CH 19: The TCA Cycle

19.1 The Citric Acid Cycle Consists of Two Stages

  • The citric acid cycle has two main purposes:

    • Stage One: Introduction of two carbon atoms via the condensation of acetyl CoA with oxaloacetate, producing citrate and generating high-energy electrons through oxidative decarboxylation.

    • Stage Two: Regeneration of oxaloacetate from succinate, allowing the cycle to continue and harvesting additional electrons.

19.2 Stage One: Oxidizes Two Carbon Atoms to Gather Energy-Rich Electrons

  • Reactions and Enzymes in Stage One:

    • Step 1: Acetyl CoA + oxaloacetate → citrate (catalyzed by citrate synthase).

    • Step 2: Citrate → isocitrate (catalyzed by aconitase, an isomerization reaction).

    • Step 3: Isocitrate decarboxylated to α-ketoglutarate (catalyzed by isocitrate dehydrogenase, an oxidation-reduction reaction).

    • Step 4: α-Ketoglutarate → succinyl CoA (via α-ketoglutarate dehydrogenase, also an oxidation-reduction and decarboxylation reaction).

  • Products of Stage One:

    • Generates 2 CO₂, 3 NADH, and 1 FADH₂.

19.3 Stage Two: Regenerates Oxaloacetate and Harvests Energy-Rich Electrons

  • Reactions and Enzymes in Stage Two:

    • Step 5: Succinyl CoA → succinate (catalyzed by succinyl CoA synthetase, producing ATP via substrate-level phosphorylation).

    • Steps 6, 7, & 8:

    • Step 6: Succinate oxidized to fumarate (producing FADH₂, an oxidation-reduction reaction).

    • Step 7: Fumarate hydrated to form malate (catalyzed by fumarase).

    • Step 8: Malate oxidized to regenerate oxaloacetate (producing NADH, an oxidation-reduction reaction).

  • Products of Stage Two:

    • Generates further high-energy electrons and recycles oxaloacetate.

Mechanism Details

  • Citrate Synthase Mechanism:

    • Citrate synthase catalyzes the condensation of acetyl CoA and oxaloacetate through a sequential mechanism involving an enzyme-substrate complex, facilitating the formation of citrate without using ATP directly due to favorable binding and resulting conformational changes.

  • Isomerization of Citrate to Isocitrate:

    • Catalyzed by aconitase, utilizing a dehydration-hydration mechanism rather than a simple isomerization, which is atypical.

  • Alpha-Ketoglutarate Dehydrogenase Mechanism:

    • Similar to PDH, the enzyme catalyzes multiple reactions to transform α-ketoglutarate, involving the decarboxylation, oxidation, and transfer of CoA to form succinyl CoA.

  • ATP Formation from Succinyl CoA:

    • Occurs in Step 5 during the conversion of succinyl CoA to succinate, producing ATP via substrate-level phosphorylation.

19.4 The Citric Acid Cycle Is Regulated

  • Control points include isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. Their activity is dependent on the energy status of the cell, responding to levels of ATP, ADP, and NADH.

19.5 Anaplerotic Reactions

  • Anaplerotic reactions replace citric acid cycle intermediates, with pyruvate carboxylase converting pyruvate to oxaloacetate when energy levels are changed. This is crucial for maintaining metabolic balance.

Summary

  • The citric acid cycle integrates energy production and biosynthesis in a regulated manner, emphasizing complexity in cellular metabolism.

Malonate, a competitive inhibitor of succinate dehydrogenase, will lead to a decrease in the conversion of succinate to fumarate in the citric acid cycle. As a result of this inhibition, the concentration of succinate will increase, while fumarate levels will decrease due to the blockage of its formation from succinate. Additionally, since less fumarate can be produced, the subsequent steps involving fumarate, such as its hydration to malate, will be affected, potentially leading to a reduction in malate concentrations as well. Overall, the immediate effect of malonate is characterized by an accumulation of succinate, and a decrease in fumarate and possibly malate concentrations, indicating a disruption in the normal flow of intermediates through the cycle.