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$\left(HC{O}_3^{-}\right)$ to phosphoenolpyruvate $\left(PEP\right)$ to form the four-carbon compound oxaloacetate and inorganic phosphate

$PEP+HC{O}_3^{-}\to oxaloacetate+Pi$