The modified
flavin coenzyme F(420) 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 F(420)-dependent
glucose-6-phosphate dehydrogenase FGD1 provides reduced
F(420) 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
F(420) and
citrate at resolutions of 1.90 and 1.95 A(,) respectively. The structure reveals a highly specific
F(420) binding mode, which is shared with several other F(420)-dependent
enzymes.
Citrate occupies the substrate binding pocket adjacent to
F(420) and is shown to be a competitive inhibitor (IC(50) 43 microm). 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
F(420) and helps promote a butterfly bend conformation that facilitates
F(420) reduction and catalysis.