The Citric Acid Cycle

The Citric Acid Cycle

Overview of the Citric Acid Cycle

  • Central metabolic pathway in cellular respiration.
  • Amphibolic nature: Connects carbohydrate, protein, and lipid metabolism.
  • Also known as:
    • Krebs Cycle, named after Sir Hans Krebs.
    • Tricarboxylic Acid Cycle (TCA Cycle).
  • Acetyl CoA serves as a common metabolic intermediate in the catabolism of carbohydrates, fatty acids, and amino acids.

Structure of Mitochondria

  • Football-shaped organelle, similar in size to bacterial cells.
  • Consists of a dual membrane:
    • Outer Mitochondrial Membrane
    • Inner Mitochondrial Membrane
      • Contains highly folded structures called cristae; contains the electron transport system and ATP synthase.
  • Spaces within mitochondria include:
    • Intermembrane space (between outer and inner membranes)
    • Matrix space (interior, containing enzymes).
  • Key processes:
    • Citric acid cycle
    • Beta-oxidation of fatty acids part of energy production.

Steps of the Citric Acid Cycle

  1. Conversion of Pyruvate to Acetyl CoA:

    • Catalyzed by pyruvate dehydrogenase complex.
    • Involves 5 coenzymes (CoA, TPP, lipoic acid, FAD, NAD+) and 3 enzymes.
    • Irreversible reaction produces 1 CO2 and 1 Acetyl-CoA.
    • Controlled by allosteric regulation and phosphorylation/dephosphorylation reactions through PDH kinase and phosphoprotein phosphatase.

  2. First Reaction of Citric Acid Cycle:

    • Acetyl group (2C) combines with oxaloacetate (4C) to form citrate (6C).
    • Enzyme: Citrate Synthase (regulated by ATP, NADH, succinyl CoA).

  3. Second Reaction:

    • Citrate is converted to isocitrate via enzyme aconitase (isomerization step).

  4. Third Reaction:

    • Isocitrate converts to α-ketoglutarate via isocitrate dehydrogenase (oxidative decarboxylation)
    • Produces NADH and CO2; controlled by ATP, NADH, ADP.

  5. Fourth Reaction:

    • α-Ketoglutarate converts to succinyl-CoA via α-ketoglutarate dehydrogenase complex (similar to PDH).
    • Additional NADH and CO2 generated; formation of high-energy thioester bond.

  6. Fifth Reaction:

    • Succinyl-CoA converts to succinate via succinyl-CoA synthetase.
    • Only step that produces direct energy (GTP or ATP) through thioester hydrolysis.

  7. Sixth Reaction:

    • Succinate is oxidized to fumarate by succinate dehydrogenase.(enzyme uses FAD).
    • Important as it generates FADH2 rather than NADH.

  8. Seventh Reaction:

    • Fumarate forms malate through water addition (catalyzed by fumarase).

  9. Eighth Reaction:

    • Malate is converted back to oxaloacetate via malate dehydrogenase, regenerating NADH and completing the cycle.

Summary of Citric Acid Cycle Outputs

  • For each acetyl CoA:
    • 3 NADH, 1 FADH2, and 1 GTP (or ATP) produced.
    • Release of 2 CO2 molecules.
    • Ultimately, large yield of ATP from reoxidation of NADH and FADH2 in the electron transport chain.

Regulation of the Citric Acid Cycle

  • Regulated at three key exergonic steps:
    1. Citrate Synthase
    2. Isocitrate Dehydrogenase
    3. α-Ketoglutarate Dehydrogenase
  • Factors influencing regulation:
    • Substrate availability
    • Product inhibition
    • Allosteric control (e.g., ions like Ca²+, AMP, ATP).

Anabolic Functions of the Citric Acid Cycle

  • Provides precursors for biosynthesis (e.g., amino acids, nucleotides).
  • Anaplerotic reactions replenish cycle intermediates (particularly oxaloacetate).
  • Connection between citric acid cycle and gluconeogenesis via pyruvate carboxylase pathway.

Role of Metabolism in Weight Management

  • Fats yield acetyl CoA but cannot convert to glucose, slowing potential weight loss.
  • Proper balance of dietary intake and physical activity is essential for weight management and maintaining blood glucose levels.
    • Exercise promotes fat as a primary energy source, helping spare protein degradation for gluconeogenesis.

Genetic Mutations Affecting Citric Acid Cycle

  • Rare mutations in cycle enzymes can lead to significant health implications, such as cancers linked to fumarase and succinate dehydrogenase deficiencies.

Conclusion

  • The citric acid cycle is fundamental to cellular energy production, metabolic regulation, and connections to anabolic processes, demonstrating its importance in both catabolism and anabolism.