Characterization of the Reduction of Selenate and Tellurite by Nitrate Reductases

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Abstract
Preliminary studies showed that the periplasmic nitrate reductase (Nap) ofRhodobacter sphaeroidesand the membrane-bound nitrate reductases ofEscherichia coliare able to reduce selenate and tellurite in vitro with benzyl viologen as an electron donor. In the present study, we found that this is a general feature of denitrifiers. Both the periplasmic and membrane-bound nitrate reductases ofRalstonia eutropha, Paracoccus denitrificans, andParacoccus pantotrophuscan utilize potassium selenate and potassium tellurite as electron acceptors. In order to characterize these reactions, the periplasmic nitrate reductase ofR. sphaeroidesf. sp.denitrificansIL106 was histidine tagged and purified. TheVmaxandKmwere determined for nitrate, tellurite, and selenate. For nitrate, values of 39 μmol · min−1· mg−1and 0.12 mM were obtained forVmaxandKm, respectively, whereas theVmaxvalues for tellurite and selenate were 40- and 140-fold lower, respectively. These low activities can explain the observation that depletion of the nitrate reductase inR. sphaeroidesdoes not modify the MIC of tellurite for this organism.