Hydroxylases catalyze the insertion of a hydroxyl functional group into unactivated aliphatic C-H bonds while halogenases catalyze the insertion of a chloride/bromide functionality into such bonds. The difference in reactivity of the two enzymes is enabled by differences in their ironbinding primary coordination sphere. Hydroxylases bind iron via a 2 His/1 Carboxyl (Asp/Glu) facial triad analogous to halogenases except that in halogenases the carboxyl ligand is replaced by a non-polar Ala or Gly residue. This amino acid substitution in halogenases enables the steric and electrostatic accommodation of an iron coordinated halide anion that is essential for halogenation reactivity. Both hydroxylases and halogenases have been suggested to operate via a similar catalytic mechanism (Fig ). Once 2OG, iron, and substrate are bound to the enzyme, a conformational change is triggered that facilitates molecular oxygen binding to the iron center. Next, oxidative decarboxylation of the 2OG co-substrate results in the cleavage of oxygen's O-O bond and the formation of a high-valent ferryl intermediate (Fe IV =O). The ferryl intermediate abstracts a hydrogen atom from the substrate, resulting in a ferric-hydroxyl (Fe III -OH) species and a substrate radical. At this point, the mechanisms for the hydroxylases and halogenases diverge. In hydroxylases, the hydroxyl group bound to the iron will "rebound" and combine with the substrate radical to give a hydroxylated product. In halogenases, the protein positions the halide ligand closer to the substrate radical than the hydroxyl ligand. This favors radical combination with the halide ligand resulting in a halogenated product, though a minor hydroxylated product is also possible through hydroxyl rebound.