P450 enzymes constitute a family of monooxygenase enzymes that are involved
in the metabolism of a wide array of endogenous and xenobiotic compounds.
Several P450 enzymes have been classified by sequence similarities
as members of the CYP1A and CYP2A subfamilies. NADPH cytochrome P450
reductase is a microsomal enzyme responsible for the transfer of electrons
from NADPH to cytochrome P450 enzymes during the P450 catalytic cycle.
NADPH cytochrome P450 reductase is localized to the endoplasmic reticulum
where it is also able to transfer electrons to heme oxygenase and cytochrome
β5. NADPH cytochrome P450 reductase is structurally related to two separate
flavoprotein families, ferredoxin nucleotide reductase (FNR) and flavodoxin.
Electron transfer of NADPH cytochrome P450 reductase requires the binding
of two flavin cofactors, FAD and FMN, to the FNR and flavodoxin domains,
respectively.