Topic 4: Krebs Cycle

Krebs Cycle

• Occurs in the mitochondrial matrix
• Occurs twice per glucose and once for each acetyl CoA made during pyruvate oxidation
• NADH and FADH2 under go OP (ETC) to form more ATP

Krebs Cycle Steps

• Step 1:
  • Acetyl-CoA bonds to a 4 carbon structure called oxaloacetate to form citrate (a 6-carbon compound)
  • Requires action of the enzyme citrate synthetase and water (hydrolysis)
  • CoA-SH is released
• Step 2:
  • Citrate then undergoes isomerization by the action of the enzyme aconitase to form isocitrate
  • The hydroxyl group is moved from one carbon to another
• Step 3:
  • Through the action of isocitrate dehydrogenase, isocitrate is oxidized by NAD+ which gains 2e-, being reduced to NADH (redox reaction)
  • Carbon dioxide is also lost to form a 5-carbon molecule called a-ketoglutarate
• Step 4:
  • Through the action of a-ketoglutarate dehydrogenase complex, a-ketoglutarate is oxidized by NAD+ which gains 2e-, being reduced to NADH (redox reaction)
  • The a-ketoglutarate loses a CO2 to form the 4-carbon molecule succinyl-CoA
• Step 5:
  • CoA is lost to form succinate using the enzyme succinyl-CoA synthetase
  • The conversion of the enzyme creates enough free energy to form and ATP molecule
  • GDP picks up an inorganic phosphate from the matrix fluid and forms GTP
  • The GTP releases the phosphate to ADP forming ATP
• Step 6:
  • Succinate is oxidized to fumarate by FAD, which gains 2e- thus being reduced to FADH2 (redox reaction)
  • This occurs through the action of the enzyme succinate dehydrogenase
• Step 7:
  • Fumarate undergoes hydrolysis (addition of water) through the action of the enzyme fumarase and malate is formed
• Step 8:
  • Malate is oxidized to oxaloacetate by NAD+ which gains 2e- and is reduced to NADH (redox reaction) through the action of the enzyme malate dehydrogenase
  • Oxaloacetate bonds to the NEXT Acetyl-CoA to form citrate and the cycle continues

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