Question

Why is the RuBisCo enzyme more appropriately called $RUBP$
Carboxylase-Oxygenase and what important role does it play in
photosynthesis?

Answer 1

RubisCo :

The RubisCo enzyme fixes atmospheric $C{O}_2$ with Ribulose$1,5$ bisphosphate $\left(RuBP\right)$. This enzyme possesses both carboxylase and oxygenase activity.
When the concentration of $C{O}_2$ reduces in the atmosphere, it switches to its oxygenase activity and fixes oxygen instead of $C{O}_2$

Role of RubisCo in photosynthesis :

RubisCo plays an important role in photosynthesis.
It is the primary enzyme that fixes atmospheric $C{O}_2$ in the Calvin cycle and reduces it for further steps involved in photosynthesis.
It is present in both $C3$ and $C4$ plants.

1. In C₃ plants, when $O_2$ concentration increases, RuBisCO acts as an oxygenase enzyme and enters $C_2$ cycle (photorespiration), where RuBP binds with $O_2$ to form phosphoglycolate $(2-$carbon) and phosphoglycerate $(3-$carbon).

2. In $C_4$ plants RuBP acts only as a carboxylase and accepts liberated $C{O}_2$ to form phosphoglyceric acid.