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 CO_2, 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.