Stereochemistry of the Enzymatic Carboxylation of Phosphoenolpyruvate

Using specifically labeled 3-3H-phosphoenolpyruvate, the stereochemistry of CO2 (or HCO3-) addition was determined for the reactions catalyzed by P-enolpyruvate carboxylase (from peanuts and Acetobacter xylinum), P-enolpyruvate carboxykinase (from pigeon liver), and P-enolpyruvate carboxytransphosphorylase (from Propionibacterium shermanii). In all cases the addition occurs from the same side of the plane of enzyme-bound P-enolpyruvate, the si side. These results relate the stereochemical course of the protons in these carboxylation reactions to that in the enolase, fumarase, and phosphoglucose isomerase reactions. Furthermore, the conservation of the addition in evolution implies that this stereochemistry may have significance to the reaction mechanism.