When CO₂ is added to PEP, the first stable product synthesised is:

The first stable product synthesized is OAA i.e. oxaloacetate.

In C4 plants, initial fixation of CO₂ or carboxylation occurs in mesophyll cells. The chloroplasts of mesophyll cells possess enzyme PEP carboxylase for initial fixation of CO₂. The primary acceptor of CO₂ is phosphoenol pyruvate. PEP combines with CO₂ in the presence of PEP carboxylase to form oxaloacetate.