Krebs Cycle Enzymes in Order, From Citrate Synthase to Malate Dehydrogenase

The cycle runs citrate synthase, aconitase, isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, malate dehydrogenase.

The eight steps

1. Citrate synthase: acetyl-CoA + oxaloacetate gives citrate.
2. Aconitase: citrate to isocitrate (isomerization).
3. Isocitrate dehydrogenase: isocitrate to alpha-ketoglutarate. Makes NADH and releases CO₂.
4. Alpha-ketoglutarate dehydrogenase: to succinyl-CoA. Makes NADH and releases CO₂.
5. Succinyl-CoA synthetase: to succinate. Makes GTP (or ATP).
6. Succinate dehydrogenase: to fumarate. Makes FADH₂. Embedded in the inner mitochondrial membrane (complex II).
7. Fumarase: adds water, fumarate to malate.
8. Malate dehydrogenase: malate to oxaloacetate. Makes NADH.

Per turn of the cycle

3 NADH, 1 FADH₂, 1 GTP, 2 CO₂. Per glucose, double it.

Regulated steps

Citrate synthase, isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase. High NADH and ATP slow them; ADP and Ca²⁺ speed them.

Common traps

• Putting FADH₂ at the wrong step. Only succinate dehydrogenase.
• Counting CO₂ at the wrong steps. Only steps 3 and 4.
• Forgetting the doubling per glucose.