outline the main stages of he Krebs cycle and state where it occurs in the cell

Answer 1

The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a crucial metabolic pathway for energy production. It takes place in the mitochondrial matrix of eukaryotic cells. Here are the main stages of the Krebs cycle:

1. Acetyl-CoA Formation:

   • Before entering the Krebs cycle, pyruvate (produced from glycolysis) is converted to acetyl-CoA in the mitochondria. This step releases carbon dioxide and produces NADH.

2. Condensation of Acetyl-CoA and Oxaloacetate:

   • Acetyl-CoA (2-carbon molecule) combines with oxaloacetate (4-carbon molecule) to form citrate (6-carbon molecule) through the action of the enzyme citrate synthase. This is the first step of the cycle.

3. Isomerization of Citrate:

   • Citrate is converted to isocitrate through the enzyme aconitase.

4. Oxidation and Decarboxylation:

   • Isocitrate is oxidized and decarboxylated to form alpha-ketoglutarate (5-carbon molecule) by the enzyme isocitrate dehydrogenase, generating NADH and releasing carbon dioxide.

5. More Oxidation and Decarboxylation:

   • Alpha-ketoglutarate is further oxidized and decarboxylated to form succinyl-CoA (4-carbon molecule) by the enzyme alpha-ketoglutarate dehydrogenase. This step also generates NADH and releases carbon dioxide.

6. Conversion to Succinate:

   • Succinyl-CoA is converted to succinate (4-carbon molecule) by the enzyme succinyl-CoA synthetase, producing GTP (or ATP) and releasing coenzyme A.

7. Oxidation of Succinate:

   • Succinate is oxidized to fumarate by the enzyme succinate dehydrogenase, producing FADH2.

8. Hydration of Fumarate:

   • Fumarate is converted to malate by the enzyme fumarase through the addition of water.

9. Final Oxidation and Regeneration of Oxaloacetate:

   • Malate is oxidized back to oxaloacetate by the enzyme malate dehydrogenase, generating another molecule of NADH.

These stages culminate in the regeneration of oxaloacetate, allowing the cycle to continue. Throughout the Krebs cycle, high-energy electron carriers NADH and FADH2 are produced, which are later used in the electron transport chain to generate ATP.