruvate dehydrogenase (PDH) phosphatase:

Dephosphorylates → Activates PDH complex

• The kinase itself can be activated by high-energy signals e.g. ATP, acetyl CoA & NADH → PDH complex is turned off. 

• The complex is also subject to product (NADH, acetyl CoA) inhibition.

•  Pyruvate - potent inhibitor of PDH kinase. If [pyruvate] elevated → E1 will be maximally active.

•  Calcium - strong activator of PDH phosphatase, stimulating E1 activity. Important in skeletal muscle where release of Ca2+ during muscle contraction stimulates PDH complex to drive energy production.

3. Steps of the Krebs Cycle

1. Synthesis of Citrate

   • Reaction: Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)

   • Enzyme: Citrate synthase

2. Isomerization of Citrate

   • Reaction: Citrate → Isocitrate (6C)

   • Enzyme: Aconitase

   • Inhibition: Fluoroacetate (toxic).

3. Oxidative Decarboxylation of Isocitrate

   • Reaction: Isocitrate (6C) → α-Ketoglutarate (5C) + NADH + CO₂

   • Enzyme: Isocitrate dehydrogenase

   • Regulation: Activated by ADP, Ca²⁺; inhibited by ATP, NADH.

4. Oxidative Decarboxylation of α-Ketoglutarate

   • Reaction: α-Ketoglutarate (5C) → Succinyl-CoA (4C) + NADH + CO₂

   • Enzyme: α-Ketoglutarate dehydrogenase complex

   • Coenzymes: Same as PDH.

5. Cleavage of Succinyl-CoA

   • Reaction: Succinyl-CoA → Succinate + GTP

   • Enzyme: Succinyl-CoA synthetase

   • Note: GTP is equivalent to ATP.

6. Oxidation of Succinate

   • Reaction: Succinate → Fumarate + FADH₂

   • Enzyme: Succinate dehydrogenase (also part of ETC).

7. Hydration of Fumarate (reversible)

   • Reaction: Fumarate → Malate

   • Enzyme: Fumarase

8. Oxidation of Malate

   • Reaction: Malate → Oxaloacetate + NADH

   • Enzyme: Malate dehydrogenase

4. Energy Yield

• Per Turn of the Cycle:

  • 3 NADH → 9 ATP

  • 1 FADH₂ → 2 ATP

  • 1 GTP → 1 ATP
    Total: 12 ATP per cycle

  • Each molecule glucose produce 2 pyruvate.

  • Total: 24 ATP + 6 ATP from each oxidative decarboxylation of pyruvate into acetyl CoA as 1 NADH is produced.

5. Regulation of the TCA Cycle

• Key Enzymes:

  1. Citrate synthase.

  2. Isocitrate dehydrogenase (rate-limiting).

  3. α-Ketoglutarate dehydrogenase.

• Activators: ADP, Ca²⁺ (low energy demand).

• Inhibitors: ATP, NADH (high energy availability).