Mechanistic studies on the intramolecular one-electron transfer between the two flavins in the human endothelial NOS reductase domain

The object of this study was to clarify the mechanism of electron transfer in the human endothelial nitric oxide synthase (eNOS) reductase domain using recombinant eNOS reductase domains; the FAD/NADPH domain containing FAD- and NADPH-binding sites and the FAD/FMN domain containing FAD/NADPH-, FMN-, and a calmodulin-binding sites. In the presence of molecular oxygen or menadione, the reduced FAD/NADPH domain is oxidized via the neutral (blue) semiquinone (FADH), which has a characteristic absorption peak at 520 nm. The FAD/NADPH and FAD/FMN domains have high activity for ferricyanide, but the FAD/FMN domain has low activity for cytochrome c. In the presence or absence of calcium/calmodulin (Ca2+/CaM), reduction of the oxidized flavins (FAD–FMN) and air-stable semiquinone (FAD–FMNH) with NADPH occurred in at least two phases in the absorbance change at 457 nm. In the presence of Ca2+/CaM, the reduction rate of both phases was significantly increased. In contrast, an absorbance change at 596 nm gradually increased in two phases, but the rate of the fast phase was decreased by approximately 50% of that in the presence of Ca2+/CaM. The air-stable semiquinone form was rapidly reduced by NADPH, but a significant absorbance change at 520 nm was not observed. These findings indicate that the conversion of FADH2–FMNH to FADH–FMNH2 is unfavorable. Reduction of the FAD moiety is activated by CaM, but the formation rate of the active intermediate, FADH–FMNH2 is extremely low. These events could cause a lowering of enzyme activity in the catalytic cycle.