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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 ?


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 chatacterised by the fact that its active site can bind both $$ CO_2$$ and $$O_2$$, hence the name. It has a great affinity for $$ CO_2$$ and $$O_2$$ and the binding is competitive. It is the relative concentration of $$CO_2$$ 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 $$CO_2$$ 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 $$CO_2$$ - this results in increasing the intracellular concentration of $$CO_2$$. So, RuBisCO functions as a carboxylase and binds with plants and carry out more carboxylation.

Biology

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