Question

RuBisCo is an enzyme that acts both as a carboxylase and oxygenase. Why do you think RuBisCO carries out more carboxylation in $C_4$ plants ?

Answer 1

RuBisCO stands for Ribulose Bisphosphate Carboxylase Oxygenase. It is formed from Ribulose bisphosphate (RuBP)- a five-carbon ketose sugar. RuBisCO is the most abundant enzyme on earth. It is chatacterised by the fact that its active site can bind both $C{O}_2$ and $O_2$, hence the name. It has a great affinity for $C{O}_2$ and $O_2$ and the binding is competitive. It is the relative concentration of $C{O}_2$ and $O_2$ that determines which of the two will bind to the enzyme. RuBisCO carries out more carboxylation in $C_4$ plants because these plants have a mechanism that increase the concentration of $C{O}_2$ at the enzyme site. During the $C_4$ pathway, when the $C_4$ acid from the mesophyll cells is broken down in the bundle sheath cells, it releases $C{O}_2$ - this results in increasing the intracellular concentration of $C{O}_2$. So, RuBisCO functions as a carboxylase and binds with plants and carry out more carboxylation.