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 is a key enzyme in the first significant phase of carbon fixation, a mechanism by which plants as well as other photosynthetic species convert ambient carbon dioxide into energy-rich molecules i.e. glucose.

Complete answer:
- RuBisCO refers to Carboxylase Oxygenase Ribulose Bisphosphate. It is formed from the five-carbon ketose sugar i.e. Ribulose Bisphosphate (RuBP).
- The most common enzyme on earth is RuBisCO. It is distinguished by the fact that both $O_2$ and $CO_2$ can be bound by its active site, thus the name. It has a strong affinity and the attachment is competitive for $O_2$ and $CO_2$.
- It is the comparative $CO_2$ and $O_2$ concentration that decides which of the two is going to bind to the enzyme. In $C_4$, RuBisCO does further carboxylation in plants because there is a mechanism for these plants which increases the $CO_2$ concentration at the enzyme site.
- As the $C_4$ acid from the mesophyll cells is degraded in the bundle sheath cells during the $C_4$ pathway, it produces $CO_2$-which results in an increased intracellular $CO_2$ concentration. RuBisCO, then, serves as a carboxylase and binds to plants and carries out further carboxylation.

Note: Biologically, RuBisCO is important as it catalyses the primary chemical response through which inorganic carbon reaches the biosphere. RuBisCO is the most major form in leaves, accounting for C3 plants for 50% of soluble leaf protein and in $C_4$ plants for 30% of soluble leaf protein.