Carboxylation is the most crucial step of Calvin cycle, which involves the fixation of CO2 into a stable organic intermediate. Ribulose 1, 5-bisphosphate combines with CO2, and fixes it to a stable organic intermediate 3C compound called 3-phosphoglycerate ( 2 molecules). This reaction is catalysed by the enzyme RuBisCO (RuBP Carboxylase-Oxygenase). Since, this enzyme has high affinity for oxygen, it shows oxygenation activity when the CO2 concentration decreases. During high temperature, the RuBP binds to O2 and act as oxygenase enzyme, which forces CO2 to enter C2 cycle thus leading to photorespiration and loss of CO2.
