Kreb’s Cycle (Citric Acid Cycle or Tricarboxylic Acid Cycle - TCA Cycle).

The Krebs cycle, or citric acid cycle, is a series of reactions in the mitochondria that generate energy by oxidizing acetyl-CoA into carbon dioxide. It produces high-energy molecules like NADH and FADH₂, which are used to produce ATP in the electron transport chain.

1. Kreb’s cycle S. Nahidha Begum II M.Sc Microbiology Sacred Heart College

2. Kreb’s cycle Kreb’s Cycle (Citric Acid Cycle or Tricarboxylic Acid Cycle - TCA Cycle) The Kreb’s cycle is a crucial metabolic pathway for energy production in the body, responsible for synthesizing about 65-70% of the ATP. This cycle involves the oxidation of acetyl-CoA to carbon dioxide (CO₂) and water (H₂O). The TCA cycle is named due to the participation of tricarboxylic acids (citrate, cis-aconitate, and isocitrate) at the beginning of the cycle. It was proposed by Hans Adolf Krebs in 1937, following studies on oxygen consumption in pigeon breast muscle, and is named in his honor. Kreb’s was awarded the Nobel Prize for Physiology or Medicine in 1953.

3. The enzymes of the TCA cycle are located in the mitochondrial matrix, near the electron transport chain. This proximity facilitates ATP synthesis via oxidative phosphorylation without hindrance. The cycle serves as the final oxidative pathway for carbohydrates, fats, and amino acids. Energy and Biosynthesis In addition to energy production, the Kreb’s cycle provides several intermediates required for synthesizing amino acids, glucose, heme, nucleotides (purines and pyrimidines), and other compounds, making it a central hub in metabolism.

4. Cyclic and Amphibolic Nature The Kreb’s cycle is a cyclic process. It is considered amphibolic because it plays dual roles Catabolism and Anabolism. Catabolic Role : It oxidizes acetyl-CoA derived from carbohydrates, lipids, and proteins to produce CO₂, H₂O, and energy in the form of ATP. Anabolic Role : Intermediates from the TCA cycle are used in the synthesis of various essential molecules like amino acids, glucose, heme, and nucleotides.

5. Steps of the Kreb’s Cycle (Citric Acid Cycle) Step 1: Formation of Citric Acid The cycle begins with the removal of the CoA portion from acetyl-CoA by enzymes. The remaining two-carbon acetyl group combines with oxaloacetic acid, forming a six-carbon molecule called citric acid. Steps 2 to 4: Oxidations and Decarboxylations Step 2: Involves the rearrangement of citric acid to form isocitric acid. Steps 3 and 4: These steps involve oxidation and decarboxylation reactions: Oxidation reactions convert NAD+ to NADH. Two carbon atoms from the original oxaloacetic acid are removed as CO₂. During the second oxidation (step 4), CoA is added back into the cycle, forming succinyl-CoA.

6. Step 5: ATP Production ATP is generated through substrate-level phosphorylation. The CoA group is removed from succinyl-CoA, resulting in the formation of succinic acid. Steps 6 to 8: Regeneration of Oxaloacetic Acid Step 6 : Involves an oxidation that produces FADH₂ from FAD. Steps 7 and 8: Further enzymatic reactions rearrange chemical bonds: These steps lead to the formation of three different molecules. The final oxidation in step 8 converts malic acid into oxaloacetic acid. Oxaloacetic acid is regenerated and ready to combine with a new acetylCoA molecule, starting the cycle again.

7. Kreb’s cycle or Citric acid cycle or TCA cycle

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