we know that the enzyme Rubisco use both oxygen and carbon dioxide as substrates,but the reasons behind such affinity for both substrates is not clear.
The enzyme, rubisco, not only initiates carbon fixation in the Calvin cycle; it also combines with oxygen to initiate photorespiration. As its name suggests (rubsiCO) the enzyme is both a carboxylase and an oxygenase. The active site of rubisco cannot distinguish the two similar substrates: O=C=O and O=O. The two reactions catalyzed by the same enzyme are diametrically oppposed to each other.