Metabolism

Master answer

The Citric Acid Cycle

Energy production

• Required for the total metabolism of glucose, for the production of approx. 38 ATP (Compared to 8 ATP production from metabolism of glucose to pyruvate)
• Also used in the metabolism of fats, ketone bodies and amino acids via Acetyl CoA
  • 12 ATP generated for each Acetyl CoA molecule
• Occurs within the mitochondrial matrix
• Pyruvate generated in glycolysis is converted to Acetyl CoA by pyruvate dehydrogenase releasing 1 NADH per pyruvate molecule (leading to the generation of 6 ATP equvalents per glucose molecule)
• Acetyl CoA condensed with oxaloacetate to for citric acid and in a number of subsequent steps forms 1GTP, 3NADH and 1FADH₂ (Total of 24 ATP equivalents per glucose molecule)
• The NADH and FADH₂ generated in the citric acid cycle is used to generate ATP with O₂ as the terminal electron carrier to form water.
  • Therefore O₂ is required for the citric acid cycle to continue
• The electron transport chain
  • Inner mitochondrial membrane
  • Series of 4 cytochrome enzymes that pump H⁺ across into intermembrane space as electrons are transferred from NADH and FADH₂
  • NAD and FAD regenerated in the process replenishing substrates for the citric acid cycle
  • Proton gradient used to generate ATP from ADP and Pi
• CO₂ in generated in the citric acid cycle

Note: Erythrocytes lack mitochondria and therefore are unable to utilize the citric acid cycle for energy production (Therefore can only utilize glucose)

Biosynthesis

• Components of the citric acid cycle are used for biosynthesis of fuels
  • Malate and oxaloacetate – used in gluconeogenesis
  • Citrate – used in fatty acid and cholesterol generation
  • Oxaloacetate and αketoglutarate – used in amino acid synthesis
  • Succinyl CoA – used in porphyrin synthesis
  • Oxaloacetate – Porphyrins

Sakurai 2016

Answer by Sakurai