Which enzyme catalyzes the initial carboxylation of phosphoenolpyruvate with bicarbonate to form oxaloacetate in the described pathway?

• A. Pyruvate carboxylase
• B. Phosphoenolpyruvate carboxylase
• C. Malate dehydrogenase
• D. Citrate synthase

Answer: (B)

Carboxylation of a phosphoenolpyruvate molecule with bicarbonate to form oxaloacetate is carried out by phosphoenolpyruvate carboxylase. This enzyme fixes CO2 (supplied as bicarbonate) directly onto PEP to yield oxaloacetate, a key anaplerotic step that replenishes TCA cycle intermediates. It operates on PEP rather than pyruvate, and does so without requiring ATP, which distinguishes it from other carboxylases.

Pyruvate carboxylase would convert pyruvate to oxaloacetate (not PEP), needing ATP. Malate dehydrogenase converts between oxaloacetate and malate, not a carboxylation step. Citrate synthase uses oxaloacetate plus acetyl-CoA to form citrate, also not a carboxylation.