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$\mathrm{RuBisCO}$ is an enzyme that acts both as carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $\mathrm{C}_4$ plants.
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RuBisCO or Ribulose bisphosphate carboxylase - oxygenase enzyme can bind to both $\mathrm{CO}_2$ and $\mathrm{O}_2$. This binding is
competitive. The relative concentration of $\mathrm{CO}_2$ and $\mathrm{O}_2$ determines which one of the two will bind to the enzyme. In $\mathrm{C}_4$ plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of $\mathrm{CO}_2$ at the enzyme site. This takes place when oxaloacetic acid is broken down in the bundle sheath cells to release $\mathrm{CO}_2$. It results in increasing intracellular concentration of $\mathrm{CO}_2$. This ensures that the RuBisCO functions as carboxylase and minimising the oxygenase activity.
competitive. The relative concentration of $\mathrm{CO}_2$ and $\mathrm{O}_2$ determines which one of the two will bind to the enzyme. In $\mathrm{C}_4$ plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of $\mathrm{CO}_2$ at the enzyme site. This takes place when oxaloacetic acid is broken down in the bundle sheath cells to release $\mathrm{CO}_2$. It results in increasing intracellular concentration of $\mathrm{CO}_2$. This ensures that the RuBisCO functions as carboxylase and minimising the oxygenase activity.
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