The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a crucial metabolic pathway involved in cellular respiration. It takes place in the mitochondria of eukaryotic cells. Below are the main stages of the Krebs cycle:
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Formation of Citrate:
- Acetyl-CoA (from pyruvate and fatty acid oxidation) combines with oxaloacetate to form citrate (citric acid).
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Isomerization:
- Citrate is rearranged to form isocitrate through a two-step process involving the intermediate cis-aconitate.
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Oxidation of Isocitrate:
- Isocitrate is oxidized to α-ketoglutarate. This reaction also reduces NAD+ to NADH and involves the release of CO2.
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Oxidation of α-Ketoglutarate:
- α-Ketoglutarate undergoes further oxidation to succinyl-CoA, generating another NADH and releasing a second molecule of CO2.
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Conversion of Succinyl-CoA to Succinate:
- Succinyl-CoA is converted to succinate, producing GTP (or ATP) through substrate-level phosphorylation and releasing CoA.
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Oxidation of Succinate:
- Succinate is oxidized to fumarate, with the reduction of FAD to FADH2.
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Hydration of Fumarate:
- Fumarate is hydrated to form malate.
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Oxidation of Malate:
- Malate is oxidized back to oxaloacetate, with the reduction of NAD+ to NADH, thus completing the cycle.
The overall reaction of the Krebs cycle produces energy-rich molecules (NADH and FADH2) that are used in the electron transport chain for ATP synthesis, along with CO2 as a waste product.