Krebs Cycle/Citric Acid Cycle - 8 Reactions

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Where is the krebs cycle located? Why is it cyclical? What major thing happens (hint: glucose completely ____)?

Where is the krebs cycle located? Why is it cyclical? What major thing happens (hint: glucose completely ____)?

Location: Mitochondrial matrix
Cyclical: Product from 8th reaction = reactant in 1st reaction
Glucose is completely oxidized! (all carbons → CO₂)

Reaction 1

Acetyl Group (2-carbon) + Oxaloacetate (4-carbon) → Citrate (6-carbon)

• Enzyme: Citrate synthase

• Condensation reaction (needs water)

Reaction 2

Citrate (6-carbon) → Isocitrate (6-carbon)

• Enzyme: Aconitase

• Isomerization

☆ Reaction 3

Isocitrate (6-carbon) → α-ketglularate (5-carbon)

• Enzyme: Isocitrate dehydrogenase

• 1st redox reaction!

  • Isocitrate oxidized, 1 carbon released as CO₂

  • NAD+ reduced → NADH

☆ Reaction 4

α-ketoglutarate (5-carbon) → Succinyl CoA (4-carbon)

• Enzyme: Isocitrate dehydrogenase

• 2nd redox reaction!

  • Isocitrate oxidized, 1 carbon released as CO₂

  • NAD+ reduced → NADH

• ALL GLUCOSE CARBONS OXIDIZED (remaining 4-carbons from Acetyl CoA)

☆ Reaction 5

Succinyl CoA (4-carbon) → Succinate (4-carbon)

• Enzyme: Succinyl CoA Synthetase

• ATP synthesis!

  • Phosphate group from matrix displaces CoA → creates Succinate

  • Energy released → phosphate group added to GDP from matrix → GTP

  • GTP’s phosphate group → ADP → ATP

☆ Reaction 6

Succinate (4-carbon) → Fumarate (4-carbon)

• Enzyme: Succinate dehydrogenase

• 3rd redox reaction!

  • Succinate oxidized, lose 2 electrons + 2 protons

  • FAD reduced → FADH₂ 

    • Only enough energy to reduce FAD, not NAD+

Reaction 7

Fumarate (4-carbon) → Malate (4-carbon)

• Enzyme: Fumarase

• Condensation reaction (needs water)

☆ Reaction 8

Malate (4-carbon) → Oxaloacetate (4-carbon)

• Enzyme: Malate dehydrogenase

• 4th redox reaction!

  • Malate oxidized

  • NAD+ reduced → NADH

Krebs Cycle Final Products (after 2 cycles/1 glucose)

2 ATP
6 NADH
2 FADH₂