text: Phosphoenolpyruvate carboxylase (PEPC) is an enzyme that combines bicarbonate and phosphoenolpyruvate (PEP) to form the four-carbon acid, oxaloacetate. It is an important enzyme in C4 photosynthesis and anaplerotic pathways. It is also responsible for the position-specific enrichment of oxaloacetate, due to the equilibrium isotope effect of converting the linear molecule CO into the trigonal planar molecule HCO-, which partitions C into bicarbonate. Inside the PEPC enzyme, HCO- reacts 1.0022 times faster than  HCO- so that PEPC has a 0.22% kinetic isotope effect. This is not enough to compensate for the 13C enrichment in bicarbonate. Thus, oxaloacetate is left with a C-enriched carbon at the C4 position. However, the C1 site experiences a small inverse secondary isotope effect due to its bonding environment in the transition state, leaving the C1 site of oxaloacetate enriched in C. In this way, PEPC simultaneously partitions C into the C4 site and C into the C1 site of oxaloacetate, an example of multiple position-specific isotope effects.