Ribulose bisphosphate carboxylase/oxygenase (Rubisco) is an enzyme found in green plants - VCE - SSCE Biology - Question 7 - 2021 - Paper 1

This observation supports the hypothesis that the same active site on Rubisco is utilized for both reactions. If increasing oxygen concentration leads to a decrease in the rate of Reaction 2, it suggests that the binding of oxygen competes with the binding of carbon dioxide at the active site, thereby inhibiting the reaction involving carbon dioxide. Such competition indicates that both reactions are indeed using the same active site on the Rubisco enzyme.