Cytochrome P450 reductase
Encyclopedia
Cytochrome P450 reductase is a membrane-bound enzyme
required for electron transfer
to cytochrome P450 in the microsome
of the eukaryotic
cell
from a FAD
- and FMN
-containing enzyme NADPH:cytochrome P450 reductase (POR; ).
The definitive evidence for the requirement of POR in cytochrome-P450-mediated reactions came from the work of Lu, Junk and Coon, 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,
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.
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.
, which catalyzes steroid 21-hydroxylation. Later on Arlt et al. identified another POR missense mutation, Y178D, and also reported three of the POR mutations (A284P, R454H, and C566Y), that were originally described by Flück et al.. In a larger study Huang et al. have examined the POR genes in 32 additional patients. 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. Huang et al.. studied 11 new POR variants: A115V, T142A, Q153R, P228L, M263V, R316W, G413S, Y459H, A503V, G504R, G539R, L565P, R616X, V631I, and F646del.
resulting in low estrogen production, which was later confirmed by lower aromatase activities caused by POR mutations. 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. Gas chromatography/mass spectrometry analysis of urinary steroids from pregnant women carrying a POR-deficient fetus supports the existence of this pathway, but the relevance of the "backdoor" pathway in the normal or CAH fetus remains unclear. The role of POR mutations beyond CAH remains unknown; and questions such as how POR mutations cause bony abnormalities and what role POR variants play in drug metabolism by hepatic P450s are unsolved. However, reports of ABS in some offsprings 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.
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. Some persons with Williams syndrome show characteristics of POR deficiency, including radio-ulnar synostosis and other skeletal abnormalities. Cases of mild impairment of cortisol
and androgen synthesis have been noted, however, despite the fact that deficient POR impairs androgen synthesis, patients with Williams syndrome often show increased androgen levels. A similar increase in testosterone has been observed in a mouse model that has globally decreased POR expression.
, 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.
, NADPH:sulfite reductase , and methionine synthase reductase .
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
required for electron transfer
Electron transfer
Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...
to cytochrome P450 in the microsome
Microsome
In cell biology, microsomes are vesicle-like artifacts re-formed from pieces of the endoplasmic reticulum when eukaryotic cells are broken-up in the laboratory; by definition, microsomes are not ordinarily present in living cells....
of the eukaryotic
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
from a FAD
FAD
In biochemistry, flavin adenine dinucleotide is a redox cofactor involved in several important reactions in metabolism. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin moiety bound to the phosphate...
- and FMN
Flavin mononucleotide
Flavin mononucleotide , or riboflavin-5′-phosphate, is a biomolecule produced from riboflavin by the enzyme riboflavin kinase and functions as prosthetic group of various oxidoreductases including NADH dehydrogenase as well as cofactor in biological blue-light photo receptors...
-containing enzyme NADPH:cytochrome P450 reductase (POR; ).
Introduction
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:| NADPH | |
|
|
|
|
|
O2 |
The definitive evidence for the requirement of POR in cytochrome-P450-mediated reactions came from the work of Lu, Junk and Coon, 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,
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.
The reduction of cytochrome P450 is not the only physiological function of POR. The final step of heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
oxidation by mammalian heme oxygenase
Heme oxygenase
This reaction can occur in virtually every cell; the classic example is the formation of a bruise, which goes through different colors as it gradually heals: red heme to green biliverdin to yellow bilirubin...
requires POR and O2. In yeast, POR affects the ferrireductase activity, probably transferring electrons to the flavocytochrome ferric reductase.
Gene organization
Human POR gene has 16 exons and the exons 2-16 code for a 677-amino acid 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).Mutations and polymorphisms
Flück et al. have reported for the first time five missense mutations (A284P, R454H, V489E, C566Y, and V605F) and a splicing mutation in the POR genes of four 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-Hydroxylase21-Hydroxylase
Steroid 21-hydroxylase is a cytochrome P450 enzyme that is involved with the biosynthesis of the steroid hormones aldosterone and cortisol.In humans, 21-Hydroxylase is encoded by the gene CYP21A2.-Names and classification:...
, which catalyzes steroid 21-hydroxylation. Later on Arlt et al. identified another POR missense mutation, Y178D, and also reported three of the POR mutations (A284P, R454H, and C566Y), that were originally described by Flück et al.. In a larger study Huang et al. have examined the POR genes in 32 additional patients. 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. Huang et al.. studied 11 new POR variants: A115V, T142A, Q153R, P228L, M263V, R316W, G413S, Y459H, A503V, G504R, G539R, L565P, R616X, V631I, and F646del.
POR Deficiency – Mixed Oxidase Disease
POR deficiency is the newest form of congenital adrenal hyperplasia first described in 2004. 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 aromataseAromatase
Aromatase is an enzyme responsible for a key step in the biosynthesis of estrogens. It is a member of the cytochrome P450 superfamily , which are monooxygenases that catalyze many reactions involved in steroidogenesis. In particular, aromatase is responsible for the aromatization of androgens into...
resulting in low estrogen production, which was later confirmed by lower aromatase activities caused by POR mutations. 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. Gas chromatography/mass spectrometry analysis of urinary steroids from pregnant women carrying a POR-deficient fetus supports the existence of this pathway, but the relevance of the "backdoor" pathway in the normal or CAH fetus remains unclear. The role of POR mutations beyond CAH remains unknown; and questions such as how POR mutations cause bony abnormalities and what role POR variants play in drug metabolism by hepatic P450s are unsolved. However, reports of ABS in some offsprings 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.
Williams Syndrome
Williams syndromeWilliams syndrome
Williams syndrome is a rare neurodevelopmental disorder characterized by a distinctive, "elfin" facial appearance, along with a low nasal bridge; an unusually cheerful demeanor and ease with strangers; developmental delay coupled with strong language skills; and cardiovascular problems, such as...
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
Cis-regulatory element
A cis-regulatory element or cis-element is a region of DNA or RNA that regulates the expression of genes located on that same molecule of DNA . This term is constructed from the Latin word cis, which means "on the same side as". These cis-regulatory elements are often binding sites for one or...
that alters expression of this gene. Some persons with Williams syndrome show characteristics of POR deficiency, including radio-ulnar synostosis and other skeletal abnormalities. Cases of mild impairment of cortisol
Cortisol
Cortisol is a steroid hormone, more specifically a glucocorticoid, produced by the adrenal gland. It is released in response to stress and a low level of blood glucocorticoids. Its primary functions are to increase blood sugar through gluconeogenesis; suppress the immune system; and aid in fat,...
and androgen synthesis have been noted, however, despite the fact that deficient POR impairs androgen synthesis, patients with Williams syndrome often show increased androgen levels. A similar increase in testosterone has been observed in a mouse model that has globally decreased POR expression.
Structure
3D crystal structure of rat POR has been solved . 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 flavodoxinFlavodoxin
Flavodoxins are electron-transfer proteins. Flavodoxin is a bacterial protein that includes flavin mononucleotide. The structure of flavodoxin is characterized by a five-stranded parallel beta sheet, surrounded by alpha helices at either side of the sheet. They have been isolated from prokaryotes,...
, 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.
POR homologs
The other enzymes containing homologs of POR are nitric oxide synthaseNitric oxide synthase
Nitric oxide synthases are a family of enzymes that catalyze the production of nitric oxide from L-arginine. NO is an important cellular signaling molecule, having a vital role in many biological processes...
, NADPH:sulfite reductase , and methionine synthase reductase .