Conformational switch triggered by α-ketoglutarate in a halogenase of curacin A biosynthesis

The CurA halogenase (Hal) catalyzes a cryptic chlorination leading to cyclopropane ring formation in the synthesis of the natural product curacin A. Hal belongs to a family of enzymes that use Fe2+, O-2 and alpha-ketoglutarate (alpha KG) to perform a variety of halogenation reactions in natural product biosynthesis. Crystal structures of the enzyme in five ligand states reveal strikingly different open and closed conformations dependent on alpha KG binding. The open form represents ligand-free enzyme, preventing substrate from entering the active site until both alpha KG and chloride are bound, while the closed form represents the holoenzyme with alpha KG and chloride coordinated to iron. Candidate amino acid residues involved in substrate recognition were identified by site-directed mutagenesis. These new structures provide direct evidence of a conformational switch driven by alpha KG leading to chlorination of an early pathway intermediate.