Phosphoenolpyruvate (PEP) is the primary C O _ 2 acceptor in:

PEP is a 3C compound which serves as primary $C{O}_{2}$ acceptor in the mesophyll cell cytoplasm of ${C}_{4}$ plants like Maize, Sugarcane, Sorghum etc.

PEP is able to rapidly fix carbon at very low concentrations of hydrogen carbonate in presence of Phosphoenolpyruvate carboxylase enzyme the addition of bicarbonate$( HC{O}_{3}^{-} )$ to phosphoenolpyruvate $( PEP )$ to form the four-carbon compound oxaloacetate and inorganic phosphate

$PEP + HC{O}_{3}^{-} \to oxaloacetate + Pi$