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Related Subjects: |Fat Metabolism |Glucose Metabolism |Protein metabolism |Glycolysis_Krebs_Electron_Transport_Chain

Glycolysis in Cell Cytoplasm

• Glycolysis results in the reaction : Glucose → 2 Pyruvate + 2 ATP + 2 NADH⁺H⁺
• It occurs in the cytosol and Oxygen is not required
• Two molecules of ATP are converted to ADP per Glucose molecule
• They are used to initially phosphorylate the glucose
• The glucose is then broken down to 2 x 3 carbon phosphorylated molecules
• Four molecules of ADP are converted to ATP per Glucose molecule
• The first reaction phosphorylates the glucose which effectively keeps it locked within the cell as there is no transport mechanism for its escape

Control

• The rate-limiting step is the conversion of Fructose 6 phosphate to Fructose 1,6 Biphosphate
• Phosphofructokinase is inhibited by ATP
• Phosphofructokinase is activated by AMP
• Phosphofructokinase is inhibited most strongly by Fructose 2,6 Biphosphate (a marker of Gluconeogenesis)
• During starvation Fructose 2,6 Biphosphate levels fall leading to increased gluconeogenesis

Krebs in Mitochondria

About

• Pyruvate enters the mitochondrial matrix through a specific transporter in the internal mitochondrial membrane.
• Here through the action of the large enzyme complex pyruvate dehydrogenase pyruvate is converted to acetyl Coenzyme A a two carbon structure. Acetyl CoA can also be provided as a product of fat metabolism. Some smaller steps have been omitted.
• The rate of cycling of the citric acid cycle usually depends upon the demand for ATP and not on the availability of substrates
• With each turn of the cycle the two carbons are lost as 2 CO₂ molecules with the production of NADH each time.

Kreb's cycle (2-10)

• 1. Pyruvate → Acetyl CoA
• 2. Acetyl CoA+ Oxaloacetate (4C)→ Citrate
• 3. Citrate → Isocitrate
• 4. Isocitrate → a-ketoglutarate (5C) + CO₂
• 5. a-ketoglutarate → Succinate (4C) + CO₂
• 6. Succinate → Succinyl COA [Used for Haem synthesis]
• 7. Succinyl COA → Succinate
• 8. Succinate → Fumarate
• 9. Fumarate → Malate
• 10. Malate → Oxaloacetate [Useful for protein synthesis]
• Now GO BACK TO STEP 2

Functions of Kreb's cycle are

• Isocitrate → a-ketoglutarate + CO₂ + NADH
• a-ketoglutarate → Succinyl CoA+ CO₂ + NADH
• Succinate → Fumarate + FADH₂
• Succinyl CoA → Succinate + GTP
• NAD⁺ and FAD act as electron carriers
• NAD⁺ + e^-→ NADH
• FAD + 2e^- → FADH₂
• Generation of intermediaries
  • Succinyl CoA for Haem synthesis
  • a-ketoglutarate for protein synthesis
  • Oxaloacetate for protein synthesis