Mechanistic Investigation on C–C Bond Cleavage of Anthraquinone Catalyzed by an Atypical Nonheme Iron-Dependent Dioxygenase BTG13

An atypical nonheme iron-dependent dioxygenase BTG13 with a rare iron coordination of four histidine residues and a carboxylated-lysine (Kcx) was recently reported to catalyze the C4a–C10 bond cleavage of anthraquinone. However, the reaction mechanism of BTG13 remains elusive. Herein, the detailed mechanism of BTG13 is studied using molecular dynamics simulations and density functional theory calculations. The comprehensive mechanistic study shows that the most favorable pathway for the C–C bond cleavage of anthraquinone involves two unusual steps: (1) a hydrogen atom abstraction (HAA) from an sp3-hybridized carbon of the substrate by FeIII–O2•– and (2) an oxygen rebound to the substrate radical via homolytic O–O bond cleavage, which activates FeIII–OOH to form FeIV═O species. Furthermore, our results reveal that Kcx could increase the electron-donating ability of the ferrous iron, thereby boosting the activation of dioxygen to form FeIII–O2•– species and facilitating the following HAA and O–O bond cleavage processes. This study advances the current knowledge of reactions catalyzed by iron-dependent oxygenases.