Abstract:
The modified flavin coenzyme F420 is found in a restricted number of microorganisms. It is widely distributed in mycobacteria, however, where it is important in energy metabolism, and in Mycobacterium tuberculosis (Mtb) is implicated in redox processes related to non-replicating persistence. In Mtb, the F420- dependent glucose-6-phosphate dehydrogenase FGD1 provides reduced F420 for the in vivo activation of the nitroimidazopyran prodrug PA-824, currently being developed for anti-tuberculosis therapy against both replicating and persistent bacteria. The structure of M. tuberculosis FGD1 has been determined by x-ray crystallography both in its apo state and in complex with F420 and citrate at resolutions of 1.90 and 1.95 A˚ , respectively. The structure reveals a highly specific F420 binding mode, which is shared with several other F420-dependent enzymes. Citrate occupies the substrate binding pocket adjacent to F420 and is shown to be a competitive inhibitor (IC50 43 M). Modeling of the binding of the glucose 6-phosphate (G6P) substrate identifies a positively charged phosphate binding pocket and shows that G6P, like citrate, packs against the isoalloxazine moiety of F420 and helps promote a butterfly bend conformation that facilitates F420 reduction and catalysis.
