RuBisCo carries on a dual role as a carboxylase as well as oxygenase. Explain both the reactions. Under what conditions does this shift in role occurs ?

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Solution

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 characterized 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. This shift in roles occurs when the concentration of either $C O_{2}$ or $O_{2}$ increases. 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 increases the concentration of $C O_{2}$ at the enzyme site. So, RuBisCO functions as a carboxylase minimizing the oxygenase activity.

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