Slow Inactivation of Ribulosebisphosphate Carboxylase during Catalysis Is Caused by Accumulation of a Slow, Tight-Binding Inhibitor at the Catalytic Site

Abstract

The slow inactivation which accompanies catalysis by higher-plant ribulose-P2 carboxylase-oxygenase (Rubisco) in vitro was only partially reversed when the enzyme was gel filtered to remove small molecules. However, gel filtration or dialysis in the presence of high SO42- concentrations induced full recovery. This suggests that the inactivation is caused by a tight-binding inhibitor whose effective affinity is reduced by competition with SO42- ions, which are known to bind at the catalytic site. The involvement of an inhibitor was confirmed by observations that supernatants obtained after acid-precipitation of inactivated Rubisco were inhibitory when applied to fresh enzyme. The inhibitor bound slowly and tightly and showed strong negative cooperativity. The inhibitor was moderately unstable at pH 8.3, decaying with a half-life of several hours, but was more stable at pH 2. It was destroyed by phosphatase treatment but not by H2O2 or o-phenylenediamine, compounds which react with vicinal dicarbonyl groups. It did not contain a carbon atom derived from substrate CO2. Possibilities concerning the identity, genesis, and physiological relevance of this inhibitor are discussed.