Citric Acid Cycle – Enzymes & Key Processes

Ten vocabulary flashcards summarizing the main enzymes and energy-relevant steps of the citric acid cycle.

CAC Reaction 1

Via citrate synthase: Acetyl-CoA + oxaloacetate + H2O → citrate + CoA-SH; irreversible aldol condensation

CAC Reaction 2

Via Aconitase: citrate ↔ cis-aconitate + H2O ↔ isocitrate; reversible dehydration/ rehydration reaction

CAC Reaction 3

Via isocitrate dehydrogenase: isocitrate + NAD(P)+ → oxalosuccinate + NAD(P)H + H+ + Mn2+ (stabilizes enol intermediate) → CO2 + intermediate (stabilized by Mn2+) → a-Ketoglutarate; irreversible oxidative decarboxylation

CAC Reaction 4

Via a-ketoglutarate dehydrogenase complex (like PDH): a-ketoglutarate + CoA-SH + NAD+ → succinyl-CoA + NADH + CO2; irreversible oxidative decarboxylation

CAC Reaction 5

Via syccinyl-CoA synthetase: Succinyl-CoA + GDP + Pi → Succinate + GTP + CoA-SH; reversible substrate-level phosphorylation

Succinyl-CoA Synthetase mechanism

Phosphate added to Succinyl-CoA, high-energy Succinyl-phosphate; Enzyme-His removes phosphate, generating 3-Phospho-His + regenerating Succinate; GDP removes P from Enzyme-His, forming GTP and re-generating free Enzyme-His

CAC Reaction 6

Via Succinate Dehydrogenase: succinate + FAD → fumarate + FADH2 (B2 vitamin); reversible dehydrogenation

CAC Reaction 7

Via Fumarase: Fumarate + OH- → Carbanion transition state + H+ → L-Malate; reversible hydration

CAC Reaction 8

Via Malate Dehydrogenase: L-Malate + NAD+ → oxaloacetate + NADH + H+

Mnemonix device for CAC

Can I Keep Selling Seashells For Money, Officer? (Citrate → Isocitrate → a-Ketoglutarate → Succinyl-CoA → Succinate → Fumarate → L-Malate → Oxaloacetate)

Net CAC Production

3 NADH; 1 FADH2; 1 GTP; 2 CO2 per Acetyl-CoA