A second FMN binding site in yeast NADPH-cytochrome P450 reductase suggests a mechanism of electron transfer by diflavin reductases

Mitochondria-endoplasmic reticulum crosstalk in apoptosis: The interactions of cytochrome c with monooxygenase and its reductase

The crosstalk between endoplasmic reticulum and mitochondria is of significance in apoptosis, in which cytochrome b5 (Cyt b5) is thought to be a major target for cytochrome c (Cyt c) upon its release from the mitochondria. In the absence of Cyt b5, the role of interactions of Cyt c with CYP-dependent monooxygenase system in apoptotic regulation was explored in this study. NADPH-dependent and Cyt c-induced formation of reactive oxygen species (ROS) and NADPH-independent Cyt c unfolding were revealed. With the aid of a CPR inhibitor and CYP antibodies, the interactions among Cyt c, cytochrome P450 reductase (CPR) and cytochrome P450 (CYP) are evidenced, which are found crucial for monooxygenase-derived ROS formation. The underlying structural basis of Cyt c-CYP complex was unveiled by molecular dynamics simulations. This study provides novel insights into how Cyt c regulates ROS formation through the interactions with CPR and CYP, and is implicated for a deeper understanding of the regulation mechanism in the mitochondria-endoplasmic reticulum apoptotic pathway.

Metabolite changes by combined treatment, ethyl formate and low temperature, in Drosophila suzukii

Although ethyl formate (EF) fumigant and low temperature applications are widely used for pest management, studies related to their mechanisms of action and subsequent metabolic changes in Drosophila suzukii models are still unclear. In this study, a comparative metabolome analysis was performed to investigate the major metabolites modified by EF and low temperature and how they are related to and affect insect physiology. Most of the identified metabolites function in metabolic pathways related to the biosynthesis of amino acids, nucleotides and cofactors. In addition, a combined treatment with EF and low temperature significantly altered the tricarboxylic acid cycle (TCA) and the levels of the purine and pyrimidine classes of metabolites. Interestingly, the levels of cytochrome P450 and glutathione metabolites involved in detoxification dramatically changed under stress conditions compared to those in the control group.

Electrochemical transformations catalyzed by cytochrome P450s and peroxidases

Cytochrome P450s (Cyt P450s) and peroxidases are enzymes featuring iron heme cofactors that have wide applicability as biocatalysts in chemical syntheses. Cyt P450s are a family of monooxygenases that oxidize fatty acids, steroids, and xenobiotics, synthesize hormones, and convert drugs and other chemicals to metabolites. Peroxidases are involved in breaking down hydrogen peroxide and can oxidize organic compounds during this process. Both heme-containing enzymes utilize active Fe^IVO intermediates to oxidize reactants. By incorporating these enzymes in stable thin films on electrodes, Cyt P450s and peroxidases can accept electrons from an electrode, albeit by different mechanisms, and catalyze organic transformations in a feasible and cost-effective way. This is an advantageous approach, often called bioelectrocatalysis, compared to their biological pathways in solution that require expensive biochemical reductants such as NADPH or additional enzymes to recycle NADPH for Cyt P450s. Bioelectrocatalysis also serves as an ex situ platform to investigate metabolism of drugs and bio-relevant chemicals. In this paper we review biocatalytic electrochemical reactions using Cyt P450s including C-H activation, S-oxidation, epoxidation, N-hydroxylation, and oxidative N-, and O-dealkylation; as well as reactions catalyzed by peroxidases including synthetically important oxidations of organic compounds. Design aspects of these bioelectrocatalytic reactions are presented and discussed, including enzyme film formation on electrodes, temperature, pH, solvents, and activation of the enzymes. Finally, we discuss challenges and future perspective of these two important bioelectrocatalytic systems.

Mitochondrial complexome reveals quality-control pathways of protein import

Mitochondria have crucial roles in cellular energetics, metabolism, signalling and quality control1-4. They contain around 1,000 different proteins that often assemble into complexes and supercomplexes such as respiratory complexes and preprotein translocases1,3-7. The composition of the mitochondrial proteome has been characterized1,3,5,6; however, the organization of mitochondrial proteins into stable and dynamic assemblies is poorly understood for major parts of the proteome1,4,7. Here we report quantitative mapping of mitochondrial protein assemblies using high-resolution complexome profiling of more than 90% of the yeast mitochondrial proteome, termed MitCOM. An analysis of the MitCOM dataset resolves >5,200 protein peaks with an average of six peaks per protein and demonstrates a notable complexity of mitochondrial protein assemblies with distinct appearance for respiration, metabolism, biogenesis, dynamics, regulation and redox processes. We detect interactors of the mitochondrial receptor for cytosolic ribosomes, of prohibitin scaffolds and of respiratory complexes. The identification of quality-control factors operating at the mitochondrial protein entry gate reveals pathways for preprotein ubiquitylation, deubiquitylation and degradation. Interactions between the peptidyl-tRNA hydrolase Pth2 and the entry gate led to the elucidation of a constitutive pathway for the removal of preproteins. The MitCOM dataset-which is accessible through an interactive profile viewer-is a comprehensive resource for the identification, organization and interaction of mitochondrial machineries and pathways.

NADPH-cytochrome P450 reductase knockdown decreases the response to precocene I in the migratory locust Locusta migratoria

Precocene I is a juvenile hormone antagonist that needs to be activated via oxidative biotransformation catalyzed by cytochrome P450 (CYP). NADPH-cytochrome P450 reductase (CPR) supplies CYP with electrons in the oxidation-reduction process; however, its functional role in the activation of precocene I remains unexplored. Here, the representative characteristics of CPRs were analyzed in the CPR gene of Locusta migratoria (LmCPR), the result of model docking indicated that the hydrogen bonds were formed between reduced nicotinamide adenine dinucleotide phosphate (NADPH), flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN) and NADPH-, FAD-, FMN-domains of LmCPR, respectively. Treating the fourth-instar nymphs with precocene I decreased the juvenile hormone titers of nymphs to 0.55-fold of that in acetone-treated controls, and extended the interval time between fourth- and fifth-instar nymphs. 68.75% of the treated fourth-instar nymphs developed into precocious adults in the fifth-instar. LmCPR knockdown decreased the response to precocene I in the nymphs, the occurrence rate of precocious adults induced by precocene I treatment reduced by 23.11%. Therefore, LmCPR may be involved in the activation of precocene I in L. migratoria. In addition, we generated an active recombinant LmCPR protein using a prokaryotic expression system, its activity in reducing cytochrome c was 33.13 ± 11.50 nmol CytCred/min/μg protein. This study lays the foundation for further research on the role of LmCPR in precocene I activation.

Biochemical and structural insights into the cytochrome P450 reductase from Candida tropicalis

Cytochrome P450 reductases (CPRs) are diflavin oxidoreductases that supply electrons to type II cytochrome P450 monooxygenases (CYPs). In addition, it can also reduce other proteins and molecules, including cytochrome c, ferricyanide, and different drugs. Although various CPRs have been functionally and structurally characterized, the overall mechanism and its interaction with different redox acceptors remain elusive. One of the main problems regarding electron transfer between CPRs and CYPs is the so-called “uncoupling”, whereby NAD(P)H derived electrons are lost due to the reduced intermediates’ (FAD and FMN of CPR) interaction with molecular oxygen. Additionally, the decay of the iron-oxygen complex of the CYP can also contribute to loss of reducing equivalents during an unproductive reaction cycle. This phenomenon generates reactive oxygen species (ROS), leading to an inefficient reaction. Here, we present the study of the CPR from Candida tropicalis (CtCPR) lacking the hydrophobic N-terminal part (Δ2–22). The enzyme supports the reduction of cytochrome c and ferricyanide, with an estimated 30% uncoupling during the reactions with cytochrome c. The ROS produced was not influenced by different physicochemical conditions (ionic strength, pH, temperature). The X-ray structures of the enzyme were solved with and without its cofactor, NADPH. Both CtCPR structures exhibited the closed conformation. Comparison with the different solved structures revealed an intricate ionic network responsible for the regulation of the open/closed movement of CtCPR.

Biochemical and structural insights into the cytochrome P450 reductase from Candida tropicalis

Cytochrome P450 reductases (CPRs) are diflavin oxidoreductases that supply electrons to type II cytochrome P450 monooxygenases (CYPs). In addition, it can also reduce other proteins and molecules, including cytochrome c, ferricyanide, and different drugs. Although various CPRs have been functionally and structurally characterized, the overall mechanism and its interaction with different redox acceptors remain elusive. One of the main problems regarding electron transfer between CPRs and CYPs is the so-called "uncoupling", whereby NAD(P)H derived electrons are lost due to the reduced intermediates' (FAD and FMN of CPR) interaction with molecular oxygen. Additionally, the decay of the iron-oxygen complex of the CYP can also contribute to loss of reducing equivalents during an unproductive reaction cycle. This phenomenon generates reactive oxygen species (ROS), leading to an inefficient reaction. Here, we present the study of the CPR from Candida tropicalis (CtCPR) lacking the hydrophobic N-terminal part (Δ2-22). The enzyme supports the reduction of cytochrome c and ferricyanide, with an estimated 30% uncoupling during the reactions with cytochrome c. The ROS produced was not influenced by different physicochemical conditions (ionic strength, pH, temperature). The X-ray structures of the enzyme were solved with and without its cofactor, NADPH. Both CtCPR structures exhibited the closed conformation. Comparison with the different solved structures revealed an intricate ionic network responsible for the regulation of the open/closed movement of CtCPR.

Short-lived neutral FMN and FAD semiquinones are transient intermediates in cryo-reduced yeast NADPH-cytochrome P450 reductase

The electron configuration of flavin cofactors, FMN and FAD, is a critical factor governing the reactivity of NADPH-cytochrome P450 reductase (CPR). The current view of electron transfer by the mammalian CPR, based on equilibrium redox potentials of the flavin cofactors, is that the two electron-reduced FMN hydroquinone (FMNH2), rather than one electron-reduced FMN semiquinone, serves as electron donor to the terminal protein acceptors. However, kinetic and thermodynamic studies on the CPR species originated from different organisms have shown that redox potentials measured at distinct electron transfer steps differ from redox potentials determined by equilibrium titration. Collectively, previous observations suggest that the short-lived transient semiquinone species may carry electrons in diflavin reductases. In this work, we have investigated spectroscopic properties of the CPR-bound FAD and FMN reduced at 77 K by radiolytically-generated thermalized electrons. Using UV-vis spectroscopy, we demonstrated that upon cryo-reduction of oxidized yeast CPR (yCPR) containing an equimolar ratio of both FAD and FMN, or FAD alone, neutral semiquinones were trapped at 77 K. During annealing at the elevated temperatures, unstable short-lived neutral semiquinones relaxed to spectroscopically distinct air-stable neutral semiquinones. This transition was independent of pH within the 6.0-10.7 range. Our data on yeast CPR are in line with the previous observations of others that the flavin short-lived transient semiquinone intermediates may have a role in the electron transfer by CPR at physiological conditions.

The Hinge Segment of Human NADPH-Cytochrome P450 Reductase in Conformational Switching: The Critical Role of Ionic Strength

NADPH-cytochrome P450 reductase (CPR) is a redox partner of microsomal cytochromes P450 and is a prototype of the diflavin reductase family. CPR contains 3 distinct functional domains: a FMN-binding domain (acceptor reduction), a linker (hinge), and a connecting/FAD domain (NADPH oxidation). It has been demonstrated that the mechanism of CPR exhibits an important step in which it switches from a compact, closed conformation (locked state) to an ensemble of open conformations (unlocked state), the latter enabling electron transfer to redox partners. The conformational equilibrium between the locked and unlocked states has been shown to be highly dependent on ionic strength, reinforcing the hypothesis of the presence of critical salt interactions at the interface between the FMN and connecting FAD domains. Here we show that specific residues of the hinge segment are important in the control of the conformational equilibrium of CPR. We constructed six single mutants and two double mutants of the human CPR, targeting residues G240, S243, I245 and R246 of the hinge segment, with the aim of modifying the flexibility or the potential ionic interactions of the hinge segment. We measured the reduction of cytochrome c at various salt concentrations of these 8 mutants, either in the soluble or membrane-bound form of human CPR. All mutants were found capable of reducing cytochrome c yet with different efficiency and their maximal rates of cytochrome c reduction were shifted to lower salt concentration. In particular, residue R246 seems to play a key role in a salt bridge network present at the interface of the hinge and the connecting domain. Interestingly, the effects of mutations, although similar, demonstrated specific differences when present in the soluble or membrane-bound context. Our results demonstrate that the electrostatic and flexibility properties of the hinge segment are critical for electron transfer from CPR to its redox partners.

Orchestrated Domain Movement in Catalysis by Cytochrome P450 Reductase

NADPH-cytochrome P450 reductase is a multi-domain redox enzyme which is a key component of the P450 mono-oxygenase drug-metabolizing system. We report studies of the conformational equilibrium of this enzyme using small-angle neutron scattering, under conditions where we are able to control the redox state of the enzyme precisely. Different redox states have a profound effect on domain orientation in the enzyme and we analyse the data in terms of a two-state equilibrium between compact and extended conformations. The effects of ionic strength show that the presence of a greater proportion of the extended form leads to an enhanced ability to transfer electrons to cytochrome c. Domain motion is intrinsically linked to the functionality of the enzyme, and we can define the position of the conformational equilibrium for individual steps in the catalytic cycle.

Functional characterization of a soluble NADPH-cytochrome P450 reductase from Fusarium graminearum

Fusarium head blight is a devastating disease in wheat caused by some fungal pathogens of the Fusarium genus mainly F. graminearum, due to accumulation of toxic trichothecenes. Most of the trichothecene biosynthetic pathway has been mapped, although some proteins of the pathway remain uncharacterized, including an NADPH-cytochrome P450 reductase. We subcloned a F. graminearum cytochrome P450 reductase that might be involved in the trichothecene biosynthesis. It was expressed heterologously in E. coli as N-terminal truncated form with an octahistidine tag for purification. The construct yielded a soluble apoprotein and its incubation with flavins yielded the corresponding monomeric holoprotein. It was characterized for activity in the pH range 5.5-9.5, using thiazolyl blue tetrazolium bromide (MTT) or cytochrome c as substrates. Binding of the small molecule MTT was weaker than for cytochrome c, however, the rate of MTT reduction was faster. Contrary to other studies of cytochrome reductase proteins, MTT reduction proceeded in a cooperative manner in our studies. Optimum kinetic activity was found at pH 7.5-8.5 for bothMTT and cytochrome c. This is the first paper presenting characterization of a cytochrome P450 reductase from F. graminearum which most likely is involved in mycotoxin biosynthesis or some primary metabolic pathway such as sterol biosynthesis in F. graminearum.

A well-balanced preexisting equilibrium governs electron flux efficiency of a multidomain diflavin reductase

Diflavin reductases are bidomain electron transfer proteins in which structural reorientation is necessary to account for the various intramolecular and intermolecular electron transfer steps. Using small-angle x-ray scattering and nuclear magnetic resonance data, we describe the conformational free-energy landscape of the NADPH-cytochrome P450 reductase (CPR), a typical bidomain redox enzyme composed of two covalently-bound flavin domains, under various experimental conditions. The CPR enzyme exists in a salt- and pH-dependent rapid equilibrium between a previously described rigid, locked state and a newly characterized, highly flexible, unlocked state. We further establish that maximal electron flux through CPR is conditioned by adjustable stability of the locked-state domain interface under resting conditions. This is rationalized by a kinetic scheme coupling rapid conformational sampling and slow chemical reaction rates. Regulated domain interface stability associated with fast stochastic domain contacts during the catalytic cycle thus provides, to our knowledge, a new paradigm for improving our understanding of multidomain enzyme function.

Molecular Cloning, Expression Pattern and Polymorphisms of NADPH-Cytochrome P450 Reductase in the Bird Cherry-Oat Aphid Rhopalosiphum padi (L.)

NADPH–cytochrome P450 reductase (CPR) plays an important role in the cytochrome P450 (CYP)-mediated metabolism of endogenous and exogenous substrates. CPR has been found to be associated with insecticide metabolism and resistance in many insects. However, information regarding CPR in the bird cherry-oat aphid, Rhopalosiphum padi, is unavailable. In the current study, a full-length cDNA (2,476 bp) of CPR (RpCPR) encoding 681 amino acids was cloned from R. padi. Nucleotide sequence and deduced amino acid sequence analysis showed that RpCPR exhibits characteristics of classical CPRs and shares high identities with those of other insects, especially with the pea aphid, Acyrthosiphon pisum. The mRNA of RpCPR was expressed at all developmental stages, with the highest expression level found in the second instar and the lowest in adult. Expression levels of RpCPR in isoprocarb-resistant and imidacloprid-resistant strains were 3.74- and 3.53-fold higher, respectively, than that of a susceptible strain. RpCPR expression could also be induced by low concentrations (LC₃₀) of isoprocarb and imidacloprid. Moreover, we sequenced the open reading frame (ORF) of RpCPR from 167 field samples collected in 11 geographical populations. Three hundred and thirty-four SNPs were detected, of which, 65 were found in more than two individuals. One hundred and ninety-four missense mutations were present in the amino acid sequence, of which, the P484S mutant had an allele frequency of 35.1%. The present results suggest that RpCPR may play an important role in the P450-mediated insecticide resistance of R. padi to isoprocarb and imidacloprid and possibly other insecticides. Meanwhile, RpCPRmaintains high genetic diversity in natural individuals, which provides the possibility of studying potential correlations between variants and certain special physiological characters.

Knockdown of NADPH-cytochrome P450 reductase results in reduced resistance to buprofezin in the small brown planthopper, Laodelphax striatellus (fallén)

NADPH-cytochrome P450 reductase (CPR) plays an important role in cytochrome P450 function, and CPR knockdown in several insects leads to increased susceptibility to insecticides. However, a putative CPR gene has not yet been fully characterized in the small brown planthopper Laodelphax striatellus, a notorious agricultural pest in rice that causes serious damage by transmitting rice stripe and rice black-streaked dwarf viruses. The objective of this study was to clone the cDNA and to knock down the expression of the gene that encodes L. striatellus CPR (LsCPR) to further determine whether P450s are involved in the resistance of L. striatellus to buprofezin. First, the full-length cDNA of LsCPR was cloned and found to contain an open reading frame (ORF) encoding a polypeptide of 679 amino acids with a calculated molecular mass and isoelectric point of 76.92kDa and 5.37, respectively. The deduced amino acid sequence shares high identity with the CPRs of other insects (98%, 97%, 75% and 68% for Sogatella furcifera, Nilaparvata lugens, Cimex lectularius and Anopheles gambiae, respectively) and possesses the characteristic features of classical CPRs, such as an N-terminal membrane anchor and conserved domains for flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate (NADPH) binding. Phylogenetic analysis revealed that LsCPR is located in a branch along with the CPRs of other hemipteran insects. LsCPR mRNA was detectable in all examined body parts and developmental stages of L. striatellus, as determined by real-time quantitative PCR (qPCR), and transcripts were most abundant in the adult abdomen and in first-instar nymphs and adults. Ingestion of 200μg/mL of LsCPR double-stranded RNA (dsLsCPR) by the planthopper for 5days significantly reduced the transcription level of LsCPR. Moreover, silencing of LsCPR caused increased susceptibility to buprofezin in a buprofezin-resistant (YN-BPF) strain but not in a susceptible (YN) strain. These data further suggested that the P450-mediated metabolic detoxification of xenobiotics might be an important mechanism for buprofezin resistance in L. striatellus.

Silencing NADPH-cytochrome P450 reductase results in reduced acaricide resistance in Tetranychus cinnabarinus (Boisduval)

Cytochrome P450 monooxygenases (P450s) are involved in metabolic resistance to insecticides and require NADPH cytochrome P450 reductase (CPR) to transfer electrons when they catalyze oxidation reactions. The carmine spider mite, Tetranychus cinnabarinus is an important pest mite of crop and vegetable plants worldwide, and its resistance to acaricides has quickly developed. However, the role of CPR on the formation of acaricide-resistance in T. cinnabarinus is still unclear. In this study, a full-length cDNA encoding CPR was cloned and characterized from T. cinnabarinus (designated TcCPR). TcCPR expression was detectable in all developmental stages of T. cinnabarinus, but it’s much lower in eggs. TcCPR was up-regulated and more inducible with fenpropathrin treatment in the fenpropathrin-resistant (FeR) strain compared with the susceptible SS strain. Feeding of double-strand RNA was effective in silencing the transcription of TcCPR in T. cinnabarinus, which resulted in decreasing the activity of P450s and increasing the susceptibility to fenpropathrin in the FeR strain but not in the susceptible strain. The current results provide first evidence that the down-regulation of TcCPR contributed to an increase of the susceptibility to fenpropathrin in resistant mites. TcCPR could be considered as a novel target for the development of new pesticides.

Isolation and Expression Analysis of CYP9A11 and Cytochrome P450 Reductase Gene in the Beet Armyworm (Lepidoptera: Noctuidae)

Cytochrome P450 monooxygenases (CYPs), as an enzyme superfamily, is widely distributed in organisms and plays a vital function in the metabolism of exogenous and endogenous compounds by interacting with its obligatory redox partner, CYP reductase (CPR). A novel CYP gene (CYP9A11) and CPR gene from the agricultural pest insect Spodoptera exigua were cloned and characterized. The complete cDNA sequences of SeCYP9A11 and SeCPR are 1,931 and 3,919 bp in length, respectively, and contain open reading frames of 1,593 and 2,070 nucleotides, respectively. Analysis of the putative protein sequences indicated that SeCYP9A11 contains a heme-binding domain and the unique characteristic sequence (SRFALCE) of the CYP9 family, in addition to a signal peptide and transmembrane segment at the N-terminal. Alignment analysis revealed that SeCYP9A11 shares the highest sequence similarity with CYP9A13 from Mamestra brassicae, which is 66.54%. The putative protein sequence of SeCPR has all of the classical CPR features, such as an N-terminal membrane anchor; three conserved domain flavin adenine dinucleotide (FAD), flavin mononucleotide (FMN), and nicotinamide adenine dinucleotide phosphate (NADPH) domain; and characteristic binding motifs. Phylogenetic analysis revealed that SeCPR shares the highest identity with HaCPR, which is 95.21%. The SeCYP9A11 and SeCPR genes were detected in the midgut, fat body, and cuticle tissues, and throughout all of the developmental stages of S. exigua. The mRNA levels of SeCYP9A11 and SeCPR decreased remarkably after exposure to plant secondary metabolites quercetin and tannin. The results regarding SeCYP9A11 and SeCPR genes in the current study provide foundation for the further study of S. exigua P450 system.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

RNA interference of NADPH-cytochrome P450 reductase of the rice brown planthopper, Nilaparvata lugens, increases susceptibility to insecticides

BACKGROUND

NADPH-cytochrome P450 reductase (CPR) is essential for numerous biological reactions catalysed by microsomal cytochrome P450 monooxygenases (P450s). Knockdown of CPR in several insects leads to developmental defects and increased susceptibility to insecticides. However, information about the role of CPR in the brown planthopper, Nilaparvata lugens, is still unavailable.

RESULTS

A full-length cDNA encoding CPR was cloned from N. lugens (NlCPR). The deduced amino acid sequence showed marked features of classical CPRs, such as an N-terminus membrane anchor, conserved domains for flavin mononucleotide, flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide phosphate binding, as well as an FAD-binding motif and catalytic residues. Phylogenetic analysis revealed that NlCPR was located in a branch along with bed bug and pea aphid hemipteran insects. NlCPR mRNA was detectable in all tissues and developmental stages of N. lugens, as determined by real-time quantitative PCR. NlCPR transcripts were most abundant in the abdomen in adults, and in first-instar nymphs. Injection of N. lugens with double-strand RNA (dsRNA) against NlCPR significantly reduced the transcription level of the mRNA, and silencing of NlCPR resulted in increased susceptibility in N. lugens to beta-cypermethrin and imidacloprid.

CONCLUSION

The results provide first evidence that NlCPR contributes to the susceptibility to beta-cypermethrin and imidacloprid in N. lugens.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Functional and structural characterisation of a viral cytochrome b5

Cytochrome b5 is a ubiquitous electron transport protein. The sequenced viral OtV-2 genome, which infects Ostreococcus tauri, was predicted to encode a putative cytochrome b5 enzyme. Using purified OtV-2 cytochrome b5 we confirm this protein has identical spectral properties to purified human cytochrome b5 and additionally that the viral enzyme can substitute for yeast cytochrome b5 in yeast cytochrome P450 51 mediated sterol 14α-demethylation. The crystal structure of the OtV-2 cytochrome b5 enzyme reveals a single domain, comprising four β sheets, four α helices and a haem moiety, which is similar to that found in larger eukaryotic cytochrome proteins. As a product of a horizontal gene transfer event involving a subdomain of the host fumarate reductase-like protein, OtV-2 cytochrome b5 appears to have diverged in function and is likely to have evolved an entirely new role for the virus during infection. Indeed, lacking a hydrophobic C-terminal anchor, OtV-2 encodes the first cytosolic cytochrome b5 characterised. The lack of requirement for membrane attachment (in contrast to all other microsomal cytochrome b5s) may be a reflection of the small size of the host cell, further emphasizes the unique nature of this virus gene product and draws attention to the potential importance of cytochrome b5 metabolic activity at the extremes of cellular scale.

PelA deacetylase activity is required for Pel polysaccharide synthesis in Pseudomonas aeruginosa

The Pel polysaccharide serves as an intercellular adhesin for the formation and maintenance of biofilms in the opportunistic pathogen Pseudomonas aeruginosa. Pel biosynthesis requires the products of a seven-gene operon, pelA-pelG, all of which are necessary for Pel-dependent biofilm formation and Pel-related phenotypes. One of the genes, pelA, encodes a protein with a predicted polysaccharide deacetylase domain. In this work, the role of the putative deacetylase domain in Pel production was examined. We first established that purified recombinant PelA hydrolyzed the pseudosubstrate p-nitrophenyl acetate in vitro, and site-specific mutations of predicted deacetylase active-site residues reduced activity greater than 10-fold. Additionally, these mutants were deficient in Pel-dependent biofilm formation and wrinkly colony morphology in vivo. Subcellular fractionation experiments demonstrate that PelA localizes to both the membrane and periplasmic fractions. Finally, antiserum against the Pel polysaccharide was generated, and PelA deacetylase mutants do not produce Pel-reactive material. Taken together, these results suggest that the deacetylase activity of PelA is important for the production of the Pel polysaccharide.

Unusual properties of the cytochrome P450 superfamily

During the early years of cytochrome P450 research, a picture of conserved properties arose from studies of mammalian forms of these monooxygenases. They included the protohaem prosthetic group, the cysteine residue that coordinates to the haem iron and the reduced CO difference spectrum. Alternatively, the most variable feature of P450s was the enzymatic activities, which led to the conclusion that there are a large number of these enzymes, most of which have yet to be discovered. More recently, studies of these enzymes in other eukaryotes and in prokaryotes have led to the discovery of unexpected P450 properties. Many are variations of the original properties, whereas others are difficult to explain because of their unique nature relative to the rest of the known members of the superfamily. These novel properties expand our appreciation of the broad view of P450 structure and function, and generate curiosity concerning the evolution of P450s. In some cases, structural properties, previously not found in P450s, can lead to enzymatic activities impacting the biological function of organisms containing these enzymes; whereas, in other cases, the biological reason for the variations are not easily understood. Herein, we present particularly interesting examples in detail rather than cataloguing them all.

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.

Microbial cytochromes P450: biodiversity and biotechnology. Where do cytochromes P450 come from, what do they do and what can they do for us?

The first eukaryote genome revealed three yeast cytochromes P450 (CYPs), hence the subsequent realization that some microbial fungal genomes encode these proteins in 1 per cent or more of all genes (greater than 100) has been surprising. They are unique biocatalysts undertaking a wide array of stereo- and regio-specific reactions and so hold promise in many applications. Based on ancestral activities that included 14α-demethylation during sterol biosynthesis, it is now seen that CYPs are part of the genes and metabolism of most eukaryotes. In contrast, Archaea and Eubacteria often do not contain CYPs, while those that do are frequently interesting as producers of natural products undertaking their oxidative tailoring. Apart from roles in primary and secondary metabolism, microbial CYPs are actual/potential targets of drugs/agrochemicals and CYP51 in sterol biosynthesis is exhibiting evolution to resistance in the clinic and the field. Other CYP applications include the first industrial biotransformation for corticosteroid production in the 1950s, the diversion into penicillin synthesis in early mutations in fungal strain improvement and bioremediation using bacteria and fungi. The vast untapped resource of orphan CYPs in numerous genomes is being probed and new methods for discovering function and for discovering desired activities are being investigated.

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.

Monitoring shifts in the conformation equilibrium of the membrane protein cytochrome P450 reductase (POR) in nanodiscs

Nanodiscs are self-assembled ∼50-nm(2) patches of lipid bilayers stabilized by amphipathic belt proteins. We demonstrate that a well ordered dense film of nanodiscs serves for non-destructive, label-free studies of isolated membrane proteins in a native like environment using neutron reflectometry (NR). This method exceeds studies of membrane proteins in vesicle or supported lipid bilayer because membrane proteins can be selectively adsorbed with controlled orientation. As a proof of concept, the mechanism of action of the membrane-anchored cytochrome P450 reductase (POR) is studied here. This enzyme is responsible for catalyzing the transfer of electrons from NADPH to cytochrome P450s and thus is a key enzyme in the biosynthesis of numerous primary and secondary metabolites in plants. Neutron reflectometry shows a coexistence of two different POR conformations, a compact and an extended form with a thickness of 44 and 79 Å, respectively. Upon complete reduction by NADPH, the conformational equilibrium shifts toward the compact form protecting the reduced FMN cofactor from engaging in unspecific electron transfer reaction.

Modeling the interfacial interactions between CrtS and CrtR from Xanthophyllomyces dendrorhous , a P450 system involved in astaxanthin production

Xanthophyllomyces dendrorhous is a natural source of astaxanthin, a carotenoid widely used in the food industry. In this yeast, astaxanthin is synthesized from β-carotene by a cytochrome P450, CrtS, which depends on CrtR, the four-domain cytochrome P450 reductase (CPR). Although Saccharomyces cerevisiae has an endogenous CPR (ScCPR), expression of CrtS does not result in astaxanthin production unless it is coexpressed with CrtR. Assuming that CrtS could interact with the FMN-binding domain of either CrtR or ScCPR (XdFMNbd and ScFMNbd, respectively), the aim of this work was to identify possible interaction differences between these alternative complexes by protein modeling and short molecular dynamics simulations. Considering the recently proposed membrane orientation of a mammalian P450, our CrtS-CrtR model predicts that both N-terminal ends stand adjacent to the membrane plane, allowing their anchoring. Compared with the possible interface between CrtS and both FMNbd, the Xanthophyllomyces system appears to be stabilized by more saline bridges.

The closed and compact domain organization of the 70-kDa human cytochrome P450 reductase in its oxidized state as revealed by NMR

The NADPH cytochrome P450 reductase (CPR), a diflavin enzyme, catalyzes the electron transfer (ET) from NADPH to the substrate P450. The crystal structures of mammalian and yeast CPRs show a compact organization for the two domains containing FMN (flavin mononucleotide) and FAD (flavin adenine dinucleotide), with a short interflavin distance consistent with fast ET from the NADPH-reduced FAD to the second flavin FMN. This conformation, referred as "closed", contrasts with the alternative opened or extended domain arrangements recently described for partially reduced or mutant CPR. Internal domain flexibility in this enzyme is indeed necessary to account for the apparently conflicting requirements of having FMN flavin accessible to both the FAD and the substrate P450 at the same interface. However, how interdomain dynamics influence internal and external ETs in CPR is still largely unknown. Here, we used NMR techniques to explore the global, domain-specific and residue-specific structural and dynamic properties of the nucleotide-free human CPR in solution in its oxidized state. Based on the backbone resonance assignment of this 70-kDa protein, we collected residue-specific (15)N relaxation and (1)H-(15)N residual dipolar couplings. Surprisingly and in contrast with previous studies, the analysis of these NMR data revealed that the CPR exists in a unique and predominant conformation that highly resembles the closed conformation observed in the crystalline state. Based on our findings and the previous observations of conformational equilibria of the CPR in partially reduced states, we propose that the large-scale conformational transitions of the CPR during the catalytic cycle are tightly controlled to ensure optimal electron delivery.

RNA interference of NADPH-cytochrome P450 reductase results in reduced insecticide resistance in the bed bug, Cimex lectularius

Background

NADPH-cytochrome P450 reductase (CPR) plays a central role in cytochrome P450 action. The genes coding for P450s are not yet fully identified in the bed bug, Cimex lectularius. Hence, we decided to clone cDNA and knockdown the expression of the gene coding for CPR which is suggested to be required for the function of all P450s to determine whether or not P450s are involved in resistance of bed bugs to insecticides.

Methodology/Principal Findings

The full length Cimex lectularius CPR (ClCPR) cDNA was isolated from a deltamethrin resistant bed bug population (CIN-1) using a combined PCR strategy. Bioinformatics and in silico modeling were employed to identify three conserved binding domains (FMN, FAD, NADP), a FAD binding motif, and the catalytic residues. The critical amino acids involved in FMN, FAD, NADP binding and their putative functions were also analyzed. No signal peptide but a membrane anchor domain with 21 amino acids which facilitates the localization of ClCPR on the endoplasmic reticulum was identified in ClCPR protein. Phylogenetic analysis showed that ClCPR is closer to the CPR from the body louse, Pediculus humanus corporis than to the CPRs from the other insect species studied. The ClCPR gene was ubiquitously expressed in all tissues tested but showed an increase in expression as immature stages develop into adults. We exploited the traumatic insemination mechanism of bed bugs to inject dsRNA and successfully knockdown the expression of the gene coding for ClCPR. Suppression of the ClCPR expression increased susceptibility to deltamethrin in resistant populations but not in the susceptible population of bed bugs.

Conclusions/Significance

These data suggest that P450-mediated metabolic detoxification may serve as one of the resistance mechanisms in bed bugs.

Homology modeling of the three membrane proteins of the dhurrin metabolon: catalytic sites, membrane surface association and protein-protein interactions

Formation of metabolons (macromolecular enzyme complexes) facilitates the channelling of substrates in biosynthetic pathways. Metabolon formation is a dynamic process in which transient structures mediated by weak protein-protein interactions are formed. In Sorghum, the cyanogenic glucoside dhurrin is derived from l-tyrosine in a pathway involving the two cytochromes P450 (CYPs) CYP79A1 and CYP71E1, a glucosyltransferase (UGT85B1), and the redox partner NADPH-dependent cytochrome P450 reductase (CPR). Experimental evidence suggests that the enzymes of this pathway form a metabolon. Homology modeling of the three membrane bound proteins was carried out using the Sybyl software and available relevant crystal structures. Residues involved in tight positioning of the substrates and intermediates in the active sites of CYP79A1 and CYP71E1 were identified. In both CYPs, hydrophobic surface domains close to the N-terminal trans-membrane anchor and between the F' and G helices were identified as involved in membrane anchoring. The proximal surface of both CYPs showed positively charged patches complementary to a negatively charged bulge on CPR carrying the FMN domain. A patch of surface exposed, positively charged amino acid residues positioned on the opposite face of the membrane anchor was identified in CYP71E1 and might be involved in binding UGT85B1 via a hypervariable negatively charged loop in this protein.

High-yield expression of a catalytically active membrane-bound protein: human P450 oxidoreductase

P450 oxidoreductase (POR) is a two-flavin protein that reduces microsomal P450 enzymes and some other proteins. Preparation of active bacterially expressed human POR for biochemical studies has been difficult because membrane-bound proteins tend to interact with column matrices. To reduce column-protein interactions and permit more vigorous washing, human POR lacking 27 N-terminal residues (N-27 POR) was modified to carry a C-terminal Gly3His6-tag (N-27 POR-G3H6). When expressed in Escherichia coli, N-27 POR-G3H6 could be purified to apparent homogeneity by a modified, single-step nickel-nitrilotriacetic acid affinity chromatography, yielding 31 mg POR per liter of culture, whereas standard purification of native N-27 POR required multiple steps, yielding 5 mg POR per liter. Both POR proteins had absorption maxima at 375 and 453 nm and both reduced cytochrome c with indistinguishable specific activities. Using progesterone as substrate for bacterially expressed purified human P450c17, the Michaelis constant for 17α-hydroxylase activity supported by N-27 POR or N-27 POR-G3H6 were 1.73 or 1.49 μm, and the maximal velocity was 0.029 or 0.026 pmol steroids per picomole P450 per minute, respectively. Using 17-hydroxypregnenolone as the P450c17 substrate, the Michaelis constant for 17,20 lyase activity using N-27 POR or N-27 POR-G3H6 was 1.92 or 1.89 μm and the maximal velocity was 0.041 or 0.042 pmol steroid per picomole P450 per minute, respectively. Thus, N-27 POR-G3H6 is equally active as native N-27 POR. This expression and purification system permits the rapid preparation of large amounts of highly pure, biologically active POR and may be generally applicable for the preparation of membrane-bound proteins.

Biochemical comparison of Anopheles gambiae and human NADPH P450 reductases reveals different 2'-5'-ADP and FMN binding traits

NADPH-cytochrome P450 oxidoreductase (CPR) plays a central role in chemical detoxification and insecticide resistance in Anopheles gambiae, the major vector for malaria. Anopheles gambiae CPR (AgCPR) was initially expressed in Eschericia coli but failed to bind 2′, 5′-ADP Sepharose. To investigate this unusual trait, we expressed and purified a truncated histidine-tagged version for side-by-side comparisons with human CPR. Close functional similarities were found with respect to the steady state kinetics of cytochrome c reduction, with rates (k_cat) of 105 s⁻¹ and 88 s⁻¹, respectively, for mosquito and human CPR. However, the inhibitory effects of 2′,5′-ADP on activity were different; the IC₅₀ value of AgCPR for 2′, 5′ –ADP was significantly higher (6–10 fold) than human CPR (hCPR) in both phosphate and phosphate-free buffer, indicative of a decrease in affinity for 2′, 5′- ADP. This was confirmed by isothermal titration calorimetry where binding of 2′,5′-ADP to AgCPR (K_d = 410±18 nM) was ∼10 fold weaker than human CPR (K_d = 38 nM). Characterisation of the individual AgFMN binding domain revealed much weaker binding of FMN (K_d = 83±2.0 nM) than the equivalent human domain (K_d = 23±0.9 nM). Furthermore, AgCPR was an order of magnitude more sensitive than hCPR to the reductase inhibitor diphenyliodonium chloride (IC₅₀ = 28 µM±2 and 361±31 µM respectively). Taken together, these results reveal unusual biochemical differences between mosquito CPR and the human form in the binding of small molecules that may aid the development of ‘smart’ insecticides and synergists that selectively target mosquito CPR.

Role of the interface between the FMN and FAD domains in the control of redox potential and electronic transfer of NADPH-cytochrome P450 reductase

CPR (NADPH-cytochrome P450 reductase) is a multidomain protein containing two flavin-containing domains joined by a connecting domain thought to control the necessary movements of the catalytic domains during electronic cycles. We present a detailed biochemical analysis of two chimaeric CPRs composed of the association of human or yeast FMN with the alternative connecting/FAD domains. Despite the assembly of domains having a relatively large evolutionary distance between them, our data support the idea that the integrity of the catalytic cycle is conserved in our chimaeric enzymes, whereas the recognition, interactions and positioning of both catalytic domains are probably modified. The main consequences of the chimaerogenesis are a decrease in the internal electron-transfer rate between both flavins correlated with changes in the geometry of chimaeric CPRs in solution. Results of the present study highlight the role of the linker and connecting domain in the recognition at the interfaces between the catalytic domains and the impact of interdomain interactions on the redox potentials of the flavins, the internal electron-transfer efficiency and the global conformation and dynamic equilibrium of the CPRs.

Distinguishing two groups of flavin reductases by analyzing the protonation state of an active site carboxylic acid

Flavin-containing reductases are involved in a wide variety of physiological reactions such as photosynthesis, nitric oxide synthesis, and detoxification of foreign compounds, including therapeutic drugs. Ferredoxin-NADP(H)-reductase (FNR) is the prototypical enzyme of this family. The fold of this protein is highly conserved and occurs as one domain of several multidomain enzymes such as the members of the diflavin reductase family. The enzymes of this family have emerged as fusion of a FNR and a flavodoxin. Although the active sites of these enzymes are very similar, different enzymes function in opposite directions, that is, some reduce oxidized nicotinamide adenine dinucleotide phosphate (NADP(+)) and some oxidize reduced nicotinamide adenine dinucleotide phosphate (NADPH). In this work, we analyze the protonation behavior of titratable residues of these enzymes through electrostatic calculations. We find that a highly conserved carboxylic acid in the active site shows a different titration behavior in different flavin reductases. This residue is deprotonated in flavin reductases present in plastids, but protonated in bacterial counterparts and in diflavin reductases. The protonation state of the carboxylic acid may also influence substrate binding. The physiological substrate for plastidic enzymes is NADP(+), but it is NADPH for the other mentioned reductases. In this article, we discuss the relevance of the environment of this residue for its protonation and its importance in catalysis. Our results allow to reinterpret and explain experimental data.

Conformational changes of NADPH-cytochrome P450 oxidoreductase are essential for catalysis and cofactor binding

The crystal structure of NADPH-cytochrome P450 reductase (CYPOR) implies that a large domain movement is essential for electron transfer from NADPH via FAD and FMN to its redox partners. To test this hypothesis, a disulfide bond was engineered between residues Asp(147) and Arg(514) in the FMN and FAD domains, respectively. The cross-linked form of this mutant protein, designated 147CC514, exhibited a significant decrease in the rate of interflavin electron transfer and large (≥90%) decreases in rates of electron transfer to its redox partners, cytochrome c and cytochrome P450 2B4. Reduction of the disulfide bond restored the ability of the mutant to reduce its redox partners, demonstrating that a conformational change is essential for CYPOR function. The crystal structures of the mutant without and with NADP(+) revealed that the two flavin domains are joined by a disulfide linkage and that the relative orientations of the two flavin rings are twisted ∼20° compared with the wild type, decreasing the surface contact area between the two flavin rings. Comparison of the structures without and with NADP(+) shows movement of the Gly(631)-Asn(635) loop. In the NADP(+)-free structure, the loop adopts a conformation that sterically hinders NADP(H) binding. The structure with NADP(+) shows movement of the Gly(631)-Asn(635) loop to a position that permits NADP(H) binding. Furthermore, comparison of these mutant and wild type structures strongly suggests that the Gly(631)-Asn(635) loop movement controls NADPH binding and NADP(+) release; this loop movement in turn facilitates the flavin domain movement, allowing electron transfer from FMN to the CYPOR redox partners.

The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders

Steroidogenesis entails processes by which cholesterol is converted to biologically active steroid hormones. Whereas most endocrine texts discuss adrenal, ovarian, testicular, placental, and other steroidogenic processes in a gland-specific fashion, steroidogenesis is better understood as a single process that is repeated in each gland with cell-type-specific variations on a single theme. Thus, understanding steroidogenesis is rooted in an understanding of the biochemistry of the various steroidogenic enzymes and cofactors and the genes that encode them. The first and rate-limiting step in steroidogenesis is the conversion of cholesterol to pregnenolone by a single enzyme, P450scc (CYP11A1), but this enzymatically complex step is subject to multiple regulatory mechanisms, yielding finely tuned quantitative regulation. Qualitative regulation determining the type of steroid to be produced is mediated by many enzymes and cofactors. Steroidogenic enzymes fall into two groups: cytochrome P450 enzymes and hydroxysteroid dehydrogenases. A cytochrome P450 may be either type 1 (in mitochondria) or type 2 (in endoplasmic reticulum), and a hydroxysteroid dehydrogenase may belong to either the aldo-keto reductase or short-chain dehydrogenase/reductase families. The activities of these enzymes are modulated by posttranslational modifications and by cofactors, especially electron-donating redox partners. The elucidation of the precise roles of these various enzymes and cofactors has been greatly facilitated by identifying the genetic bases of rare disorders of steroidogenesis. Some enzymes not principally involved in steroidogenesis may also catalyze extraglandular steroidogenesis, modulating the phenotype expected to result from some mutations. Understanding steroidogenesis is of fundamental importance to understanding disorders of sexual differentiation, reproduction, fertility, hypertension, obesity, and physiological homeostasis.

FMN binding site of yeast NADPH-cytochrome P450 reductase exposed at the surface is highly specific

NADPH-cytochrome P450 reductase (CPR) transfers two reducing equivalents derived from NADPH via FAD and FMN to microsomal P450 monooxygenases in one-electron transfer steps. The crystal structure of yeast CPR (yCPR) contains a surface-exposed FMN binding site (FMN2 site) at the interface of the FMN binding and connecting domains, in addition to the single buried site that has been observed in rat CPR. This finding provides a testable hypothesis of how intramolecular (between FAD and FMN) and intermolecular (between FMN and P450) electron transfer may occur in CPR. To verify that occupancy of the FMN2 site is not an artifact of crystallization, a surface plasmon resonance (SPR) biosensor technique has been applied to probe the selectivity of this site under functional conditions. A series of kinetic and equilibrium binding experiments involving yCPR immobilized on different sensor chip surfaces was performed using FMN and FAD, as well as FMN-derived compounds, including riboflavin, dimethylalloxazine, and alloxazine, and other molecules that resemble the planar isoalloxazine ring structure. Only FMN and FAD showed stoichiometric binding responses. Binding affinity for FMN was in the submicromolar range, 30 times higher than that for FAD. Association kinetic rates for the yCPR/FMN complex were up to 60-fold higher than for the yCPR/FAD complex. Taken together, these data indicate that (i) the surface-exposed site in yCPR is highly selective toward binding flavins, (ii) binding of FMN in this site is notably favored, and finally, (iii) both the phosphate group and the isoalloxazine ring of FMN are essential for binding.

Candida albicans NADPH-P450 reductase: expression, purification, and characterization of recombinant protein

Candida albicans is responsible for serious fungal infections in humans. Analysis of its genome identified NCP1 gene coding for a putative NADPH-P450 reductase (NPR) enzyme. This enzyme appears to supply reducing equivalents to cytochrome P450 or heme oxygenase enzymes for fungal survival and virulence. In this study, we report the characterization of the functional features of NADPH-P450 reductase from C. albicans. The recombinant C. albicans NPR protein harboring a 6x(His)-tag was expressed heterologously in Escherichia coli, and was purified. Purified C. albicans NPR has an absorption maximum at 453 nm, indicating the feature of an oxidized flavin cofactor, which was decreased by the addition of NADPH. It also evidenced NADPH-dependent cytochrome c or nitroblue tetrazolium reducing activity. This purified reductase protein was successfully able to substitute for purified mammalian NPR in the reconstitution of the human P450 1A2-catalyzed O-deethylation of 7-ethoxyresorufin. These results indicate that purified C. albicans NPR is an orthologous reductase protein that supports cytochrome P450 or heme oxygenase enzymes in C. albicans.

Modulation of the cytochrome P450 reductase redox potential by the phospholipid bilayer

Cytochrome P450 reductase (CPR) is a tethered membrane protein which transfers electrons from NADPH to microsomal P450s. We show that the lipid bilayer has a role in defining the redox potential of the CPR flavin domains. In order to quantitate the electrochemical behavior of this central redox protein, full-length CPR was incorporated into soluble nanometer scale discoidal membrane bilayers (nanodiscs), and potentials were measured using spectropotentiometry. The redox potentials of both FMN and FAD were found to shift to more positive values when in a membrane bilayer as compared to a solubilized version of the reductase. The potentials of the semiquinone/hydroquinone couple of both FMN and FAD are altered to a larger extent than the oxidized/semiquinone couple which is understood by a simple electrostatic model. When anionic lipids were used to change the membrane composition of the CPR-nanodisc, the redox potential of both flavins became more negative, favoring electron transfer from CPR to cytochrome P450.

Structure of the open conformation of a functional chimeric NADPH cytochrome P450 reductase

Two catalytic domains, bearing FMN and FAD cofactors, joined by a connecting domain, compose the core of the NADPH cytochrome P450 reductase (CPR). The FMN domain of CPR mediates electron shuttling from the FAD domain to cytochromes P450. Together, both enzymes form the main mixed-function oxidase system that participates in the metabolism of endo- and xenobiotic compounds in mammals. Available CPR structures show a closed conformation, with the two cofactors in tight proximity, which is consistent with FAD-to-FMN, but not FMN-to-P450, electron transfer. Here, we report the 2.5 A resolution crystal structure of a functionally competent yeast-human chimeric CPR in an open conformation, compatible with FMN-to-P450 electron transfer. Comparison with closed structures shows a major conformational change separating the FMN and FAD cofactors from 86 A.

Structure and function of an NADPH-cytochrome P450 oxidoreductase in an open conformation capable of reducing cytochrome P450

NADPH-cytochrome P450 oxidoreductase (CYPOR) catalyzes the transfer of electrons to all known microsomal cytochromes P450. A CYPOR variant, with a 4-amino acid deletion in the hinge connecting the FMN domain to the rest of the protein, has been crystallized in three remarkably extended conformations. The variant donates an electron to cytochrome P450 at the same rate as the wild-type, when provided with sufficient electrons. Nevertheless, it is defective in its ability to transfer electrons intramolecularly from FAD to FMN. The three extended CYPOR structures demonstrate that, by pivoting on the C terminus of the hinge, the FMN domain of the enzyme undergoes a structural rearrangement that separates it from FAD and exposes the FMN, allowing it to interact with its redox partners. A similar movement most likely occurs in the wild-type enzyme in the course of transferring electrons from FAD to its physiological partner, cytochrome P450. A model of the complex between an open conformation of CYPOR and cytochrome P450 is presented that satisfies mutagenesis constraints. Neither lengthening the linker nor mutating its sequence influenced the activity of CYPOR. It is likely that the analogous linker in other members of the diflavin family functions in a similar manner.

Cloning, purification, crystallization and preliminary X-ray analysis of a chimeric NADPH-cytochrome P450 reductase

NADPH-cytochrome P450 reductase (CPR) is the favoured redox partner of microsomal cytochromes P450. This protein is composed of two flavin-containing domains (FMN and FAD) connected by a structured linker. An active CPR chimera consisting of the yeast FMN and human FAD domains has been produced, purified and crystallized. The crystals belonged to the monoclinic space group C2 and contained one molecule per asymmetric unit. Molecular replacement was performed using the published rat and yeast structures as search models. The initial electron-density maps revealed that the chimeric enzyme had crystallized in a conformation that differed from those of previously solved structures.