Members of the cytochrome P450 family catalyze a variety of mono-oxygenase reactions, and for the eukaryotic membrane-bound members, NADPH is typically used as the reducing agent. The flavoprotein NADPH-cytochrome P450 reductase (CPR) enables electron transfer from NADPH to cytochrome P450 via its flavin cofactors. ATR2 is one of the two authentic CPR genes in the genome of the model plant Arabidopsis thaliana, and its product has been physiologically and kinetically characterized. Here, we report the 2.3 angstrom structure of Arabidopsis thaliana NADPH-cytochrome P450 reductase 2 (ATR2) and find that the position of the two flavin cofactors differs from that of other known CPR structures. Mutation of residues related to possible interflavin electron transfer retains the reductase activity of ATR2, which suggests a direct electron transfer pathway between the flavins. In contrast, mutation of a single residue (R708) mediating interdomain interaction abolishes this activity. Because this residue is only conserved in plant CPRs, we speculate a plant-specific working mechanism as observed in ATR2
