TCA cycle

Overview of Metabolism and Mitochondria

• Oxidation of Pyruvate to Acetyl CoA: Process of converting pyruvate to acetyl CoA and its metabolism through the Tricarboxylic Acid (TCA) cycle.

• Location: All processes occur in the mitochondria, crucial for aerobic respiration.

• Function of Mitochondria: Key role in glucose and fat degradation, ATP production, and gluconeogenesis.

• Mitochondrial Structure: Matrix contains enzymes for pyruvate to acetyl CoA conversion, TCA cycle; the folded membranes contain electron transport system (ETS) components.

Pyruvate to Acetyl CoA Conversion

• Pyruvate Dehydrogenase Complex: Large multi-enzyme complex controlling pyruvate entry into TCA cycle.

  • Subunits:

    • E1 (Pyruvate Dehydrogenase): Catalyzes decarboxylation of pyruvate (removal of carbon dioxide).

      • Requires thiamine pyrophosphate (TPP) as a cofactor.

    • E2 (Dihydrolipyl Transferase): Attaches coenzyme A to the decarboxylated pyruvate.

      • Requires lipamide as a cofactor.

    • E3 (Dihydrolipide Dehydrogenase): Reoxidizes reduced lipamide.

      • Converts FAD to FADH2.

• Complex Size: Composed of approximately 30 E1 subunits, 12 E2, and 12 E3, larger than ribosomes.

Mechanism of Acetyl CoA Formation

• Decarboxylation:

  • TPP reacts with pyruvate leading to carbon loss (CO2), forming a TPP-pyruvate complex.

  • Converts keto form of pyruvate to the alcohol form.

• Redox Reaction:

  • Lipamide adds to TPP-pyruvate complex.

  • Alcohol is oxidized to a ketone, forming an acetyl group attached to lipamide.

• CoA Exchange:

  • Coenzyme A substitutes for lipamide, resulting in Acetyl CoA synthesis and reduced lipamide.

• Reoxidation of Lipamide:

  • Utilizes FAD to convert reduced lipamide back to oxidized form and produces FADH2.

  • FADH2 is further processed to NADH, entering the ETS.

Tricarboxylic Acid Cycle (TCA) Overview

• Function: Central in metabolism, involved in both catabolic and anabolic pathways.

• Cycle Structure: Comprised of eight enzymatic steps, divided into four main stages for analysis:

  • Stage 1: Condensation of acetyl CoA and oxaloacetate to form citrate (catalyzed by citrate synthase).

  • Stage 2: Two decarboxylation steps, resulting in loss of 2 CO2 molecules and production of NADH.

  • Stage 3: Conversion of succinyl CoA to GTP (analogous to ATP).

  • Stage 4: Regeneration of oxaloacetate from succinate with three oxidation steps producing NADH and FADH2.

• Outputs: Acetyl CoA oxidized to carbon dioxide, producing:

  • 3 NADH

  • 1 FADH2

  • 1 GTP

• Aerobic Requirement: TCA cycle operates only under aerobic conditions due to reliance on oxygen for NADH and FADH2 reoxidation in ETS.

Detailed Steps of the TCA Cycle

Stage 1: Condensation and Rearrangement

• Condensation: Acetyl CoA and oxaloacetate condense to form citrate, losing coenzyme A.

• Rearrangement: Citrate is rearranged to isocitrate.

Stage 2: Decarboxylation Steps

• NAD+ Reduction: Isocitrate undergoes oxidation forming a beta-keto acid then loses CO2 to produce alpha-ketoglutarate.

• Conversion: Alpha-ketoglutarate converted to succinyl CoA, generating another NADH.

Stage 3: GTP Formation

• GTP Production: Succinyl CoA is converted to GTP through phosphate substitution (analogous reaction to glycolysis).

Stage 4: Regeneration of Oxaloacetate

• Oxidative Reactions:

  • Succinate to Fumarate: FAD is reduced to FADH2, and a double bond forms.

  • Fumarate to Malate: Water adds across the double bond.

  • Malate to Oxaloacetate: NAD+ oxidizes malate back to oxaloacetate, a thermodynamically unfavorable step that depends on the rate of the cycle.

Conclusion

• Cycle Dynamics: TCA cycle's rate dependent on acetyl CoA availability, producing NADH and FADH2 for further aerobic respiration processes.