Which intermediate of the TCA cycle is regenerated at the end of each cycle, and how many carbon atoms does it contain?

Explanation

## TCA Cycle: The Regenerated Intermediate ### Oxaloacetate as the Cycle Catalyst **Key Point:** Oxaloacetate (OAA) is the 4-carbon intermediate that is regenerated at the end of each TCA cycle turn. It is not consumed; it acts catalytically to condense with acetyl-CoA and drive the cycle forward. ### Role in the Cycle **High-Yield:** - **Entry:** Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C) via citrate synthase - **Exit:** Malate (4C) → Oxaloacetate (4C) via malate dehydrogenase - **Net result:** One molecule of oxaloacetate is regenerated for every turn of the cycle ### Why This Matters **Clinical Pearl:** Because oxaloacetate is regenerated, the TCA cycle is catalytic, not stoichiometric. A small amount of oxaloacetate can oxidize unlimited acetyl-CoA. This is why oxaloacetate depletion (e.g., during prolonged fasting or starvation) impairs the cycle — gluconeogenesis consumes oxaloacetate, and it must be replenished via anaplerotic reactions (pyruvate carboxylase). ### Carbon Tracking Through the Cycle ```mermaid flowchart TD A["Acetyl-CoA<br/>(2C)"]:::outcome --> B["Citrate<br/>(6C)"]:::outcome C["Oxaloacetate<br/>(4C)"]:::outcome --> B B --> D["Isocitrate<br/>(6C)"]:::outcome D --> E["α-Ketoglutarate<br/>(5C)"]:::outcome E --> F["Succinyl-CoA<br/>(4C)"]:::outcome F --> G["Succinate<br/>(4C)"]:::outcome G --> H["Fumarate<br/>(4C)"]:::outcome H --> I["Malate<br/>(4C)"]:::outcome I --> C ``` **Mnemonic:** **"CAN'T REMEMBER KREBS?"** — Citrate, Aconitase, isocitrate, α-Ketoglutarate, succinyl-CoA, Succinate, Fumarate, Malate → back to Oxaloacetate (the 4C regenerated intermediate). ### Anaplerotic Reactions When oxaloacetate is depleted: - **Pyruvate carboxylase:** Pyruvate → Oxaloacetate (primary anaplerotic enzyme) - **Glutamate dehydrogenase:** Glutamate → α-ketoglutarate (indirect replenishment) - **PEP carboxykinase:** Also contributes in some tissues