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The citric acid cycle has eight steps, each

catalyzed by a specific enzyme
•
explain these 8 steps of citric acid cycle with what they produced in each steps

1 answer

The citric acid cycle (also known as the Krebs cycle or TCA cycle) is a crucial metabolic pathway that occurs in the mitochondria of cells. It plays a key role in energy production through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The cycle consists of eight steps, each catalyzed by a specific enzyme, leading to the production of energy-rich molecules.

Here’s a detailed overview of each step, including the enzymes involved and the products produced:

  1. Formation of Citrate:

    • Enzyme: Citrate synthase
    • Reactants: Acetyl-CoA and oxaloacetate (OAA)
    • Product: Citrate
    • Description: The acetyl group from acetyl-CoA condenses with oxaloacetate to form citrate. This step is irreversible and initiates the cycle.

  2. Formation of Isocitrate:

    • Enzyme: Aconitase
    • Reactant: Citrate
    • Product: Isocitrate
    • Description: Citrate is converted into isocitrate through a rearrangement reaction. This occurs via an intermediary called cis-aconitate.

  3. Oxidation of Isocitrate:

    • Enzyme: Isocitrate dehydrogenase
    • Reactant: Isocitrate
    • Products: α-Ketoglutarate, NADH, and CO₂
    • Description: Isocitrate is oxidized to form α-ketoglutarate. In this process, NAD⁺ is reduced to NADH and a molecule of CO₂ is released.

  4. Formation of Succinyl-CoA:

    • Enzyme: α-Ketoglutarate dehydrogenase
    • Reactants: α-Ketoglutarate and CoA
    • Products: Succinyl-CoA, NADH, and CO₂
    • Description: α-Ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA, while another molecule of NAD⁺ is reduced to NADH and another CO₂ is released.

  5. Conversion of Succinyl-CoA to Succinate:

    • Enzyme: Succinyl-CoA synthetase
    • Reactants: Succinyl-CoA
    • Products: Succinate, CoA, and GTP (or ATP)
    • Description: Succinyl-CoA is converted into succinate, generating energy that is used to produce GTP (or ATP), and releasing CoA.

  6. Oxidation of Succinate:

    • Enzyme: Succinate dehydrogenase
    • Reactants: Succinate
    • Products: Fumarate and FADH₂
    • Description: Succinate is oxidized to fumarate, and in this process, FAD is reduced to FADH₂.

  7. Hydration of Fumarate:

    • Enzyme: Fumarase (fumarate hydratase)
    • Reactant: Fumarate
    • Product: Malate
    • Description: Fumarate is hydrated (addition of water) to form malate.

  8. Oxidation of Malate:

    • Enzyme: Malate dehydrogenase
    • Reactant: Malate
    • Products: Oxaloacetate and NADH
    • Description: Malate is oxidized back to oxaloacetate, reducing NAD⁺ to NADH. This regenerates oxaloacetate, allowing the cycle to continue.

Summary of Products:

Overall, for each acetyl-CoA that enters the citric acid cycle, the following products are generated:

  • 3 NADH
  • 1 FADH₂
  • 1 GTP (or ATP)
  • 2 CO₂

These reduced coenzymes (NADH and FADH₂) are then utilized in the electron transport chain to produce ATP, while the CO₂ is expelled as a waste product of cellular respiration.