Reaction of the Catalytic Subunit of Escherichia coli Aspartate Transcarbamylase with Permanganate Ion, a Reactive Structural Analogue of Phosphate Ion

A reaction between the catalytic subunit of Escherichia coli aspartate transcarbamylase and potassium permanganate is described. The reaction is accompanied by a rapid loss of the enzyme's catalytic activity even when stoichiometric amounts of permanganate and enzyme are used. Characterization of the inactivated enzyme has shown that when stoichiometric amounts of permanganate are used for the inactivation the only chemical change detected is oxidation of the enzyme's three cysteine thiol groups to sulfonate groups. The rate of inactivation of the enzyme by permanganate was found to be greatly decreased by the substrate carbamyl phosphate and to a lesser extent by the inhibitors phosphate, pyrophosphate, and succinate. Competition experiments show that the thiol groups of the catalytic subunit are approximately 45 times as reactive toward permanganate as the thiol group of 2-mercaptoethanol. This result is to be contrasted to the results of Vanaman and Stark (J. Biol. Chem., 245, 3565 (1970)) which showed that the thiol groups of catalytic subunit are hyporeactive or unreactive toward the many thiol reagents that they tested. The facility with which permanganate reacts with the protein's thiol groups may be a result of the structural resemblance between permanganate and the competitive inhibitor phosphate ion, a resemblance which is not shared by the other thiol reagents. Certain chemical properties of permanganate ion are pointed out which recommend it as a potential affinity reagent for phosphate binding sites in other proteins.