Cytochrome P450 reductase (also known as NADPH:ferrihemoprotein oxidoreductase, NADPH:hemoprotein oxidoreductase, NADPH:P450 oxidoreductase, P450 reductase, POR, CPR, CYPOR) is a membrane-bound enzyme required for electron transfer from NADPH to cytochrome P450 [5] and other heme proteins including heme oxygenase in the endoplasmic reticulum [6] of the eukaryotic cell.
Human POR gene has 16 exons and the exons 2-16 code for a 677-amino acid [7] POR protein (NCBI NP_000932.2). There is a single copy of 50 kb POR gene (NCBI NM_000941.2) in humans on chromosome 7 (7q11.23).
Paralogs of POR include nitric oxide synthase (EC 1.14.13.39), NADPH:sulfite reductase (EC 1.8.1.2), and methionine synthase reductase (EC 1.16.1.8).[ citation needed ]
The 3D crystal structure of human POR has been determined. [8] The molecule is composed of four structural domains: the FMN-binding domain, the connecting domain, the FAD-binding domain, and NADPH-binding domain. The FMN-binding domain is similar to the structure of FMN-containing protein flavodoxin, whereas the FAD-binding domain and NADPH-binding domains are similar to those of flavoprotein ferredoxin-NADP+ reductase (FNR). The connecting domain is situated between the flavodoxin-like and FNR-like domains. Conformation flexibility of POR is a key requirement for interaction with different redox partners like Cytochrome P450 proteins, and biasing the conformation of POR with small molecule ligands may be a way to control interaction with partner proteins and influence metabolism. [9]
In Bacillus megaterium and Bacillus subtilis, POR is a C-terminal domain of CYP102, a single-polypeptide self-sufficient soluble P450 system (P450 is an N-terminal domain). The general scheme of electron flow in the POR/P450 system is:
The definitive evidence for the requirement of POR in cytochrome-P450-mediated reactions came from the work of Lu, Junk and Coon, [10] who dissected the P450-containing mixed function oxidase system into three constituent components: POR, cytochrome P450, and lipids.
Since all microsomal P450 enzymes require POR for catalysis, it is expected that disruption of POR would have devastating consequences. POR knockout mice are embryonic lethal, [11] probably due to lack of electron transport to extrahepatic P450 enzymes since liver-specific knockout of POR yields phenotypically and reproductively normal mice that accumulate hepatic lipids and have remarkably diminished capacity of hepatic drug metabolism. [12]
The reduction of cytochrome P450 is not the only physiological function of POR. The final step of heme oxidation by mammalian heme oxygenase requires POR and O2. In yeast, POR affects the ferrireductase activity, probably transferring electrons to the flavocytochrome ferric reductase. [13]
More than 200 variations in POR gene have been identified. [14] [15]
Five missense mutations (A287P, R457H, V492E, C569Y, and V608F) and a splicing mutation in the POR genes have been found in patients who had hormonal evidence for combined deficiencies of two steroidogenic cytochrome P450 enzymes - P450c17 CYP17A1, which catalyzes steroid 17α-hydroxylation and 17,20 lyase reaction, and P450c21 21-hydroxylase, which catalyzes steroid 21-hydroxylation. [16] Another POR missense mutation Y181D has also been identified. [17] Fifteen of nineteen patients having abnormal genitalia and disordered steroidogenesis were homozygous or apparent compound heterozygous for POR mutations that destroyed or dramatically inhibited POR activity. [18]
POR deficiency is the newest form of congenital adrenal hyperplasia first described in 2004. [16] The index patient was a newborn 46,XX Japanese girl with craniosynostosis, hypertelorism, mid-face hypoplasia, radiohumeral synostosis, arachnodactyly and disordered steroidogenesis. However, the clinical and biochemical characteristics of patients with POR deficiency are long known in the literature as so-called mixed oxidase disease, as POR deficiency typically shows a steroid profile that suggests combined deficiencies of steroid 21-hydroxylase and 17α-hydroxylase/17,20 lyase activities. The clinical spectrum of POR deficiency ranges from severely affected children with ambiguous genitalia, adrenal insufficiency, and the Antley-Bixler skeletal malformation syndrome (ABS) to mildly affected individuals with polycystic ovary syndrome-like features. Some of the POR patients were born to mothers who became virilized during pregnancy, suggesting deficient placental aromatization of fetal androgens due to a lesion in microsomal aromatase resulting in low estrogen production, which was later confirmed by lower aromatase activities caused by POR mutations. [19] [20] However, it has also been suggested that fetal and maternal virilization in POR deficiency might be caused by increased dihydrotestosterone synthesis by the fetal gonad through an alternative "backdoor pathway" first described in the marsupials and later confirmed in humans. [21] Gas chromatography/mass spectrometry analysis of urinary steroids from pregnant women carrying a POR-deficient fetus described in an earlier report also supports the existence of this pathway, [22] [23] and the relevance of the backdoor pathway along with POR dependent steroidogenesis have become clearer from recent studies. [21] The role of POR mutations beyond CAH are being investigated; and questions such as how POR mutations cause bony abnormalities and what role POR variants play in drug metabolism by hepatic P450s are being addressed in recent publications. [24] [25] [26] [27] [28] However, reports of ABS in some offspring of mothers who were treated with fluconazole, an antifungal agent which interferes with cholesterol biosynthesis at the level of CYP51 activity - indicate that disordered drug metabolism may result from deficient POR activity. [29]
Williams syndrome is a genetic disorder characterized by the deletion of genetic material approximately 1.2 Mb from the POR gene (POR). Cells with this genetic deletion show reduced transcription of POR, it seems, due to the loss of a cis-regulatory element that alters expression of this gene. [30] Some persons with Williams syndrome show characteristics of POR deficiency, including radioulnar synostosis and other skeletal abnormalities. [31] Cases of mild impairment of cortisol and androgen synthesis have been noted, [32] however, despite the fact that deficient POR impairs androgen synthesis, patients with Williams syndrome often show increased androgen levels. [33] A similar increase in testosterone has been observed in a mouse model that has globally decreased POR expression. [34]
Israel Hanukoglu is a Turkish-born Israeli scientist. He is a full professor of biochemistry and molecular biology at Ariel University and former science and technology adviser to the prime minister of Israel (1996–1999). He is founder of Israel Science and Technology Directory.
Cytochromes P450 are a superfamily of enzymes containing heme as a cofactor that mostly, but not exclusively, function as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various compounds, as well as for hormone synthesis and breakdown. In 1963, Estabrook, Cooper, and Rosenthal described the role of CYP as a catalyst in steroid hormone synthesis and drug metabolism. In plants, these proteins are important for the biosynthesis of defensive compounds, fatty acids, and hormones.
Congenital adrenal hyperplasia due to 17α-hydroxylase deficiency is an uncommon form of congenital adrenal hyperplasia resulting from a defect in the gene CYP17A1, which encodes for the enzyme 17α-hydroxylase. It causes decreased synthesis of cortisol and sex steroids, with resulting increase in mineralocorticoid production. Thus, common symptoms include mild hypocortisolism, ambiguous genitalia in genetic males or failure of the ovaries to function at puberty in genetic females, and hypokalemic hypertension (respectively). However, partial (incomplete) deficiency is notable for having inconsistent symptoms between patients, and affected genetic (XX) females may be wholly asymptomatic except for infertility.
Flavoproteins are proteins that contain a nucleic acid derivative of riboflavin. These proteins are involved in a wide array of biological processes, including removal of radicals contributing to oxidative stress, photosynthesis, and DNA repair. The flavoproteins are some of the most-studied families of enzymes.
Cytochrome P450 17A1 is an enzyme of the hydroxylase type that in humans is encoded by the CYP17A1 gene on chromosome 10. It is ubiquitously expressed in many tissues and cell types, including the zona reticularis and zona fasciculata of the adrenal cortex as well as gonadal tissues. It has both 17α-hydroxylase and 17,20-lyase activities, and is a key enzyme in the steroidogenic pathway that produces progestins, mineralocorticoids, glucocorticoids, androgens, and estrogens. More specifically, the enzyme acts upon pregnenolone and progesterone to add a hydroxyl (-OH) group at carbon 17 position (C17) of the steroid D ring, or acts upon 17α-hydroxyprogesterone and 17α-hydroxypregnenolone to split the side-chain off the steroid nucleus.
3β-Hydroxysteroid dehydrogenase/Δ5-4 isomerase (3β-HSD) is an enzyme that catalyzes the biosynthesis of the steroid progesterone from pregnenolone, 17α-hydroxyprogesterone from 17α-hydroxypregnenolone, and androstenedione from dehydroepiandrosterone (DHEA) in the adrenal gland. It is the only enzyme in the adrenal pathway of corticosteroid synthesis that is not a member of the cytochrome P450 family. It is also present in other steroid-producing tissues, including the ovary, testis and placenta. In humans, there are two 3β-HSD isozymes encoded by the HSD3B1 and HSD3B2 genes.
Cholesterol side-chain cleavage enzyme is commonly referred to as P450scc, where "scc" is an acronym for side-chain cleavage. P450scc is a mitochondrial enzyme that catalyzes conversion of cholesterol to pregnenolone. This is the first reaction in the process of steroidogenesis in all mammalian tissues that specialize in the production of various steroid hormones.
Steroid 21-hydroxylase is an enzyme that hydroxylates steroids at the C21 position and is involved in biosynthesis of aldosterone and cortisol. The enzyme converts progesterone and 17α-hydroxyprogesterone into 11-deoxycorticosterone and 11-deoxycortisol, respectively, within metabolic pathways that ultimately lead to aldosterone and cortisol. Deficiency in the enzyme may cause congenital adrenal hyperplasia.
Steroid 11β-hydroxylase, also known as steroid 11β-monooxygenase, is a steroid hydroxylase found in the zona glomerulosa and zona fasciculata of the adrenal cortex. Named officially the cytochrome P450 11B1, mitochondrial, it is a protein that in humans is encoded by the CYP11B1 gene. The enzyme is involved in the biosynthesis of adrenal corticosteroids by catalyzing the addition of hydroxyl groups during oxidation reactions.
The human gene SRD5A2 encodes the 3-oxo-5α-steroid 4-dehydrogenase 2 enzyme, also known as 5α-reductase type 2 (5αR2), one of three isozymes of 5α-reductase.
In enzymology, a NADPH—hemoprotein reductase is an enzyme that catalyzes the chemical reaction
Adrenodoxin reductase, was first isolated from bovine adrenal cortex where it functions as the first enzyme in the mitochondrial P450 systems that catalyze essential steps in steroid hormone biosynthesis. Examination of complete genome sequences revealed that adrenodoxin reductase gene is present in most metazoans and prokaryotes.
Antley–Bixler syndrome is a rare, severe autosomal recessive congenital disorder characterized by malformations and deformities affecting the majority of the skeleton and other areas of the body.
Aromatase deficiency is a rare condition characterized by extremely low levels or complete absence of the enzyme aromatase activity in the body. It is an autosomal recessive disease resulting from various mutations of gene CYP19 (P450arom) which can lead to ambiguous genitalia and delayed puberty in females, continued linear growth into adulthood and osteoporosis in males and virilization in pregnant mothers. As of 2020, fewer than 15 cases have been identified in genetically male individuals and at least 30 cases in genetically female individuals.
An inborn error of steroid metabolism is an inborn error of metabolism due to defects in steroid metabolism.
Walter L. Miller is an American endocrinologist and professor emeritus of pediatrics at the University of California, San Francisco (UCSF). Miller is expert in the field of human steroid biosynthesis and disorders of steroid metabolism. Over the past 40 years Miller's group at UCSF has described molecular basis of several metabolic disorders including, congenital adrenal hyperplasia, pseudo vitamin D dependent rickets, severe, recessive form of Ehlers-Danlos syndrome, 17,20 lyase deficiency caused by CYP17A1 defects, P450scc deficiency caused by CYP11A1 defects, P450 oxidoreductase deficiency.
Steroidogenic enzymes are enzymes that are involved in steroidogenesis and steroid biosynthesis. They are responsible for the biosynthesis of the steroid hormones, including sex steroids and corticosteroids, as well as neurosteroids, from cholesterol. Steroidogenic enzymes are most highly expressed in classical steroidogenic tissues, such as the testis, ovary, and adrenal cortex, but are also present in other tissues in the body.
Cytochrome P450 oxidoreductase deficiency (PORD) is a rare disease and inborn error of metabolism caused by deficiency of cytochrome P450 oxidoreductase (POR). POR is a 2-flavin protein that is responsible for the transfer of electrons from NADPH to all 50 microsomal cytochrome P450 (CYP450) enzymes. This includes the steroidogenic enzymes CYP17A1 (17α-hydroxylase/17,20-lyase), CYP19A1 (aromatase), and CYP21A2 (21-hydroxylase); CYP26B1 ; and the hepatic drug-metabolizing CYP450 enzymes, among many other CYP450 enzymes. Symptoms of severe forms of PORD include ambiguous genitalia in males and females, congenital adrenal hyperplasia, cortisol deficiency, and Antley–Bixler skeletal malformation syndrome (ABS), while symptoms of mild forms include polycystic ovary syndrome in women and hypogonadism in men. Maternal virilization also occurs in severe forms, due to aromatase deficiency in the placenta. Virilization of female infants in PORD may also be caused by alternative biosynthesis of 5α-dihydrotestosterone via the so-called "androgen backdoor pathway". The ABS component of severe forms of PORD is probably caused by CYP26B1 deficiency, which results in retinoic acid excess and defects during skeletal embryogenesis. All forms of PORD in humans are likely partial, as POR knockout in mice results in death during prenatal development.
The androgen backdoor pathway is a collective name for all metabolic pathways where clinically relevant androgens are synthesized from 21-carbon steroids (pregnanes) by their 5α-reduction with roundabout of testosterone and/or androstenedione.
5α-Pregnane-3α,17α-diol-20-one, also known as 17α-hydroxyallopregnanolone (17-OH-allo) is an endogenous steroid.
CPR was shown to be localized in the endoplasmic reticulum in the early 1960s (Williams and Kamin, 1962).
