| Cytochrome P450 reductase (CPR, also termed NADPH–cytochrome P450 oxidoreductase, POR) is a microsomal diflavin oxidoreductase that belongs to the family of FAD/FMN‑containing electron‑transfer enzymes and serves as the obligate electron donor to essentially all endoplasmic‑reticulum cytochrome P450 isoforms, thereby controlling a wide spectrum of xenobiotic and endogenous oxidative reactions. The enzyme is anchored to the cytosolic face of the endoplasmic reticulum via an N‑terminal membrane segment and contains a cytosolic catalytic region organized into distinct FAD‑ and FMN‑binding domains connected by a flexible hinge, with the FAD‑binding domain receiving a hydride equivalent from NADPH and the FMN‑binding domain docking with redox partners such as P450 and cytochrome b5. Electron transfer follows a defined pathway in which NADPH reduces the FAD cofactor, which then reduces FMN, and FMN passes single electrons one at a time to the heme iron of microsomal P450s or to alternative acceptors; mechanistic analyses support a catalytic cycle in which CPR cycles between oxidized, two‑electron‑reduced and one‑electron‑reduced FMN semiquinone states, enabling sequential one‑electron delivery to P450 required for oxygen activation. CPR thereby supports P450‑catalyzed mono‑oxygenation reactions involved in phase I drug metabolism, environmental xenobiotic detoxification, and biosynthesis of steroids, bile acids, cholesterol‑derived mediators, and fatty acid metabolites, and also transfers electrons to other redox enzymes including heme oxygenase, squalene monooxygenase, and some cytochrome b5‑dependent pathways, extending its influence to lipid homeostasis and heme turnover. CPR–P450 interactions depend on complementary electrostatic surfaces near the FMN domain and on the hinge‑mediated domain movements that bring FMN into optimal distance for rapid electron transfer, and CPR binding can also regulate P450 catalytic cycles by modulating water and proton channels at the proximal face of the heme domain. CPR activity contributes to the generation of diffusible reduced oxygen species, since electron transfer to xenobiotic acceptors such as quinones or to molecular oxygen via redox cycling can yield superoxide and other reactive oxygen species, linking CPR‑dependent metabolism to oxidative stress and cytotoxicity under conditions of high xenobiotic load or impaired P450 coupling. In hepatic physiology, POR expression level and activity are major determinants of total microsomal P450 capacity, and interindividual variability or polymorphisms in POR influence the clearance, efficacy, and toxicity profiles of many drugs whose biotransformation depends on CYP3A, CYP2C, and other POR‑dependent isoforms, creating a critical determinant of pharmacokinetic variability. CPR also regulates steroid hormone and bile acid biosynthetic P450s in adrenal and gonadal tissues, and POR deficiency or mutation causes disordered steroidogenesis and skeletal malformations through impaired activities of 21‑hydroxylase, 17α‑hydroxylase/17,20‑lyase, and other steroidogenic cytochromes P450. |
