Introduction
One turn of citric acid cycle produces a specific set of energy‑rich molecules that are essential for cellular respiration. In each cycle, acetyl‑CoA derived from carbohydrates, fats, or proteins is oxidized, releasing carbon dioxide while reducing NAD⁺ and FAD and generating a molecule of guanosine triphosphate (GTP). Understanding exactly what one turn of citric acid cycle produces helps students grasp how the cell extracts energy from nutrients and connects glycolysis, the citric acid cycle, and oxidative phosphorylation into a coherent metabolic pathway Nothing fancy..
Steps
The citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) consists of eight enzymatic reactions that transform acetyl‑CoA and oxaloacetate back into oxaloacetate, completing one turn. Below is a concise list of the major steps and the molecules produced or consumed at each stage:
- Condensation – Acetyl‑CoA (2‑carbon) combines with oxaloacetate (4‑carbon) to form citrate (6‑carbon), catalyzed by citrate synthase.
- Isomerization – Citrate is rearranged to isocitrate by aconitase.
- First Oxidative Decarboxylation – Isocitrate dehydrogenase oxidizes isocitrate, producing NADH, releasing CO₂, and forming α‑ketoglutarate (5‑carbon).
- Second Oxidative Decarboxylation – α‑Ketoglutarate dehydrogenase complex oxidizes α‑ketoglutarate, yielding another NADH, a second CO₂, and succinyl‑CoA (4‑carbon).
- Substrate‑Level Phosphorylation – Succinyl‑CoA synthetase converts succinyl‑CoA to succinate, generating GTP (which can be readily converted to ATP).
- Oxidation – Succinate dehydrogenase oxidizes succinate to fumarate, reducing FAD to FADH₂.
- Hydration – Fumarase adds water to fumarate, producing malate.
- Second Oxidation – Malate dehydrogenase oxidizes malate to oxaloacetate, producing the third NADH of the cycle.
At the end of these eight reactions, oxaloacetate is regenerated, allowing the cycle to begin again
