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 Ca2+ (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
Table
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-YieldNEET PG
Citrate synthase is regulated but is NOT the rate-limiting enzyme — this is a common NEET PG trap.
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