Sulfolobus tokodaii ST2133 is characterized as a thioredoxin reductase-like ferredoxin:NADP⁺ oxidoreductase

The putative gene (st2133) for ferredoxin:NADP⁺ oxidoreductase (FNR) from Sulfolobus tokodaii, a thermoacidophilic crenarchaeon, was heterologously expressed. About 90 % of the purified product was a homodimer containing 0.46 mol FAD/mol subunit, and showing NADPH:DCPIP oxidoreductase activity, V _max being 1.38 and 21.8 U/mg (70 °C) in the absence and presence of 1 mM FMN. NADPH was a much better electron donor than NADH with various electron acceptors, such as oxygen, hydrogen peroxide, DCPIP, cytochrome c, and dithiobisnitrobenzoate. Most of the reactions were activated by 15- to 140-fold on addition of FMN, while FAD was 5-10 times less effective. Ferredoxin (Fd) from S. tokodaii served as an electron carrier in both Fd-dependent NADPH formation and NADPH-dependent Fd reduction. ST2133 belongs to the thioredoxin reductase-like protein family, which is slightly distantly related to FNR family proteins from bacteria, plants and man. This is the first report on FNR from a crenarchaeon, providing a clue to the recycling of Fd during archaeal metabolism.