Both carbon dioxide and oxygen have affinity with the enzyme of Rubisco. This is competitive, however. It is more closely related to CO₂ than to oxygenation. It is the relative CO₂ and O₂ concentration that affects Rubisco’s binding to a particular molecule. Since it acts as both carboxylase and oxygenase, RUBP carboxylase–Oxygenase is better referred to.

Rubisco binds CO₂ to 3PGA in C3 plants. In C3 plants, Rubisco is binding with O₂ in a process known photorespiration to generate phosphoglycerate and phosphoglycerate. Photo vacuum reduces carbon attachment in C3 plants

Rubisco catalyses the production of oxaloacetic acid in C4 plants due to the increase in carbon dioxide levels in the vascular bundle. In this case, photorespiration is avoided and carbon fixation is more efficient.