## Citrate Synthase: The Gateway Enzyme ### Role in TCA Cycle Initiation Citrate synthase catalyzes the condensation of acetyl-CoA (2-carbon unit) with oxaloacetate (4-carbon) to form citrate (6-carbon), marking the entry point of acetyl units into the TCA cycle. ### Key Biochemical Features - **Reaction type:** Aldol condensation followed by hydrolysis - **Cofactor:** No cofactor required; uses the energy from thioester bond cleavage - **Regulation:** Allosterically inhibited by ATP, NADH, and succinyl-CoA (feedback inhibition); activated by ADP and Ca²⁺ - **Location:** Mitochondrial matrix **Key Point:** Citrate synthase is the rate-limiting enzyme of the TCA cycle and represents the primary control point for cycle flux. **High-Yield:** This is the first irreversible step of the cycle; the reaction is energetically favorable due to hydrolysis of the high-energy thioester bond of acetyl-CoA. ### Thermodynamics The reaction is highly exergonic (ΔG°' ≈ −31.4 kJ/mol), making it essentially irreversible under physiological conditions. **Mnemonic:** **CIT** — **C**ondense **I**nitial **T**wo-carbon unit (acetyl-CoA with oxaloacetate). 
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