Flavodiiron Proteins: Nitric Oxide and/or Oxygen Reductases

Flavodiiron proteins (FDPs) have been isolated from and identified in genomes of bacteria, archaea, and a restricted group of pathogenic protozoa, mostly anaerobes or microaerophiles. The current knowledge on this protein family suggests alternative or complementary roles in nitric oxide (NO) and/or oxygen detoxification in anaerobes. These enzymes couple substrate (NO, O2) reduction to the oxidation of NAD(P)H or F420H2 through electron transfer chains with varying complexity. The first crystallographic structure of an FDP, the Desulfovibrio gigas rubredoxin:oxygen oxidoreductase (Dg_ROO), revealed the structural prototype of this family and the identification of common amino acid sequence motifs. The common functional unit of the FDP family is a ‘head-to-tail' homodimer, each monomer comprising two structural domains: an N-terminal metallo-β-lactamase-like domain, where a nonheme diiron center is embedded (being the active site of NO/O2 reduction), and a C-terminal flavodoxin-like domain, harboring a flavin mononucleotide (FMN) moiety. The homodimer ‘head-to-tail' arrangement places the diiron from each monomer in close proximity with the FMN from its opposing monomer, allowing efficient electron transfer between cofactors, otherwise impaired by their large distance within the same monomer. Herein are summarized structural and functional features of FDPs from several organisms, which provide clues to an understanding of their role in microbial nitrosative and/or oxidative stress resistance.