Fig. 2.

Structure of the FMN-binding pocket. (A) Conformation of the purine-rich region surrounding the isoalloxazine chromophore, as observed in the structures of apo-6WJR (cyan) and holo-3F2Q (magenta). In the absence of FMN, junction residues U61, A48, and A49 reposition to form base stacking interactions (G47, A48, and A49), stabilized by non-WC base-pairing between A48 and U61, (B-C) FMN-phosphate-coordinating side of the binding site depicting hydrogen-bond interactions coordinating a sulfate (B) or phosphate (C) ion, as observed in the respective crystal structures of apo-6WJR and apo-6WJS, which mimic the phosphate moiety of FMN in the holo structure. (D) Structure of the FMN-binding pocket of holo-3F2Q (magenta), showing the coordination of the FMN ligand and a Mg²⁺ ion (black sphere). The superimposed structure of apo-6WJR (cyan) is included for reference. FMN is depicted as yellow ball-and-stick. Hydrogen-bond and ion-coordinating interactions are drawn as a black dashed lines.