Through the carboxylation of phosphoenolpyruvate (PEP) it forms oxaloacetate, a four-carbon dicarboxylic acid source for the tricarboxylic acid cycle. oxaloacetate + phosphate = hydrogencarbonate + phosphoenolpyruvate By light-reversible phosphorylation. Photosynthesis; C4 acid pathway. Homotetramer. Tobacco, a typical C3 plant, shows characteristics of C4 photosynthesis in cells of stems and petioles that surround the xylem and phloem. Belongs to the PEPCase type 1 family.