Formation of the tight-binding inhibitor, 3-ketoarabinitol-1,5-bisphosphate by ribulose-1,5-bisphosphate carboxylase/oxygenase is O2-dependent

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) (EC 4.1.1.39) not only catalyzes carboxylation and oxygenation of ribulose-1,5-bisphosphate (RuBP), but it can also act either as an epimerase or isomerase converting RuBP into xylulose-1,5-bisphosphate (XuBP) or 3-ketoarabinitol-1,5-bisphosphate (KABP), respectively, a process called misfire. XuBP is formed as a result of misprotonation at C3 of the RuBP-enediol. It is released from Rubisco active sites and accumulates in the reaction mixture. Increasing the amounts of CO₂ or O₂ decreases XuBP production. However, KABP synthesis, which has been proposed to be only a product due to C2 misprotonation of the RuBP-endiol, is dependent upon the presence of O₂. KABP remains tightly bound to Rubisco active sites after its formation, causing the loss of Rubisco activity (fallover). The results suggest that the non-stabilized form of the peroxy-intermediate in the oxygenase reaction can be converted in a backreaction to KABP and molecular oxygen. The stabilization of the peroxy-intermediate due to the presence of Mn²⁺ instead of Mg²⁺ eliminates the formation of KABP.