Citrate synthase catalyzes the condensation of which two substrates to form citrate?

• A. Oxaloacetate and Acetyl-CoA
• B. Pyruvate and Acetyl-CoA
• C. Malate and CoA
• D. Succinate and CoA

Answer: (A)

At the start of the citric acid cycle, citrate synthase catalyzes a condensation between oxaloacetate and acetyl-CoA. The two-carbon acetyl unit from acetyl-CoA combines with the four-carbon oxaloacetate to form citrate, and CoA-SH is released. This gives citrate as a six-carbon molecule and sets up the cycle for the subsequent oxidation steps. The other pairings don’t fit this step: pyruvate and acetyl-CoA wouldn’t directly yield citrate, malate with CoA isn’t the substrates for this condensation, and succinate with CoA would lead to a different intermediate in a later reaction.