RuBisCo enzymes acts as both carboxylase and oxygenase. It carries out more carboxylation in the C4 plants as these plants have special mechanism that increases the concentration of the CO2 at the enzyme site. There is no photorespiration in the C4 plants and thus oxygen does not bind to the RuBisCo enzyme. All this minimizes the oxygenase activity of the enzyme and increase the carboxylase activity.