Flavodoxin (FDX) is a bacterial protein which contains flavin mononucleotide (FMN) or flavin adenine dinucleotide (FAD).  It participates in nitrogen metabolism and reduces N2 to NH3 while changing from its oxidized quinone state FDX-ox via semiquinone FDX-sq to its reduced hydroquinone state FDX-red[1].  NrdI is a FDX which is involved in ribonucleotide reduction.  WrbA (Tryptophan Repressor-Binding Protein) is also FDX.  In E. coli there are 2 distinct flavodoxins: FDX I and II.
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- . Water molecules shown as red spheres.
  3D Structures of Flavodoxin 
Flavodoxin 3D structures