Role of enzymes involved in the TCA cycle

Lesson 20 : The Citric Acid Cycle

Role of enzymes involved in the TCA cycle

  1. Citrate synthase
  2. Irreversible reaction

    Acetyl CoA reacts with oxaloacetate --> citrate and CoA

  3. Aconitase
  4. Citrate --> isocitrate

  5. Isocitrate dehydrogenase
  6. Irreversible reaction

    Substrate first oxidized (2e- and H+ given to NAD+), then decarboxylated

    Isocitrate -->?-ketoglutarate + CO2 + NADH + H+

  7. ?-ketoglutarate dehydrogenase complex
  8. -ketoglutarate first decarboxylated, oxidized (2e- and H+ given to NAD+), and HS-CoA added

    Product is succinyl CoA

    Enzyme complex similar the PDH, but has dihydrolipoamide succinyltransferase instead of acetyltransferase.

  9. Succinyl CoA synthetase or succinate thiokinase
  10. Succinyl CoA --> succinate

    Substrate has high energy thioester bond; that energy is stored as nucleoside triphosphate via substrate level phosphorylation

    GDP +Pi --> GTP mammals

    ADP +Pi --> ATP plants and bacteria

  11. Succinate dehydrogenase complex
  12. Enzyme is embedded in inner mitochondrial membrane.

    Has FAD covalently bound to it (prosthetic group).

    Converts succinate --> fumarate with generation of FADH2 --> ETS

    FAD is regenerated by reduction of a mobile molecule called ubiquinone (coenzyme Q) --> QH2.

  13. Fumarase

    fumarate --> malate

  14. Malate dehydrogenase

    L-malate --> oxaloacetate

    2e- and H+ given to NAD+ --> NADH

Last modified: Saturday, 21 January 2012, 9:20 AM