The interaction of NADPH-P450 reductase with P450: an electrochemical study of the role of the flavin mononucleotide-binding domain

Multifaceted personality and roles of heme enzymes in industrial biotechnology

Heme enzymes are the most prominent category of iron-containing metalloenzymes with the capability of catalyzing an astonishingly wide range of reactions like epoxidation, hydroxylation, demethylation, desaturation, reduction, sulfoxidation, and decarboxylation. Various enzymes in this category are P450s, heme peroxidases, catalases, myoglobin, cytochrome C, and others. Besides this, the natural promiscuity and amenability of these enzymes to protein engineering and evolution have also added several non-native reactions such as C-H, N-H, S-H insertions, cyclopropanation, and other industrially important reactions to their capabilities. Surprisingly, all of these reactions and their wide substrate scopes are attributed to changes in the active site scaffold of different heme enzymes as the center of all enzymes is constituted by a porphyrin ring containing iron. Multiple prominent research groups across the world, including 2018, Nobel Laureate Frances Arnold's group, have shown keen interest in engineering and evolving these enzymes for utilizing their industrial potential. Besides engineering the active site, researchers have also explored the possibility of these enzymes catalyzing non-native reactions by replacing the center porphyrin ring with other cofactors or by changing the iron in the porphyrin ring with other metal ions along with engineering the active site and thereby creating novel artificial metalloenzymes. Thus, in this mini-review from our group, for the first time, we are trying to catalog various activities catalyzed by heme enzymes and their engineered variants and their active usage in various industries along with shedding light on their potential for use in various applications in the future.

Dandelion Extract Alleviated Lipopolysaccharide-Induced Oxidative Stress through the Nrf2 Pathway in Bovine Mammary Epithelial Cells

In practical dairy production, cows are frequently subjected to inflammatory diseases, such as high-grain diet-induced subacute ruminal acidosis (SARA) as well as mastitis and metritis. Under the circumstances, lipopolysaccharide (LPS) induces oxidative stress within the cow and in the mammary epithelial cells. It has implications in practical production to alleviate oxidative stress and to optimize the lactational function of the mammary epithelial cells. This study thus aimed to investigate the antioxidative effects of dandelion aqueous extract (DAE) on LPS-induced oxidative stress and the mechanism of DAE as an antioxidant to alleviate oxidative stress through the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway in the bovine mammary epithelial cell line MAC-T cells. The cells were cultured for 48 h in six treatments including control (without LPS and DAE), LPS (100 ng/mL), DAE10 (100 ng/mL LPS and 10 μg/mL DAE), DAE50 (100 ng/mL LPS and 50 μg/mL DAE), DAE100 (100 ng/mL LPS and 100 μg/mL DAE), and DAE200 (100 ng/mL LPS and 200 μg/mL DAE), respectively. The results showed that cell viability was reduced by LPS, and the adverse effect of LPS was suppressed with the supplementation of DAE. Lipopolysaccharide-induced oxidative stress through enhancing reactive oxygen species (ROS) production, resulted in increases in oxidative damage marker concentrations, while 10 and 50 μg/mL DAE alleviated the LPS-induced oxidative stress via scavenging cellular ROS and improving antioxidant enzyme activity. The upregulation of antioxidative gene expression in DAE treatments was promoted through activating the Nrf2 signaling pathway, with DAE at a concentration of 50 μg/mL exhibiting the highest effect. Overall, DAE acted as an effective antioxidant to inhibit LPS-induced oxidative stress and as a potential inducer of the Nrf2 signaling pathway.

Phosphorylation of human placental aromatase CYP19A1

Aromatase CYP19A1 catalyzes the synthesis of estrogens in endocrine, reproductive and central nervous systems. Higher levels of 17β-estradiol (E2) are associated with malignancies and diseases of the breast, ovary and endometrium, while low E2 levels increase the risk for osteoporosis, cardiovascular diseases and cognitive disorders. E2, the transcriptional activator of the estrogen receptors, is also known to be involved in non-genomic signaling as a neurotransmitter/neuromodulator, with recent evidence for rapid estrogen synthesis (RES) within the synaptic terminal. Although regulation of brain aromatase activity by phosphorylation/dephosphorylation has been suggested, it remains obscure in the endocrine and reproductive systems. RES and overabundance of estrogens could stimulate the genomic and non-genomic signaling pathways, and genotoxic effects of estrogen metabolites. Here, by utilizing biochemical, cellular, mass spectrometric, and structural data we unequivocally demonstrate phosphorylation of human placental aromatase and regulation of its activity. We report that human aromatase has multiple phosphorylation sites, some of which are consistently detectable. Phosphorylation of the residue Y361 at the reductase-coupling interface significantly elevates aromatase activity. Other sites include the active site residue S478 and several at the membrane interface. We present the evidence that two histidine residues are phosphorylated. Furthermore, oxidation of two proline residues near the active site may have implications in regulation. Taken together, the results demonstrate that aromatase activity is regulated by phosphorylation and possibly other post-translational modifications. Protein level regulation of aromatase activity not only represents a paradigm shift in estrogen-mediated biology, it could also explain unresolved clinical questions such as aromatase inhibitor resistance.

Whole-cell biocatalysis using cytochrome P450 monooxygenases for biotransformation of sustainable bioresources (fatty acids, fatty alkanes, and aromatic amino acids)

Cytochrome P450s (CYPs) are heme-thiolated enzymes that catalyze the oxidation of CH bonds in a regio and stereoselective manner. Activation of the non-activated carbon atom can be further enhanced by multistep chemo-enzymatic reactions; moreover, several useful chemicals can be synthesized to provide alternative organic synthesis routes. Given their versatile functionality, CYPs show promise in a number of biotechnological fields. Recently, various CYPs, along with their sequences and functionalities, have been identified owing to rapid developments in sequencing technology and molecular biotechnology. In addition to these discoveries, attempts have been made to utilize CYPs to industrially produce biochemicals from available and sustainable bioresources such as oil, amino acids, carbohydrates, and lignin. Here, these accomplishments, particularly those involving the use of CYP enzymes as whole-cell biocatalysts for bioresource biotransformation, will be reviewed. Further, recently developed biotransformation pathways that result in gram-scale yields of fatty acids and fatty alkanes as well as aromatic amino acids, which depend on the hosts used for CYP expression, and the nature of the multistep reactions will be discussed. These pathways are similar regardless of whether the hosts are CYP-producing or non-CYP-producing; the limitations of these methods and the ways to overcome them are reviewed here.

Dehydroepiandrosterone (DHEA)-SO4 Depot and Castration-Resistant Prostate Cancer

Dehydroepiandrosterone (DHEA)-SO₄ of adrenal origin is the major C19 steroid in the serum. It is a precursor of intratumoral androgen biosynthesis in patients with advanced prostate cancer following chemical or surgical castration. DHEA is a product of the P450c17 (17α-hydroxylase-17,20-lyase) enzyme. Despite inhibition of P450c17 with new agents, e.g., Abiraterone acetate, Orterenol, and Galeterone, the level of enzyme inhibition rarely exceeds 90% leaving behind a significant depot for androgen biosynthesis within the tumor. For DHEA-SO₄ to be utilized there is uptake by organic anion transporter polypeptides, deconjugation catalyzed by steroid sulfatase, and adaptive upregulation of prostate steroidogenic enzymes that will convert DHEA into either testosterone or dihydrotestosterone. The depot of DHEA-SO₄ that remains after P450c17 inhibition and the adaptive responses that occur within the tumor to promote DHEA utilization contribute to mechanisms of drug resistance observed with P450c17 inhibitors. Knowledge of these mechanisms identify new targets for therapeutics that could be used to surmount drug resistance in prostate cancer.

Cytochrome P450 17A1 Interactions with the FMN Domain of Its Reductase as Characterized by NMR

To accomplish key physiological processes ranging from drug metabolism to steroidogenesis, human microsomal cytochrome P450 enzymes require the sequential input of two electrons delivered by the FMN domain of NADPH-cytochrome P450 reductase. Although some human microsomal P450 enzymes can instead accept the second electron from cytochrome b5, for human steroidogenic CYP17A1, the cytochrome P450 reductase FMN domain delivers both electrons, and b5 is an allosteric modulator. The structural basis of these key but poorly understood protein interactions was probed by solution NMR using the catalytically competent soluble domains of each protein. Formation of the CYP17A1·FMN domain complex induced differential line broadening of the NMR signal for each protein. Alterations in the exchange dynamics generally occurred for residues near the surface of the flavin mononucleotide, including 87-90 (loop 1), and for key CYP17A1 active site residues. These interactions were modulated by the identity of the substrate in the buried CYP17A1 active site and by b5. The FMN domain outcompetes b5 for binding to CYP17A1 in the three-component system. These results and comparison with previous NMR studies of the CYP17A1·b5 complex suggest a model of CYP17A1 enzyme regulation.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

NADPH P450 oxidoreductase: structure, function, and pathology of diseases

Cytochrome P450 oxidoreductase (POR) is an enzyme that is essential for multiple metabolic processes, chiefly among them are reactions catalyzed by cytochrome P450 proteins for metabolism of steroid hormones, drugs and xenobiotics. Mutations in POR cause a complex set of disorders that often resemble defects in steroid metabolizing enzymes 17α-hydroxylase, 21-hydroxylase and aromatase. Since our initial reports of POR mutations in 2004, more than 200 different mutations and polymorphisms in POR gene have been identified. Several missense variations in POR have been tested for their effect on activities of multiple steroid and drug metabolizing P450 proteins. Mutations in POR may have variable effects on different P450 partner proteins depending on the location of the mutation. The POR mutations that disrupt the binding of co-factors have negative impact on all partner proteins, while mutations causing subtle structural changes may lead to altered interaction with specific partner proteins and the overall effect may be different for each partner. This review summarizes the recent discoveries related to mutations and polymorphisms in POR and discusses these mutations in the context of historical developments in the discovery and characterization of POR as an electron transfer protein. The review is focused on the structural, enzymatic and clinical implications of the mutations linked to newly identified disorders in humans, now categorized as POR deficiency.

Dynamic control of electron transfers in diflavin reductases

Diflavin reductases are essential proteins capable of splitting the two-electron flux from reduced pyridine nucleotides to a variety of one electron acceptors. The primary sequence of diflavin reductases shows a conserved domain organization harboring two catalytic domains bound to the FAD and FMN flavins sandwiched by one or several non-catalytic domains. The catalytic domains are analogous to existing globular proteins: the FMN domain is analogous to flavodoxins while the FAD domain resembles ferredoxin reductases. The first structural determination of one member of the diflavin reductases family raised some questions about the architecture of the enzyme during catalysis: both FMN and FAD were in perfect position for interflavin transfers but the steric hindrance of the FAD domain rapidly prompted more complex hypotheses on the possible mechanisms for the electron transfer from FMN to external acceptors. Hypotheses of domain reorganization during catalysis in the context of the different members of this family were given by many groups during the past twenty years. This review will address the recent advances in various structural approaches that have highlighted specific dynamic features of diflavin reductases.

Diverse biological activities of dandelion

Dandelion (Taraxacum officinale Weber) is a member of the Asteraceae (Compositae) family, native to Europe but widely distributed in the warmer temperate zones of the Northern Hemisphere. Dandelion and its parts are habitually consumed as plant foods in several areas of the world, where they are also employed in phytotherapy. Indeed, dandelion contains a wide array of phytochemicals whose biological activities are actively being explored in various areas of human health. In particular, emerging evidence suggests that dandelion and its constituents have antioxidant and anti-inflammatory activities that result in diverse biological effects. The present review provides a comprehensive analysis of the constituents of dandelion, an assessment of the pharmacological properties of dandelion, and a description of relevant studies that support the use of dandelion as a medicinal plant.

Review - Flavins as photoreceptors of blue light and their spectroscopic properties

This review describes 1) the development of studies on flavin photoreceptors as blue light photoreceptors in many living organisms: their kinds and functions; 2) the studies on spectroscopic properties of flavins, both their dimers and monomers; 3) nonradiative excitation energy transport in the presence of monomers and fluorescent/nonflurescent FMN dimers (excitation traps). The existence equilibrated luminescent FMN centers, energy migration and excitation sink to FMN dimers are taken into account.

Cloning and expression of koala (Phascolarctos cinereus) liver cytochrome P450 reductase

The cloning, expression and characterization of hepatic NADPH-cytochrome P450 reductase (CPR) from koala (Phascolarctos cinereus) is described. Two 2059 bp koala liver CPR cDNAs, designated CPR1 and CPR2, were cloned by reverse transcription-polymerase chain reaction and rapid amplification of cDNA ends. The koala CPR cDNAs encode proteins of 678 amino acids and share 85% amino acid sequence identity to human CPR. Transfection of the koala CPR cDNAs into Cos-7 cells resulted in the expression of proteins, which were recognized by a goat-antihuman CPR antibody. The koala CPR1 and 2 cDNA-expressed enzymes catalysed cytochrome c reductase at the rates of 4.9 +/- 0.5 and 2.6 +/- 0.4 nmol/min/mg protein (mean +/- SD, n = 3), respectively which were comparable to that of rat CPR cDNA-expressed enzyme. The apparent Km value for CPR activity in koala liver microsomes was 11.61 +/- 6.01 microM, which is consistent with that reported for rat CPR enzyme. Northern analysis detected a CPR mRNA band of approximately 2.6 kb. Southern analysis suggested a single PCR gene across species. The present study provides primary molecular data regarding koala CPR1 and CPR2 genes in this unique marsupial species.

Genetic variation in human P450 oxidoreductase

Catalysis by all 50 Type II (microsomal) P450 enzymes, including steroidogenic P450c17, P450c21, and P450aro and hepatic drug-metabolizing enzymes requires electron donation from P450 oxidoreductase (POR). POR knockout mice are embryonic lethal, but human POR mutations cause a complex disorder of steroidogenesis. Disorders of hepatic drug metabolism in human POR deficiency have not yet been described. To understand the potential contribution of POR to pharmacogenetics, we sequenced the POR gene in 842 normal persons from 4 ethnic groups. We detected 140 single nucleotide sequence variants of which 43 were in >1% of alleles, including 15 missense mutants; this brings the total of known POR missense mutants to 35. A503V was found on 28% of alleles, varying from 19% in African Americans to 37% in Chinese Americans. We expressed all 35 missense mutants in E. coli and assayed their activities to: oxidize NADPH, reduce cytochrome c, support the 17alpha-hydroxylase and 17,20 lyase activities of bacterially expressed human P450c17, and support the metabolism of fluorogenic EOMCC by bacterially expressed human CYP1A2 and CYP2C19. These data show that there are great differences in the activities of some POR mutants depending on the electron recipient assayed; for example, Q153R causes severely impaired steroid biosynthesis in human patients and in vitro, but is a gain-of-function mutant with CYP1A2 and 2C19. A503V reduces both activities of P450c17 in half, but had no effect on CYP1A2 or 2C19. POR variants are a previously unappreciated source of genetic variation in patterns of steroid synthesis and drug metabolism.

Clinical, structural and functional implications of mutations and polymorphisms in human NADPH P450 oxidoreductase

Cytochrome P450 proteins are involved in metabolism of drugs and xenobiotics. In the endoplasmic reticulum a single nicotinamide adenine dinucleotide phosphate (NADPH) P450 oxidoreductase (POR) supplies electrons to all microsomal P450s for catalytic activity. POR is a flavoprotein that contains both flavin mononucleotide and flavin adenine dinucleotide as cofactors and uses NADPH as the source of electrons. We have recently reported a number of POR mutations in the patients with disordered steroidogenesis. In the first report we had described missense mutations (A287P, R457H, V492E, C569Y, and V608F) identified in four patients with defects in steroid production. Each POR variant was produced as recombinant N-27 form of the enzyme in bacteria and as full-length form in yeast. Membranes from bacteria or yeast expressing normal or variant POR were purified and their activities were characterized in cytochrome c and CYP17A1 assays. Later we have published a larger study that described a whole range of POR mutations and characterized the mutants/polymorphisms A115V, T142A, M263V, Y459H, A503V, G539R, L565P, R616X, V631I, and F646del from the sequencing of patient DNA. We also studied POR variants Y181D, P228L, R316W, G413S, and G504R that were available in public databases or published literature. Three-dimensional structure of rat POR is known and we have used this structure to deduce the structure-function correlation of POR mutations in human. The missense mutations found in patients with disordered steroidogenesis are generally in the co-factor binding and functionally important domains of POR and the apparent polymorphisms are found in regions with lesser structural importance. A variation in POR can alter the activity of all microsomal P450s, and therefore, can affect the metabolism of drugs and xenobiotics even when the P450s involved are otherwise normal. It is important to study the genetic and biochemical basis of POR variants in human population to gain information about possible differences in P450 mediated reactions among the individuals carrying a variant or polymorphic form of POR that could impact their metabolism.

Biochemical analysis of mutations in P450 oxidoreductase

All microsomal P450s require POR (cytochrome P450 reductase) for catalytic activity. Most of the clinically used drugs are metabolized by a small number of P450s and polymorphisms in the cytochrome P450s are known to cause changes in drug metabolism. We have recently found a number of POR missense mutations in the patients with disordered steroidogenesis. Our initial report described five missense mutations (A284P, R454H, V489E, C566Y and V605F) identified in four patients. We built bacterial expression vectors for each POR variant, purified the membranes expressing normal or variant POR and characterized their activities with cytochrome c and P450c17 assays. We have recently completed an extensive study of the range of POR mutations and characterized the mutants/polymorphisms A112V, T139A, M260V, Y456H, A500V, G536R, L562P, R613X, V628I and F643del from sequencing of patient DNA. We also studied POR variants Y179D, P225L, R313W, G410S and G501R that were available in databases or the published literature. We analysed the mutations with a three-dimensional model of human POR that was based on an essentially similar rat POR with known crystal structure. The missense mutations found in patients with disordered steroidogenesis mapped to functionally important domains of POR and the apparent polymorphisms mapped to less crucial regions. Since a variation in POR can alter the activity of all microsomal P450s, it can also affect the drug metabolism even with a normal P450. Understanding the genetic and biochemical basis of POR-mediated drug metabolism will provide valuable information about possible differences in P450-mediated reactions among the individuals carrying a variant or polymorphic form of POR.

P450 oxidoreductase deficiency: a new form of congenital adrenal hyperplasia

PURPOSE OF REVIEW

P450 oxidoreductase deficiency--a newly described form of congenital adrenal hyperplasia--typically presents a steroid profile suggesting combined deficiencies of steroid 21-hydroxylase and 17alpha-hydroxylase/17,20-lyase activities. These and other enzymes require electron donation from P450 oxidoreductase. The clinical spectrum of P450 oxidoreductase deficiency ranges from severely affected children with ambiguous genitalia, adrenal insufficiency and the Antley-Bixler skeletal malformation syndrome to mildly affected individuals with polycystic ovary syndrome. We review current knowledge of P450 oxidoreductase deficiency and its broader implications.

RECENT FINDINGS

Since the first report in 2004, at least 21 P450 oxidoreductase mutations have been reported in over 40 patients. The often subtle manifestations of P450 oxidoreductase deficiency suggest it may be relatively common. P450 oxidoreductase deficiency, with or without Antley-Bixler syndrome, is autosomal recessive, whereas Antley-Bixler syndrome without disordered steroidogenesis is caused by autosomal dominant fibroblast growth factor receptor 2 mutations. In-vitro assays of P450 oxidoreductase missense mutations based on P450 oxidoreductase-supported P450c17 activities provide excellent genotype/phenotype correlations. The causal connection between P450 oxidoreductase deficiency and disordered bone formation remains unclear.

SUMMARY

P450 oxidoreductase mutations cause combined partial deficiency of 17alpha-hydroxylase and 21-hydroxylase. Individuals with an Antley-Bixler syndrome-like phenotype presenting with sexual ambiguity or other abnormalities in steroidogenesis should be analyzed for P450 oxidoreductase deficiency.

P450 oxidoreductase deficiency: a new disorder of steroidogenesis

Microsomal P450 enzymes, which metabolize drugs and catalyze steroid biosynthesis require electron donation from NADPH via P450 oxidoreductase (POR). POR knockout mice are embryonically lethal, but we found recessive human POR missense mutations causing disordered steroidogenesis and Antley-Bixler syndrome (ABS), a skeletal malformation syndrome featuring craniosynostosis. Dominant mutations in exons 8 and 10 of fibroblast growth factor receptor 2 (FGFR2) cause phenotypically related craniosynostosis syndromes and were reported in patients with ABS and normal steroidogenesis. Sequencing POR and FGFR2 exons in 32 patients with ABS and/or hormonal findings suggesting POR deficiency showed complete genetic segregation of POR and FGFR2 mutations. Fifteen patients carried POR mutations on both alleles, four carried POR mutations on 1 allele, nine carried FGFR2/3 mutations on one allele and no mutation was found in three patients. The 34 affected POR alleles included 10 with A287P, 7 with R457H, 9 other missense mutations and 7 frameshifts. These 11 missense mutations and 10 others identified by database mining were expressed in E. coli, purified to apparent homogeneity, and their catalytic capacities were measured in four assays: reduction of cytochrome c, oxidation of NADPH, and support of the 17alpha-hydroxylase and 17,20 lyase activities of human P450c17. As assessed by Vmax/Km, 17,20 lyase activity provided the best correlation with clinical findings. Modeling human POR on the X-ray crystal structure of rat POR shows that these mutant activities correlate well with their locations in the structure. POR deficiency is a new disease, distinct from the craniosynostosis syndromes caused by FGFR mutations.

Laboratory evolution of P450 BM-3 for mediated electron transfer

Preparative synthesis with P450 monooxygenases is hampered in cell-free systems by the requirement for cofactors such as NAD(P)H as reduction equivalents. A validated medium-throughput screening system was designed for improving P450 monooxygenases by mediated electron transfer with zinc/cobalt(III)sepulchrate (Zn/Co(III)sep) as an alternative and cost-effective cofactor system. The monooxygenase P450 BM-3 F87A was used as a model system for developing the screening system in a 96-well format. A coefficient of variation of less than 10% was achieved under optimized screening conditions. The mediator evolution screen was validated by comparing the activity of P450 BM-3 to P450 BM-3 F87A and by screening a saturation mutagenesis library at amino acid position R47. For mediated electron transfer, two double mutants P450 BM-3(F87A R47F) and P450 BM-3 (F87A R47Y) were identified with a two-threefold increased catalytic efficiency (up to 32 microM(-1) min(-1) for P450 BM-3(F87A R47F) and 34 microM(-1) min(-1) for P450 BM-3 (F87A R47Y)) compared to P450 BM-3 F87A. The kinetic constants of the double mutants are, in contrast to those of P450 BM-3 F87A, dependent on Co(III)sep concentration in the presence of NADPH. kcat increases from 145 min(-1) (0.25 mM Co(III)sep) to 197 min(-1) (0.5 mM Co(III)sep), and Km decreases simultaneously from 7.0 microM to 3.7 microM, for P450 BM-3 (F87A R47F). For P450 BM-3 (F87A R47Y), kcat increases from 138 min(-1) (0.25 mM Co(III)sep) up to 187 min(-1) (0.5 mM Co(III)sep), and Km decreases from 8.2 microM to 4.2 microM. Due to lower Km values, the catalytic efficiencies were improved six times for P450 BM-3 (F87A R47F) and three times for P450 BM-3 (F87A R47Y), when comparing catalytic efficiencies of the mediated electron-transfer system to the natural reduction equivalent NADPH.

Uncoupling of cytochrome P450 1A and stimulation of reactive oxygen species production by co-planar polychlorinated biphenyl congeners

The non-ortho-polychlorinated biphenyl (PCB) congener 3,3'4,4'-tetrachlorobiphenyl (PCB 77) can uncouple the catalytic cycle of fish (scup) cytochrome P4501A (CYP1A) and mammalian (rat, human) CYP1A1, stimulating release of reactive oxygen species (ROS). PCB 77 also inactivates CYP1A in an NADPH-, oxygen-, and time-dependent process, linked to uncoupling. We addressed a hypothesis that planar halogenated hydrocarbons generally will uncouple CYP1A. Thus, additional PCB congeners including non-ortho-3,3',4,4',5'-pentachlorobiphenyl (PCB 126) and 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169), mono-ortho-2,3,3',4,4'-pentachlorobiphenyl (PCB 105) and di-ortho-2,2',5,5'-tetrachlorobiphenyl (PCB 52), as well as the polycyclic aromatic hydrocarbon benzo[a]pyrene (B[a]P), were examined for their ability to stimulate microsomal ROS production and to inactivate CYP1A. Incubated without NADPH, non-ortho-PCB 126 and -PCB 169 both inhibited microsomal CYP1A activity (ethoxyresorufin O-deethylase; EROD). When NADPH was included, these congeners caused a progressive inactivation of CYP1A, in addition to the inhibition. The determined K(Inact) values for inactivation were 0.14 and 0.08 microM, respectively, for PCB 126 and PCB 169, similar to the 0.05 microM for PCB 77 previously reported. The mono-ortho-PCB 105 weakly inhibited and weakly inactivated CYP1A. The di-ortho-PCB 52 neither inhibited nor inactivated CYP1A. Alone, B[a]P strongly inhibited CYP1A, but when NADPH was added that inhibition was reversed, apparently by metabolic depletion of the substrate, and there was no inactivation. PCB 126 and PCB 169 stimulated release of ROS from induced liver microsomes, while B[a]P, PCB 52 and PCB 105 did not. ROS release and CYP1A inactivation stimulated by the non-ortho-PCB 126 and PCB 169 indicate an uncoupling of CYP1A like that previously shown with PCB 77. The uncoupling and release of ROS further suggest a participation of CYP1A in the oxidative stress associated with some planar halogenated aryl hydrocarbon receptor agonists.

Enzyme Electrochemistry — Biocatalysis on an Electrode

Oxidoreductase enzymes catalyze single- or multi-electron reduction/oxidation reactions of small molecule inorganic or organic substrates, and they are integral to a wide variety of biological processes including respiration, energy production, biosynthesis, metabolism, and detoxification. All redox enzymes require a natural redox partner such as an electron-transfer protein (e.g. cytochrome, ferredoxin, flavoprotein) or a small molecule cosubstrate (e.g. NAD(P)H, dioxygen) to sustain catalysis, in effect to balance the substrate/product redox half-reaction. In principle, the natural electron-transfer partner may be replaced by an electrochemical working electrode. One of the great strengths of this approach is that the rate of catalysis (equivalent to the observed electrochemical current) may be probed as a function of applied potential through linear sweep and cyclic voltammetry, and insight to the overall catalytic mechanism may be gained by a systematic electrochemical study coupled with theoretical analysis. In this review, the various approaches to enzyme electrochemistry will be discussed, including direct and indirect (mediated) experiments, and a brief coverage of the theory relevant to these techniques will be presented. The importance of immobilizing enzymes on the electrode surface will be presented and the variety of ways that this may be done will be reviewed. The importance of chemical modification of the electrode surface in ensuring an environment conducive to a stable and active enzyme capable of functioning natively will be illustrated. Fundamental research into electrochemically driven enzyme catalysis has led to some remarkable practical applications. The glucose oxidase enzyme electrode is a spectacularly successful application of enzyme electrochemistry. Biosensors based on this technology are used worldwide by sufferers of diabetes to provide rapid and accurate analysis of blood glucose concentrations. Other applications of enzyme electrochemistry are in the sensing of macromolecular complexation events such as antigen–antibody binding and DNA hybridization. The review will include a selection of enzymes that have been successfully investigated by electrochemistry and, where appropriate, discuss their development towards practical biotechnological applications.

Coexpression in yeast of Taxus cytochrome P450 reductase with cytochrome P450 oxygenases involved in Taxol biosynthesis

To maximize redox coupling efficiency with recombinant cytochrome P450 hydroxylases from yew (Taxus) species installed in yeast for the production of the anticancer drug Taxol, a cDNA encoding NADPH:cytochrome P450 reductase from T. cuspidata was isolated. This single-copy gene (2,154 bp encoding a protein of 717 amino acids) resembles more closely other reductases from gymnosperms (approximately 90% similarity) than those from angiosperms (<80% similarity). The recombinant reductase was characterized and compared to other reductases by heterologous expression in insect cells and was shown to support reconstituted taxoid 10beta-hydroxylase activity with an efficiency comparable to that of other plant-derived reductases. Coexpression in yeast of the reductase along with T. cuspidata taxoid 10beta-hydroxylase, which catalyzes an early step of taxoid biosynthesis, demonstrated significant enhancement of hydroxylase activity compared to that supported by the endogenous yeast reductase alone. Functional transgenic coupling of the Taxus reductase with a homologous cytochrome P450 taxoid hydroxylase represents an important initial step in reconstructing Taxol biosynthesis in a microbial host.

Minireview: regulation of steroidogenesis by electron transfer

Cytochrome P450 enzymes catalyze the degradation of drugs and xenobiotics, but also catalyze a wide variety of biosynthetic processes, including most steps in steroidogenesis. The catalytic rate of a P450 enzyme is determined in large part by the rate of electron transfer from its redox partners. Type I P450 enzymes, found in mitochondria, receive electrons from reduced nicotinamide adenine dinucleotide (NADPH) via the intermediacy of two proteins-ferredoxin reductase (a flavoprotein) and ferredoxin (an iron/sulfur protein). Type I P450 enzymes include the cholesterol side-chain cleavage enzyme (P450scc), the two isozymes of 11-hydroxylase (P450c11beta and P450c11AS), and several vitamin D-metabolizing enzymes. Disorders of these enzymes, but not of the two redox partners, have been described. Type II P450 enzymes, found in the endoplasmic reticulum, receive electrons from NADPH via P450 oxidoreductase (POR), which contains two flavin moieties. Steroidogenic Type II P450 enzymes include 17alpha-hydroxylase/17,20 lyase (P450c17), 21-hydroxylase (P450c21), and aromatase (P450aro). All P450 enzymes catalyze multiple reactions, but P450c17 appears to be unique in that the ratio of its activities is regulated at a posttranslational level. Three factors can increase the degree of 17,20 lyase activity relative to the 17alpha-hydroxylase activity by increasing electron flow from POR: a high molar ratio of POR to P450c17, serine phosphorylation of P450c17, and the presence of cytochrome b(5), acting as an allosteric factor to promote the interaction of POR with P450c17. POR is required for the activity of all 50 human Type II P450 enzymes, and ablation of the Por gene in mice causes embryonic lethality. Nevertheless, mutation of the human POR gene is compatible with life, causing multiple steroidogenic defects and a skeletal dysplasia called Antley-Bixler syndrome.

Diversity and function of mutations in p450 oxidoreductase in patients with Antley-Bixler syndrome and disordered steroidogenesis

P450 oxidoreductase (POR) is the obligatory flavoprotein intermediate that transfers electrons from reduced nicotinamide adenine dinucleotide phosphate (NADPH) to all microsomal cytochrome P450 enzymes. Although mouse Por gene ablation causes embryonic lethality, POR missense mutations cause disordered steroidogenesis, ambiguous genitalia, and Antley-Bixler syndrome (ABS), which has also been attributed to fibroblast growth factor receptor 2 (FGFR2) mutations. We sequenced the POR gene and FGFR2 exons 8 and 10 in 32 individuals with ABS and/or hormonal findings that suggested POR deficiency. POR and FGFR2 mutations segregated completely. Fifteen patients carried POR mutations on both alleles, 4 carried mutations on only one allele, 10 carried FGFR2 or FGFR3 mutations, and 3 patients carried no mutations. The 34 affected POR alleles included 10 with A287P (all from whites) and 7 with R457H (four Japanese, one African, two whites); 17 of the 34 alleles carried 16 "private" mutations, including 9 missense and 7 frameshift mutations. These 11 missense mutations, plus 10 others found in databases or reported elsewhere, were recreated by site-directed mutagenesis and were assessed by four assays: reduction of cytochrome c, oxidation of NADPH, support of 17alpha-hydroxylase activity, and support of 17,20 lyase using human P450c17. Assays that were based on cytochrome c, which is not a physiologic substrate for POR, correlated poorly with clinical phenotype, but assays that were based on POR's support of catalysis by P450c17--the enzyme most closely associated with the hormonal phenotype--provided an excellent genotype/phenotype correlation. Our large survey of patients with ABS shows that individuals with an ABS-like phenotype and normal steroidogenesis have FGFR mutations, whereas those with ambiguous genitalia and disordered steroidogenesis should be recognized as having a distinct new disease: POR deficiency.

Electrochemical reduction of cytochrome P450 as an approach to the construction of biosensors and bioreactors

In the present review an attempt was made to present an up-to-date amount of the data on electrochemical reduction of the hemoprotein cytochrome P450. The concept and potentialities of enzyme electrodes--transducers--as the main element for construction of electrochemical biosensors were discussed. Different types of electrodes for bioelectrochemistry were analysed. New nanotechnological approaches to cytochrome P450 immobilisation were reported. It was shown that nanobiotechnology in electrochemistry has potential application in manufacturing biosensors and bioreactors for clinical medicine and pharmacology.

Stereospecific biocatalytic epoxidation: the first example of direct regeneration of a FAD-dependent monooxygenase for catalysis

Catalysis for chemical synthesis by cell-free monooxygenases necessitates an efficient and robust in situ regeneration system to supply the enzyme with reducing equivalents. We report on a novel approach to directly regenerate flavin-dependent monooxygenases. The organometallic complex [CpRh(bpy)(H(2)O)](2+) catalyzes the transhydrogenation reaction between formate and isoalloxazine-based cofactors such as FAD and FMN. Coupling this FADH(2) regeneration reaction to the FADH(2)-dependent styrene monooxygenase (StyA) resulted in a chemoenzymatic epoxidation reaction where the organometallic compound substitutes for the native reductase (StyB), the nicotinamide coenzyme (NAD), and an artificial NADH regeneration system such as formate dehydrogenase. Various styrene derivatives were converted into the essentially optically pure (S)-epoxides (ee > 98%). In addition, StyA was shown to be capable of performing sulfoxidation reactions. The productivity of the chemoenzymatic epoxidation reaction using 6.5 microM StyA reached up to 6.4 mM/h, corresponding to approximately 70% of a comparable fully enzymatic reaction using StyB, NADH, and formate dehydrogenase for regeneration. The coupling efficiency of the nonenzymatic regeneration reaction to enzymatic epoxidation was examined in detail, leading to an optimized reaction setup with minimized quenching of the electron supply for the epoxidation reaction. Thus, up to 60% of the reducing equivalents provided via [CpRh(bpy)(H(2)O)](2+) could be channeled into epoxide rather than hydrogen peroxide formation, allowing selective synthesis with high yields.

Organization of multiple cytochrome P450s with NADPH-cytochrome P450 reductase in membranes

Microsomal P450-mediated monooxygenase activity supported by NADPH requires an interaction between flavoprotein NADPH-cytochrome P450 reductase and cytochrome P450. These proteins have been identified as the simplest system (with the inclusion of a phospholipid (PL) component) that possesses monooxygenase function; however, little is known about the organization of these proteins in the microsomal membrane. Although reductase and P450 are known to form a 1:1 functional complex, there exists a 10- to 20-fold excess of P450 over the reductase. This raises several questions including "How are the enzymes of the P450 system organized in the microsomal membrane?" and "Can one P450 enzyme affect the functional characteristics of another P450?" This review summarizes evidence supporting the potential for enzymes involved in the P450 system to interact, focusing on the interactions between reductase and P450 and interactions between multiple P450 enzymes. Studies on the aggregation characteristics of P450 as well as on rotational diffusion are detailed, with a special emphasis on the potential for P450 enzymes to produce oligomeric complexes and to suggest the environment in which P450 exists in the endoplasmic reticulum. Finally, more recent studies describing the potential for multiple P450s to exist as complexes and their effect on P450 function are presented, including studies using reconstituted systems as well as systems where two P450s are coexpressed in the presence of reductase. An understanding of the interactions among reductase and multiple P450s is important for predicting conditions where the drug disposition may be altered by the direct effects of P450-P450 complex formation. Furthermore, the potential for one P450 enzyme to affect the behavior of another P450 may be extremely important for drug screening and development, requiring metabolic screening of a drug with reconstituted systems containing multiple P450s rather than simpler systems containing only a single form.

The in vitro effect of Helichrysi flos on microsomal lipid peroxidation

Helichrysum arenarium (L.) Moench has long been known as a medicinal plant in Europe for its cholagogue, choleretic, hepatoprotective and detoxifying activities. Antioxidant properties of its main phenolics, flavonoids may be supposed to be responsible for these effects. The aim of this study was to verify the antioxidant properties of lyophilized water extracts with different polyphenol and flavonoid contents from inflorescences. The effects of natural extracts on microsomal fraction of rat liver were examined. Enzymatically induced lipid peroxidation and NADPH cytochrome c reductase activity in liver microsomes were measured by spectrophotometric methods. Results were compared with the activity of silibinin flavonoid, the main agent of well-known milk thistle (Silybum marianum L.). The natural plant extracts diminish the enzymatically induced lipid peroxidation in a concentration-dependent manner and reduce the cytochrome c dose dependently. The sample with higher polyphenol and flavonoid contents showed more stimulation of NADPH cytochrome c reductase. The lyophilized Helichrysi flos extracts proved to be more effective compared to silibinin in examined concentrations.

Evolutionarily divergent electron donor proteins interact with P450MT2 through the same helical domain but different contact points

We have investigated the sites of N-terminally truncated cytochrome P4501A1 targeted to mitochondria (P450MT2) which interact with adrenodoxin (Adx), cytochrome P450 reductase (CPR) and bacterial flavodoxin (Fln). The binding site was mapped by a combination of in vitro mutagenesis, in vivo screening with a mammalian two-hybrid system, spectral analysis, reconstitution of enzyme activity and homology-based structural modeling. Our results show that part of an aqueous accessible helix (putative helix G, residues 264-279) interacts with all three electron donor proteins. Mutational studies revealed that Lys267 and Lys271 are crucial for binding to Adx, while Lys268 and Arg275 are important for binding to CPR and FLN: Additive effects of different electron donor proteins on enzyme activity and models on protein docking show that Adx and CPR bind in a non-overlapping manner to the same helical domain in P450MT2 at different angular orientations, while CPR and Fln compete for the same binding site. We demonstrate that evolutionarily divergent electron donor proteins interact with the same domain but subtly different contact points of P450MT2.

Protein engineering of cytochromes P-450

The cytochromes P-450 are an immensely important superfamily of heme-containing enzymes. They catalyze the monooxygenation of an enormous range of substrates. In bacteria, cytochromes P-450 are known to catalyze the hydroxylation of environmentally significant substrates such as camphor, phenolic compounds and many herbicides. In eukaryotes, these enzymes perform key roles in the synthesis and interconversion of steroids, while in mammals hepatic cytochromes P-450 are vital for the detoxification of many drugs. As such, the cytochromes P-450 are of considerable interest in medicine and biotechnology and are obvious targets for protein engineering. The purpose of this article is to illustrate the ways in which protein engineering has been used to investigate and modify the properties of cytochromes P-450. Illustrative examples include: the manipulation of substrate selectivity and regiospecificity, the alteration of membrane binding properties, and probing the route of electron transfer.

P450 in biotechnology: zinc driven omega-hydroxylation of p-nitrophenoxydodecanoic acid using P450 BM-3 F87A as a catalyst

Cytochrome P450 enzymes require the delivery of two electrons to the heme protein for their enzymatic function. NADPH or NADH are usually used as reduction equivalents. In the absence of a substrate, NADPH may inactivate P450 enzymes. Furthermore, it is expensive, making it unsuitable for the preparative synthesis of fine chemicals. Approaches for replacing NADPH with an electrochemically generated reduction by using platinum-electrodes and different mediators are known. In the present study, NADPH was substituted by the mediator cobalt(III)sepulchrate and zinc dust that serves as an electron source. The mutated fatty acid hydroxylase P450 BM-3 F87A from Bacillus megaterium was chosen as a catalyst, since it shows a three-fold higher sensitivity and a nearly five-fold higher activity for p-nitrophenoxydodecanoic acid (12-pNCA) than the wild-type enzyme. The formation of p-nitrophenolate can easily be monitored using a photometer at 410 nm. The turnover rate of the zinc/cobalt(III)sepulchrate system reaches 20% of the NADPH activity. Compared to the electrochemical approaches the activity is at least 77% higher (turnover 125 eq min-1). The presented alternative cofactor system can be used instead of NADPH or expensive electrochemical devices (platinum electrodes) for fine chemical synthesis.

Oxidative inactivation of cytochrome P-450 1A (CYP1A) stimulated by 3,3',4,4'-tetrachlorobiphenyl: production of reactive oxygen by vertebrate CYP1As

Microsomal cytochrome P-450 1A (CYP1A) in a vertebrate model (the teleost fish scup) is inactivated by the aryl hydrocarbon receptor agonist 3,3',4,4'-tetrachlorobiphenyl (TCB). Here, the mechanism of CYP1A inactivation and its relationship to reactive oxygen species (ROS) formation were examined by using liver microsomes from scup and rat and expressed human CYP1As. In vitro inactivation of scup CYP1A activity 7-ethoxyresorufin O-deethylation by TCB was time dependent, NADPH dependent, oxygen dependent, and irreversible. TCB increased microsomal NADPH oxidation rates, and CYP1A inactivation was lessened by adding cytochrome c. CYP1A inactivation was accompanied by loss of spectral P-450, a variable loss of heme and a variable appearance of P-420. Rates of scup liver microsomal metabolism of TCB were < 0.5 pmol/min/mg, 25-fold less than the rate of P-450 loss. Non-heme iron chelators, antioxidant enzymes, and ROS scavengers had no influence on inactivation. Inactivation was accelerated by H(2)O(2) and azide but not by hydroxylamine or aminotriazole. TCB also inactivated rat liver microsomal CYP1A, apparently CYP1A1. Adding TCB to scup or rat liver microsomes containing induced levels of CYP1A, but not control microsomes, stimulated formation of ROS; formation rates correlated with native CYP1A1 content. TCB stimulated ROS formation by baculovirus-expressed human CYP1A1 but not CYP1A2. The results indicate that TCB uncouples the catalytic cycle of CYP1A, ostensibly CYP1A1, resulting in formation of ROS within the active site. These ROS may inactivate CYP1A or escape from the enzyme. ROS formed by CYP1A1 may contribute to the toxicity of planar halogenated aromatic hydrocarbons.

Treatment of mammalian cells with the endoplasmic reticulum-proliferator compactin strongly induces recombinant and endogenous xenobiotic metabolizing enzymes and 3-hydroxy-3-methylglutaryl-CoA reductase in vitro

Some xenobiotics induce membrane-bound drug metabolizing enzymes (Xme) and a profound proliferation of the endoplasmic reticulum (ER) in vivo. However these effects are much weaker in vitro, possibly due to absence of certain transcription factors. We tested the possibility that ER proliferation can affect the level of ER-resident enzymes even in the absence of transcriptional activation. For this purpose we analysed the effects of compactin, which has been shown to induce ER proliferation in vitro, on recombinant Xme, which were expressed from a constitutive viral promoter. High levels of recombinant UDP-glucuronosyltransferase UGT1A6 were achieved by amplification of the UGT1A6 cDNA using the dihydrofolate reductase cDNA as selectable marker in DHFR- CHO cells. Treatment of the resulting cell lines with lipoprotein-deficient serum in the absence and presence of compactin for 5 days resulted in a 1.3- and 2.3-fold, respectively, increase of the UGT enzyme activity towards 4-methylumbelliferone, paralleled by an induction of immunoreactive UGT1A6 protein. Similarly, treatment with this 3-hydroxy-3-methylglutaryl-CoA reductase inhibitor increased the endogenous P450 reductase activity 2.6-fold, concomitant with an increase of immunodetectable protein. As expected compactin induced the level of 3-hydroxy-3-methylglutaryl-CoA reductase. Increased levels of this protein have been associated with a proliferation of the ER. Compactin treatment of a separate cell line that expressed recombinant human P450 reductase increased this enzyme activity fivefold. Pulse-chase experiments revealed that the induction of the recombinant Xme by compactin was most likely due to decreased protein degradation. Our results show that enzyme systems unrelated to those involved in cholesterol biosynthesis are affected by compounds known to affect membrane biogenesis. Since this effect extends to heterologously expressed enzymes, it also provides an efficient means by which to increase the levels of recombinant ER proteins.

Escherichia coli flavodoxin sepharose as an affinity resin for cytochromes P450 and use to identify a putative cytochrome P450c17/3beta-hydroxysteroid dehydrogenase interaction

Flavodoxin Sepharose (Fld Sepharose), a reagent originally developed to demonstrate an interaction between native Escherichia coli Fld and cytochrome P450c17, has been synthesized, using highly expressed (7 micromol Fld/liter E. coli culture) recombinant E. coli Fld, for use as an affinity resin for microsomal cytochromes P450. As a test of the specificity of Fld Sepharose, we have examined the utility of this resin for purification of P450c17 and P450c21 from a relatively crude mixture of solubilized adrenocortical microsomal proteins. Chromatography of this mixture on Fld Sepharose resulted in a threefold enrichment of cytochrome P450 specific content without spectrally detectable P450 denaturation. Electrophoretic and immunoblot analyses of fractions eluted from the Fld Sepharose column revealed the presence of P450c17 and P450c21, both of which were sufficiently pure, after SDS-PAGE, for identification by N-terminal sequence analysis. Intriguingly, a major protein copurifying with P450c17 and P450c21 was identified as 3beta-hydroxysteroid dehydrogenase (3beta-HSD) which was subsequently found not to directly bind Fld Sepharose. Purified bovine 3beta-HSD covalently linked to Sepharose can bind recombinant bovine P450c17, an interaction which is partially disrupted upon mild heat denaturation of P450c17 or by the nonionic detergent Emulgen. This interaction, however, does not appear to affect P450c17 hydroxylase and lyase activities as measured in vitro. From these results, we propose that 3beta-HSD and P450c17 may associate, perhaps as part of a steroidogenic complex, in the endoplasmic reticulum.

Expression of drug-metabolizing enzymes

A principal advance in the production of drug-metabolizing enzymes has been the development of catalytically self-sufficient cytochrome P450 systems, including additional P450-reductase fusion proteins and Escherichia coli and baculovirus coexpression constructs. Continuing work with glutathione transferases has resulted in the identification of important residues by random mutagenesis screening techniques, as well as in the engineering of model Salmonella typhimurium strains for genotoxicity analysis.

Negatively charged anabaena flavodoxin residues (Asp144 and Glu145) are important for reconstitution of cytochrome P450 17alpha-hydroxylase activity

Catalysis by microsomal cytochromes P450 requires the membrane-bound enzyme NADPH-cytochrome P450 reductase (P450 reductase), which transfers electrons to the P450 heme via a flavodoxin-like domain. Previously, we reported that Escherichia coli flavodoxin (Fld), a soluble electron transfer protein, directly interacts with bovine cytochrome P450 17alpha-hydroxylase/17,20-lyase (P450c17) and donates electrons to this enzyme when reconstituted with NADPH-ferredoxin (flavodoxin) reductase (FNR) (Jenkins, C. M., and Waterman, M. R. (1994) J. Biol. Chem. 269, 27401-27408). To investigate whether flavodoxins can serve as useful models of the analogous domain in P450 reductase, we have examined the FNR-Fld system from the cyanobacterium Anabaena. Mutagenesis of two acidic Anabaena Fld residues (D144A and E145A) significantly decreased flavodoxin-supported P450c17 progesterone 17alpha-hydroxylase activity. Specifically, D144A exhibited only 15% of the activity of wild-type Fld, whereas the adjacent mutation, E145A, caused a 40% loss in activity. P450-dependent hydrogen peroxide/superoxide production by wild-type FNR-Fld was measurably higher than that generated by FNR-D144A or FNR-E145A, indicating that the mutations do not lead to P450 heme-mediated electron uncoupling. Interestingly, the D144A and E145A mutants bind with equal or even greater affinity to P450c17 than wild-type Fld. Furthermore, these mutations (D144A and E145A) actually increased cytochrome c reductase activity (35 and 100% higher than wild type). Anabaena Fld residues Asp144 and Glu145 align closely with rat P450 reductase residue Asp208, which has been shown by mutagenesis to be important in electron transfer to P4502B1 but not to cytochrome c (Shen, A. L., and Kasper, C. B. (1995) J. Biol. Chem. 270, 27475-27480). Thus, these residues in flavodoxins and P450 reductase appear to have similar functions in P450 recognition and/or electron transfer, supporting the hypothesis that flavodoxins represent valid models for the FMN-binding domain of P450 reductase.

Three-dimensional structure of NADPH-cytochrome P450 reductase: prototype for FMN- and FAD-containing enzymes

Microsomal NADPH-cytochrome P450 reductase (CPR) is one of only two mammalian enzymes known to contain both FAD and FMN, the other being nitric-oxide synthase. CPR is a membrane-bound protein and catalyzes electron transfer from NADPH to all known microsomal cytochromes P450. The structure of rat liver CPR, expressed in Escherichia coli and solubilized by limited trypsinolysis, has been determined by x-ray crystallography at 2.6 A resolution. The molecule is composed of four structural domains: (from the N- to C- termini) the FMN-binding domain, the connecting domain, and the FAD- and NADPH-binding domains. The FMN-binding domain is similar to the structure of flavodoxin, whereas the two C-terminal dinucleotide-binding domains are similar to those of ferredoxin-NADP+ reductase (FNR). The connecting domain, situated between the FMN-binding and FNR-like domains, is responsible for the relative orientation of the other domains, ensuring the proper alignment of the two flavins necessary for efficient electron transfer. The two flavin isoalloxazine rings are juxtaposed, with the closest distance between them being about 4 A. The bowl-shaped surface near the FMN-binding site is likely the docking site of cytochrome c and the physiological redox partners, including cytochromes P450 and b5 and heme oxygenase.

The function of recombinant cytochrome P450s in intact Escherichia coli cells: the 17 alpha-hydroxylation of progesterone and pregnenolone by P450c17

Studies are reported showing that recombinant P450c17, coexpressed with rat NADPH-P450 reductase or expressed as a fusion protein containing the domain of the P450 linked to the domain of NADPH-P450 reductase, function effectively in intact Escherichia coli cells. Progesterone is rapidly hydroxylated by transformed E. coli cells at rates as rapid as 50 nmol of steroid hydroxylated/min/nmol of P450 at 37 degrees C. This rate measured in vivo equals or exceeds the best rates we have measured when reconstituting progesterone hydroxylase activity in vitro using purified recombinant bovine P450c17 and purified recombinant rat NADPH-P450 reductase. The limits imposed in vivo by the availability of reducing equivalents (NADPH) and molecular oxygen are identified by showing the nearly fivefold increase in hydroxylation activity when glucose is present and the tendency for the constitutive respiratory activity of E. coli to limit the availability of oxygen required for the P450-catalyzed reaction. The rate of progesterone metabolism is about 200 times faster by P450c17 coexpressed with NADPH-P450 reductase than when P450c17 functions with the constitutive electron transfer system of E. coli (flavodoxin and flavodoxin reductase). Expression of the fusion protein, termed rF450[mBov17A/mRatOR]L1, results in a rate of progesterone metabolism in vivo at 37 degrees C of about 15 nmol of steroid hydroxylated/min/nmol of P450. Pregnenolone is actively metabolized to dehydroepiandrosterone at rates similar to those seen when the P450 activity is reconstituted in vitro with cytochrome b5. Experiments are described showing that the limited solubility of progesterone in water imposes a limit on the extent of steroid hydroxylated. The practicality of this type of P450-containing system for the bioconversion of large amounts of a chemical for the manufacture of speciality chemicals is discussed.