All of the following TCA cycle intermediates can be replenished by anaplerotic reactions EXCEPT:

Explanation

## Anaplerotic Reactions: Replenishing TCA Cycle Intermediates **Key Point:** Anaplerotic reactions ('ana' = up; 'plerotic' = to fill) replenish TCA cycle intermediates that are withdrawn for biosynthetic pathways. Acetyl-CoA is NOT a TCA intermediate that can be replenished — it is a substrate that enters the cycle. ### Correct Anaplerotic Pathways (Options 0, 1, 2) | Intermediate | Anaplerotic Pathway | Enzyme(s) | Substrate | |--------------|-------------------|-----------|----------| | **Oxaloacetate** | Pyruvate → OAA | Pyruvate carboxylase | Pyruvate + CO₂ + ATP | | **Succinyl-CoA** | Odd-chain amino acids & propionyl-CoA | Propionyl-CoA carboxylase → Methylmalonyl-CoA mutase | Propionyl-CoA (from odd-chain fatty acids, valine, isoleucine, methionine, threonine) | | **α-Ketoglutarate** | Glutamate → α-KG | Glutamate dehydrogenase | Glutamate (from amino acid catabolism) | **Option 0: Oxaloacetate via pyruvate carboxylase** - ✓ Correct anaplerotic reaction - Most important anaplerotic pathway - Catalyzes: Pyruvate + CO₂ + ATP → Oxaloacetate + ADP + Pi - Biotin-dependent carboxylase **Option 1: Succinyl-CoA via propionyl-CoA carboxylase and methylmalonyl-CoA mutase** - ✓ Correct anaplerotic reaction - Pathway: Propionyl-CoA → Methylmalonyl-CoA → Succinyl-CoA - Methylmalonyl-CoA mutase requires B₁₂ (cobalamin) as cofactor - Sources: odd-chain fatty acids, branched-chain amino acids (valine, isoleucine), methionine, threonine **Option 2: α-Ketoglutarate via glutamate dehydrogenase** - ✓ Correct anaplerotic reaction - Catalyzes: Glutamate + NAD^+^ ⇄ α-Ketoglutarate + NADH + NH₄^+^ - Reversible reaction - Glutamate comes from amino acid catabolism ### Incorrect Statement (Option 3) — THE ANSWER **Option 3: Acetyl-CoA via pyruvate dehydrogenase** - ~~Acetyl-CoA is NOT a TCA cycle intermediate~~ - **Acetyl-CoA is a substrate/entry point into the TCA cycle**, not an intermediate - Pyruvate dehydrogenase catalyzes: Pyruvate + CoA + NAD^+^ → Acetyl-CoA + CO₂ + NADH - This is an oxidative decarboxylation, NOT an anaplerotic reaction - **Anaplerotic reactions replenish intermediates withdrawn from the cycle; acetyl-CoA is not withdrawn — it is continuously supplied from carbohydrate, fat, and amino acid metabolism** - Acetyl-CoA cannot be converted back to pyruvate or other TCA intermediates (no gluconeogenic pathway from acetyl-CoA in animals) ## Why Anaplerotic Reactions Matter **Clinical Pearl:** When TCA intermediates are withdrawn for biosynthesis (gluconeogenesis, amino acid synthesis, fatty acid synthesis), the cycle would slow down. Anaplerotic reactions replenish these intermediates so the cycle can continue functioning at full capacity. **High-Yield:** Acetyl-CoA is a **substrate**, not a **TCA intermediate**. Do not confuse pyruvate dehydrogenase (which generates acetyl-CoA) with anaplerotic enzymes (which replenish oxaloacetate, succinyl-CoA, and α-ketoglutarate). **Mnemonic:** **OSAG** = Oxaloacetate, Succinyl-CoA, α-Ketoglutarate, Glutamate — these are the intermediates and amino acids that can be replenished anaplerically.