Redox properties of the oxygen-detoxifying flavodiiron protein from the human parasite Giardia intestinalis

Flavodiiron proteins (FDPs) are enzymes identified in prokaryotes and a few pathogenic protozoa, which protect microorganisms by reducing O(2) to H(2)O and/or NO to N(2)O. Unlike most prokaryotic FDPs, the protozoan enzymes from the human pathogens Giardia intestinalis and Trichomonas vaginalis are selective towards O(2). UV/vis and EPR spectroscopy showed that, differently from the NO-consuming bacterial FDPs, the Giardia FDP contains an FMN with reduction potentials for the formation of the single and the two-electron reduced forms very close to each other (E(1) = -66 +/- 15 mV and E(2) = -83 +/- 15 mV), a condition favoring destabilization of the semiquinone radical. Giardia FDP contains also a non-heme diiron site with significantly up-shifted reduction potentials (E(1) = +163 20 mV and E(2) = +2 +/- 20 mV). These properties are common to the Trichomonas hydrogenosomal FDP, and likely reflect yet undetermined subtle structural differences in the protozoan FDPs. accounting for their marked O(2) specificity. (C) 2009 Elsevier Inc. All rights reserved.