Rubisco is an enzyme that acts both as a carboxylase and oxygenase. Why do you think Rubisco carries out more carboxylation in C4 plants?
Rubisco, i.e., ribulose biphophate carboxylase and oxygenase. This is because its active site can bind to both CO2 and O2. But actually rubisco has much greater affinity for CO2, than O2. The relative concentration of CO2 and O2 determines which of the two will bind to the enzyme. In case of C4 plants, since there is a high concentration of CO2 plants, at the enzyme site, photorespiration does not occur. Therefore rubisco carries out more carboxylation in C4 plants.