Citric Acid Cycle

Step 1 of Citric Acid Cycle

• Acetyle CoA + oxaloacetate → citrate + CoA

• Main enzyme is citrate synthase

• Nonreversible

Step 2 of Citric Acid Cycle

• Citrate ↔ isocitrate

• Main enzyme is aconitase

Step 3 of Citric Acid Cycle

• Isocitrate + NAD⁺ ↔ alpha-ketoglutarate + NADH + H⁺ + CO₂

• Main enzyme is isocitrate dehydrogenase

Step 4 of Citric Acid Cycle

• Alpha-ketoglutarate + CoA + NAD⁺ → succinyl CoA + NADH + H⁺ + CO₂

• Main enzyme is alpha-ketoglutarate dehydrogenase

• Nonreversible

Step 5 of Citric Acid Cycle

• Succinyl CoA + ADP + Pi ↔ succinate + CoA + ATP

• Main enzyme is succinate thiokinase

Step 6 of Citric Acid Cycle

• Succinate + FAD ↔ fumarate + FADH₂

• Main enzyme is succinate dehydrogenase

Step 7 of Citric Acid Cycle

• Fumarate + H₂O ↔ malate

• Main enzyme is fumarase

Step 8 of Citric Acid Cycle

• Malate + NAD⁺ ↔ oxaloacetate + NADH + H⁺

• Main enzyme is malate dehydrogenase

• Oxaloacetate is reformed to start step 1 of the cycle again

Citric Acid Cycle

• Aerobic process

• Acetyl CoA + ADP + Pi + 3NAD⁺ + FAD + H₂O → 2CO₂ + 3NADH + 3H⁺ + FADH₂ + ATP + CoA

• Main purpose is to convert acetyl CoA to NADH and FADH₂ (electron carriers) that are used in the respiratory chains to produce many ATP

• If energy is not needed, then acetyl CoA can used for fatty acids synthesis

• Other intermediates play a major roles in gluconeogenesis, transamination