Bifunctional Substrate Activation via an Arginine Residue Drives Catalysis in Chalcone Isomerases
Chalcone isomerases are plant enzymes that perform enantioselective oxa-Michael cyclizations of 2'-hydroxy-chalcones into flavanones. An X-ray crystal structure of an enzyme-product complex and molecular dynamics simulations reveal an enzyme mechanism wherein the guanidinium ion of a conserved arginine positions the nucleophilic phenoxide and activates the electrophilic enone for cyclization through Bronsted and Lewis acid interactions. The reaction terminates by asymmetric protonation of the carbanion intermediate syn to the guanidinium. Interestingly, bifunctional guanidine- and urea-based chemical reagents, increasingly used for asymmetric organocatalytic applications, are synthetic counterparts to this natural system. Comparative protein crystal structures and molecular dynamics simulations further demonstrate how two active site water molecules coordinate a hydrogen bond network that enables expanded substrate reactivity for 6'-deox-ychalcones in more recently evolved type-2 chalcone isomerases.