All of the following statements about the TCA cycle are correct EXCEPT:

Explanation

## Analysis of TCA Cycle Regulatory Enzymes ### Correct Statements (Options 0, 1, 2) **Option 0: Isocitrate dehydrogenase regulation** - Allosterically activated by ADP (energy demand signal) - Inhibited by NADH (product inhibition) - This is a classic regulatory mechanism in the cycle **Option 1: Succinyl-CoA synthetase (also called succinate thiokinase)** - Catalyzes: Succinyl-CoA + GDP + Pi → Succinate + GTP + CoA - Generates GTP (or ATP in some tissues), a high-energy phosphate bond - This is the only substrate-level phosphorylation in the TCA cycle - ✓ Correct statement **Option 2: Malate dehydrogenase** - Catalyzes: Malate + NAD^+^ ⇄ Oxaloacetate + NADH + H^+^ - Final step of the cycle, regenerating oxaloacetate - Reaction is reversible (ΔG°' ≈ +29.7 kJ/mol, but driven forward by removal of products) - ✓ Correct statement ### Incorrect Statement (Option 3) — THE ANSWER **Option 3: Citrate synthase as rate-limiting enzyme** - Citrate synthase catalyzes the first committed step: Acetyl-CoA + Oxaloacetate → Citrate + CoA - **Inhibited by ATP, NADH, and succinyl-CoA** (negative feedback) - **Activated by ADP and Ca^2+^** (energy demand) - ~~Citrate synthase is NOT the primary rate-limiting enzyme of the TCA cycle~~ - **Isocitrate dehydrogenase is the primary rate-limiting enzyme**, not citrate synthase - Although citrate synthase is regulated, it is not THE rate-limiting step ## Key Regulatory Points | Enzyme | Substrate-Level Phosphorylation | Primary Rate-Limiting | Key Inhibitors | |--------|----------------------------------|----------------------|----------------| | Citrate synthase | No | Secondary | ATP, NADH, succinyl-CoA | | Isocitrate dehydrogenase | No | **Yes** | NADH, ATP | | α-Ketoglutarate dehydrogenase | No | Secondary | NADH, ATP, succinyl-CoA | | Succinyl-CoA synthetase | **Yes (GTP)** | No | — | **Key Point:** The TCA cycle has THREE regulatory enzymes (citrate synthase, isocitrate dehydrogenase, α-ketoglutarate dehydrogenase), but isocitrate dehydrogenase is the PRIMARY rate-limiting step. **High-Yield:** Citrate synthase is regulated but is NOT the rate-limiting enzyme — this is a common NEET PG trap.