Project Summary Iron and 2-oxoglutarate-dependent (Fe/2OG) enzymes, representing a superfamily of non-heme mononuclear iron-containing (NHM-Fe) enzymes, have garnered strong research interests from fundamental enzyme mechanism studies to bioengineering/biocatalysis explorations in recent years due to their exceedingly diverse catalytic reactivities and simple enzyme architectures. Radical halogenation reactions via C-H bond activation catalyzed by Fe/2OG halogenases are particularly attractive for chemical synthesis and biocatalysis applications, since these enzymes can install carbon-halide bonds in a regio- and stereo-specific manner, a feat that has yet to be achieved by organic synthetic methodology. As revealed by the mechanistic studies of carrier protein dependent Fe/2OG halogenases, the key step in the radical halogenation mechanism is the selective halide radical transfer from the hydroxo-Fe(III)-halide intermediate to the substrate radical generated by the key reactive species, the ferryl (Fe(IV)=O) intermediate. However, a consensus mechanism to explain the selective halide transfer in Fe/2OG halogenases has not been reached, particularly the controlling factors to avoid hydroxyl radical transfer to lead to hydroxylation reaction are not fully revealed. Additionally, the reasons why Fe/2OG enzymes cannot perform fluorination reaction are completely unknown. In this project, we will bridge these knowledge gaps by studying two newly discovered carrier protein-independent Fe/2OG halogenases that catalyze chlorination reactions to generate halogenated nucleotide natural products and halogenated freestanding amino acids. By using an integrative approach consisting of mechanistic probe design and synthesis, enzyme product structural determination via LC-MS and NMR analysis, transient enzyme kinetics, advanced spectroscopic characterization and molecular dynamics simulations, we will elucidate the influence of protein substrate interactions and dynamics in controlling efficient halogenation, explore the effect of different iron-bound anions (e.g. Cl- vs. F-) to the electronic structure and the reactivity of the ferryl intermediate, test new chemical strategies to enable fluorination in Fe/2OG enzymes, and expand the substrate scope of these enzymes for potential synthetic applications. Given the importance of halogen-containing organic molecules in the modern pharmaceutical and agrochemical applications, mechanistic elucidation of these newly discovered halogenases will lay scientific foundation for future biocatalytic applications of these unique enzymes.