Archaeoglobus fulgidus neelaredoxin (Nlr) is an electron donor: superoxide oxidoreductase. The reaction of superoxide with reduced Nlr is almost diffusion-limited, but the overall efficiency for detoxifying superoxide in vivo depends on the rate of reduction of Nlr by electron donors. Here, we report the purification and characterization of the two type I rubredoxins from A. fulgidus (AF0880 and AF1349) and show that they act as efficient electron donors for neelaredoxin, in vitro, with a second-order rate constant of 10(7) M-1 s(-1) at 10 degrees C and PH 7.2. (c) 2005 Elsevier Inc. All rights reserved.