Mechanism of action of chalcone isomerase

Abstract

The pH profile of the rate of isomerization of 4,2′,4′-trihydroxychalcone catalyzed by chalcone isomerase shows dependence on the basic form of a group with a p K of 7.25. The same pH dependence is seen for the reverse reaction. Enzyme activity is lost in the presence of diethylpyrocarbonate at pH 6.0. In the presence of 20% formamide in imidazole buffers, the p K for the forward reaction is modified by a second p K of 7.1. This behavior represents a perturbed p K of a neutral acid group and is attributable to the 2′ hydroxyl of the chalcone substrate. These results suggest a mechanism of enzyme action involving nucleophilic addition of an imidazole group in the active site to the double bond followed by nucleophilic attack by the 2′ phenolate group, resulting in ring closure with inversion of configuration at C-2.