how can RuBisCO combine to both CO2 and O2?

RuBP Carboxylase-Oxygenase  (Ribulose- 1, 5-bisphosphate Carboxylase-Oxygenase), also called RUBISCO has dual function. it has both carboxylasse and oxygenase activity. It is the most abundant  enzyme complex found on the earth. It can act on the two substrates: Carbon dioxide (carboxylase activity- for carbon dioxide assimilation into glucose during photosynthesis) and Oxygen (oxygenase activity-causing photorespiration, decreasing the photosynthetic efficiency of plants). can combine to either CO​₂ or O₂. 

There are several factors which cause RuBisCo to choose its substrate. e.g. during low tempertaure and high CO₂ : O₂, CO₂ is the substrate for this enzyme while high temperature and low CO₂ : O₂ ratio cause RuBisCO to take up oxygen and perform photorespiration. During Photorespiration, already fixed carbon (by Calvin cycle) are used and energy is wasted.

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