Name the two important enzymes of $C_3$ and $C_4$ pathways, respectively. What important role do they play in fixing $CO_2$?

(N/A) In $C_3$ plants, the enzyme $RuBisCO$ (Ribulose-1,5-bisphosphate carboxylase-oxygenase) is present. It catalyzes the carboxylation of $5C$ $RuBP$ to form $3$-phosphoglycerate $(PGA)$, which is the first stable product of the $C_3$ pathway.
$5C \text{ } RuBP + CO_2 \xrightarrow{RuBisCO} 2 \text{ } PGA$
In $C_4$ plants, the enzyme $PEP$ carboxylase $(PEPCase)$ is present. It catalyzes the carboxylation of phosphoenolpyruvate $(PEP)$ to form oxaloacetic acid $(OAA)$, a $4C$ compound, which is the first stable product of the $C_4$ pathway.
$PEP + CO_2 \xrightarrow{PEPCase} OAA$