Expression of bovine cytochrome P450c21 and its fused enzymes with yeast NADPH-cytochrome P450 reductase in Saccharomyces cerevisiae

Functional expression of eukaryotic cytochrome P450s in yeast

Cytochrome P450 enzymes (P450s) are a superfamily of heme-thiolate proteins widely existing in various organisms. Due to their key roles in secondary metabolism, degradation of xenobiotics, and carcinogenesis, there is a great demand to heterologously express and obtain a sufficient amount of active eukaryotic P450s. However, most eukaryotic P450s are endoplasmic reticulum-localized membrane proteins, which is the biggest challenge for functional expression to high levels. Furthermore, the functions of P450s require the cooperation of cytochrome P450 reductases for electron transfer. Great efforts have been devoted to the heterologous expression of eukaryotic P450s, and yeasts, particularly Saccharomyces cerevisiae are frequently considered as the first expression systems to be tested for this challenging purpose. This review discusses the strategies for improving the expression and activity of eukaryotic P450s in yeasts, followed by examples of P450s involved in biosynthetic pathway engineering.

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.

Molecular cloning and characterization of a cytochrome P450 in sanguinarine biosynthesis from Eschscholzia californica cells

Benzophenanthridine alkaloids, such as sanguinarine, are produced from reticuline, a common intermediate in benzylisoquinoline alkaloid biosynthesis, via protopine. Four cytochrome P450s are involved in the biosynthesis of sanguinarine from reticuline; i.e. cheilanthifoline synthase (CYP719A5; EC 1.14.21.2.), stylopine synthase (CYP719A2/A3; EC 1.14.21.1.), N-methylstylopine hydroxylase (MSH) and protopine 6-hydroxylase (P6H; EC 1.14.13.55.). In this study, a cDNA of P6H was isolated from cultured Eschscholzia californica cells, based on an integrated analysis of metabolites and transcript expression profiles of transgenic cells with Coptis japonica scoulerine-9-O-methyltransferase. Using the full-length candidate cDNA for P6H (CYP82N2v2), recombinant protein was produced in Saccharomyces cerevisiae for characterization. The microsomal fraction containing recombinant CYP82N2v2 showed typical reduced CO-difference spectra of P450, and production of dihydrosanguinarine and dihydrochelerythrine from protopine and allocryptopine, respectively. Further characterization of the substrate-specificity of CYP82N2v2 indicated that 6-hydroxylation played a role in the reaction.

Tuning microbial hosts for membrane protein production

The last four years have brought exciting progress in membrane protein research. Finally those many efforts that have been put into expression of eukaryotic membrane proteins are coming to fruition and enable to solve an ever-growing number of high resolution structures. In the past, many skilful optimization steps were required to achieve sufficient expression of functional membrane proteins. Optimization was performed individually for every membrane protein, but provided insight about commonly encountered bottlenecks and, more importantly, general guidelines how to alleviate cellular limitations during microbial membrane protein expression. Lately, system-wide analyses are emerging as powerful means to decipher cellular bottlenecks during heterologous protein production and their use in microbial membrane protein expression has grown in popularity during the past months.

This review covers the most prominent solutions and pitfalls in expression of eukaryotic membrane proteins using microbial hosts (prokaryotes, yeasts), highlights skilful applications of our basic understanding to improve membrane protein production. Omics technologies provide new concepts to engineer microbial hosts for membrane protein production.

Engineering and assaying of cytochrome P450 biocatalysts

Cytochrome P450s constitute a highly fascinating superfamily of enzymes which catalyze a broad range of reactions. They are essential for drug metabolism and promise industrial applications in biotechnology and biosensing. The constant search for cytochrome P450 enzymes with enhanced catalytic performances has generated a large body of research. This review will concentrate on two key aspects related to the identification and improvement of cytochrome P450 biocatalysts, namely the engineering and assaying of these enzymes. To this end, recent advances in cytochrome P450 development are reported and commonly used screening methods are surveyed.

Molecular cloning and characterization of CYP80G2, a cytochrome P450 that catalyzes an intramolecular C-C phenol coupling of (S)-reticuline in magnoflorine biosynthesis, from cultured Coptis japonica cells

Cytochrome P450s (P450) play a key role in oxidative reactions in plant secondary metabolism. Some of them, which catalyze unique reactions other than the standard hydroxylation, increase the structural diversity of plant secondary metabolites. In isoquinoline alkaloid biosyntheses, several unique P450 reactions have been reported, such as methylenedioxy bridge formation, intramolecular C-C phenol-coupling and intermolecular C-O phenol-coupling reactions. We report here the isolation and characterization of a C-C phenol-coupling P450 cDNA (CYP80G2) from an expressed sequence tag library of cultured Coptis japonica cells. Structural analysis showed that CYP80G2 had high amino acid sequence similarity to Berberis stolonifera CYP80A1, an intermolecular C-O phenol-coupling P450 involved in berbamunine biosynthesis. Heterologous expression in yeast indicated that CYP80G2 had intramolecular C-C phenol-coupling activity to produce (S)-corytuberine (aporphine-type) from (S)-reticuline (benzylisoquinoline type). Despite this intriguing reaction, recombinant CYP80G2 showed typical P450 properties: its C-C phenol-coupling reaction required NADPH and oxygen and was inhibited by a typical P450 inhibitor. Based on a detailed substrate-specificity analysis, this unique reaction mechanism and substrate recognition were discussed. CYP80G2 may be involved in magnoflorine biosynthesis in C. japonica, based on the fact that recombinant C. japonica S-adenosyl-L-methionine:coclaurine N-methyltransferase could convert (S)-corytuberine to magnoflorine.

Molecular cloning and characterization of methylenedioxy bridge-forming enzymes involved in stylopine biosynthesis inEschscholzia californica

(S)-stylopine is an important intermediate in the biosynthesis of benzophenanthridine alkaloids, such as sanguinarine. Stylopine biosynthesis involves the sequential formation of two methylenedioxy bridges. Although the methylenedioxy bridge-forming P450 (CYP719) involved in berberine biosynthesis has been cloned from Coptis japonica[Ikezawa N, Tanaka M, Nagayoshi M, Shinkyo R, Sakaki T, Inouye K & Sato F (2003) J Biol Chem278, 38557-38565], no information is available regarding the genes for methylenedioxy bridge-forming enzymes in stylopine biosynthesis. Two cytochrome P450 cDNAs involved in stylopine biosynthesis were isolated using degenerate primers designed for C. japonica CYP719 from cultured Eschscholzia californica cells. Heterologous expression in Saccharomyces cerevisiae showed that both CYP719A2 and CYP719A3 had stylopine synthase activity to catalyze methylenedioxy bridge-formation from cheilanthifoline to stylopine, but not cheilanthifoline synthase activity to convert scoulerine to cheilanthifoline. Functional differences and expression patterns of CYP719A2 and CYP719A3 were examined to investigate their physiological roles in stylopine biosynthesis. Enzymatic analysis showed that CYP719A2 had high substrate affinity only toward (R,S)-cheilanthifoline, whereas CYP719A3 had high affinity toward three similar substrates (R,S)-cheilanthifoline, (S)-scoulerine, and (S)-tetrahydrocolumbamine. An expression analysis in E. californica plant tissues showed that CYP719A2 and CYP719A3 exhibited expression patterns similar to those of three stylopine biosynthetic genes (CYP80B1, berberine bridge enzyme, and S-adenosyl-l-methionine : 3'-hydroxy-N-methylcoclaurine 4'-O-methyltransferase), whereas the specific expression of CYP719A3 in root was notable. Treatment of E. californica seedlings with methyl jasmonate resulted in the coordinated induction of CYP719A2 and CYP719A3 genes. The physiological roles of CYP719A2 and CYP719A3 in stylopine biosynthesis are discussed.

Increased TCA cycle activity and reduced oxygen consumption during cytochrome P450-dependent biotransformation in fission yeast

Cytochrome P450s are haem-containing monooxygenases that catalyse a variety of oxidations utilizing a large substrate spectrum and are therefore of interest for biotechnological applications. We expressed human CYP21 in fission yeast Schizosaccharomyces pombe as a eukaryotic model for P450-dependent whole-cell biotransformation. The resulting strain displayed strong steroid hydroxylase activity that was accompanied by contrary effects on respiration and non-respiratory oxygen consumption, which combined to a significant decline in total oxygen consumption of the cells. While production of ROS (reactive oxygen species) decreased, the TCA cycle activity increased, as was shown by metabolic flux (METAFoR) analysis. Pentose phosphate pathway (PPP) activity was found to be negligible, regardless of growth phase, CYP21 expression or biocatalytic activity, indicating that NADPH levels in Sz. pombe are sufficiently high to support an exogenous P450 without adaptations of central carbon metabolism. We conclude from these data that neither oxygen supply nor NADPH availability are limiting factors in P450-dependent biocatalysis in Sz. pombe.

Hydrocortisone made in yeast: metabolic engineering turns a unicellular microorganism into a drug-synthesizing factory

Inspired by the successful work of converting Saccharomyces cerevisiae into an microorganism capable of synthesizing hydrocortisone, a 27-carbon molecule, from ethanol, a 2-carbon molecule, this review provides an overview of the potential of yeast as a recombinant organism in the 21st century. Yeast has been used by man for more than 6,000 years, and is still paving the way to new discoveries. It was the first eukaryotic organism to be sequenced, in 1996, and the first to produce hydrocortisone in 2003. In addition, extensive genome-wide analyses have been performed with yeast. In this review, we discuss the pros and cons of using yeast to produce small therapeutic molecules. It is obvious that S. cerevisiae has a cutting edge advantage of being a well-known organism and time will tell if yeast "biohydrocortisone" is a unique example or the beginning of a long list of yeast bioproducts. Other organisms, such as plants and bacteria, are competing with yeast. Bacteria produce a wealth of marketed molecules and plants are capable of producing extremely complex molecules with an unbeatable yield. However, S. cerevisiae offers a unique mix of the simplicity of a recombinant organism combined with the complexity of a eukaryote.

Practical application of mammalian cytochrome P450

Heterologous expression systems play an important role in the analysis of structure-function relationships of mammalian P450s. In addition, these expression systems allow practical application of mammalian P450s. Genetically engineered fused enzymes between mammalian P450 and yeast NADPH-P450 reductase have possible applications in bioconversion processes. Combined use of techniques reported thus far could produce steroid hormones in the recombinant yeast cells harboring four P450 species, CYP11A1, CYP17A1, CYP21B1 and CYP11B1. In an Escherichia coli expression system, the technology of the construction of the mitochondrial P450 electron transport chain has been established. The recombinant E. coli cells expressing CYP27B1, adrenodoxin and NADPH-adrenodoxin reductase would be applicable to a bioconversion process to produce 1alpha,25-dihydroxyvitamin D3. We also demonstrated the usefulness of heterologous expression systems for human liver microsomal P450s for the prediction of drug metabolism in the human body. Microsomal fractions prepared from recombinant yeast, insect and mammalian cells are commercially available and play an important role in preclinical drug development. Application of mammalian P450 to bioremediation with genetic engineering has also been developed. Thus, mammalian P450s appear to have great potential for a wide range of practical applications.

A virus-directed enzyme prodrug therapy (VDEPT) strategy for lung cancer using a CYP2B6/NADPH-cytochrome P450 reductase fusion protein

Virus-directed enzyme prodrug therapy (VDEPT) is an emerging strategy against cancer. Our approach is a P450-based VDEPT that consists of using cyclophosphamide (CPA) as a prodrug and a Cytochrome P450 2B6/NADPH cytochrome P450 reductase fusion protein (CYP2B6/RED) as a prodrug-activating enzyme. Due to the heterogenous expression of proteins in tumor cells, basal reductase activity may not be sufficient to supply CYP2B6 with electrons, the fusion protein should enable the expression of both proteins at high levels in tumor cells. CYP/RED fusion proteins have never been previously expressed in mammalian cells, to enable expression the fusion protein was cloned into an adenoviral vector and subsequently several pulmonary tumor cell lines were infected. The CYP2B6/RED fusion protein was detected by Western blot, its mRNA by Northern blot, and its heme incorporation into an active form by spectral analysis. Infection with the fusion gene increased RED activity in microsomes by a factor of 3 compared to the control. After infection and treatment with CPA, in cell lines with low endogenous RED, the fusion protein mediated significantly higher CPA-induced cytotoxicity compared to cells expressing solely CYP2B6. In conclusion, the fusion protein is functional for VDEPT by providing one protein for higher levels of CPA metabolism.

Molecular cloning and characterization of CYP719, a methylenedioxy bridge-forming enzyme that belongs to a novel P450 family, from cultured Coptis japonica cells

Two cytochrome P450 (P450) cDNAs involved in the biosynthesis of berberine, an antimicrobial benzylisoquinoline alkaloid, were isolated from cultured Coptis japonica cells and characterized. A sequence analysis showed that one C. japonica P450 (designated CYP719) belonged to a novel P450 family. Further, heterologous expression in yeast confirmed that it had the same activity as a methylenedioxy bridge-forming enzyme (canadine synthase), which catalyzes the conversion of (S)-tetrahydrocolumbamine ((S)-THC) to (S)-tetrahydroberberine ((S)-THB, (S)-canadine). The other P450 (designated CYP80B2) showed high homology to California poppy (S)-N-methylcoclaurine-3'-hydroxylase (CYP80B1), which converts (S)-N-methylcoclaurine to (S)-3'-hydroxy-N-methylcoclaurine. Recombinant CYP719 showed typical P450 properties as well as high substrate affinity and specificity for (S)-THC. (S)Scoulerine was not a substrate of CYP719, indicating that some other P450, e.g. (S)-cheilanthifoline synthase, is needed in (S)-stylopine biosynthesis. All of the berberine biosynthetic genes, including CYP719 and CYP80B2, were highly expressed in selected cultured C. japonica cells and moderately expressed in root, which suggests coordinated regulation of the expression of biosynthetic genes.

Formation and functioning of fused cholesterol side-chain cleavage enzymes

We studied the properties of various fused combinations of the components of the mitochondrial cholesterol side-chain cleavage system including cytochrome P450scc, adrenodoxin (Adx), and adrenodoxin reductase (AdR). When recombinant DNAs encoding these constructs were expressed in Escherichia coli, both cholesterol side-chain cleavage activity and sensitivity to intracellular proteolysis of the three-component fusions depended on the species of origin and the arrangement of the constituents. To understand the assembly of the catalytic domains in the fused molecules, we analyzed the catalytic properties of three two-component fusions: P450scc-Adx, Adx-P450scc, and AdR-Adx. We examined the ability of each fusion to carry out the side-chain cleavage reaction in the presence of the corresponding missing component of the whole system and examined the dependence of this reaction on the presence of exogenously added individual components of the double fusions. This analysis indicated that the active centers in the double fusions are either unable to interact or are misfolded; in some cases they were inaccessible to exogenous partners. Our data suggest that when fusion proteins containing P450scc, Adx, and AdR undergo protein folding, the corresponding catalytic domains are not formed independently of each other. Thus, the correct folding and catalytic activity of each domain is determined interactively and not independently.

Dehydroepiandrosterone (DHEA) metabolism in Saccharomyces cerevisiae expressing mammalian steroid hydroxylase CYP7B: Ayr1p and Fox2p display 17beta-hydroxysteroid dehydrogenase activity

We have engineered recombinant yeast to perform stereospecific hydroxylation of dehydroepiandrosterone (DHEA). This mammalian pro-hormone promotes brain and immune function; hydroxylation at the 7alpha position by P450 CYP7B is the major pathway of metabolic activation. We have sought to activate DHEA via yeast expression of rat CYP7B enzyme. Saccharomyces cerevisiae was found to metabolize DHEA by 3beta-acetylation; this was abolished by mutation at atf2. DHEA was also toxic, blocking tryptophan (trp) uptake: prototrophic strains were DHEA-resistant. In TRP(+) atf2 strains DHEA was then converted to androstene-3beta,17beta-diol (A/enediol) by an endogenous 17beta-hydroxysteroid dehydrogenase (17betaHSD). Seven yeast polypeptides similar to human 17betaHSDs were identified: when expressed in yeast, only AYR1 (1-acyl dihydroxyacetone phosphate reductase) increased A/enediol accumulation, while the hydroxyacyl-CoA dehydrogenase Fox2p, highly homologous to human 17betaHSD4, oxidized A/enediol to DHEA. The presence of endogenous yeast enzymes metabolizing steroids may relate to fungal pathogenesis. Disruption of AYR1 eliminated reductive 17betaHSD activity, and expression of CYP7B on the combination background (atf2, ayr1, TRP(+)) permitted efficient (>98%) bioconversion of DHEA to 7alpha-hydroxyDHEA, a product of potential medical utility.

Expression, purification, and physical properties of recombinant flavocytochrome fusion proteins containing rat cytochrome b(5) linked to NADPH-cytochrome P450 reductase by different membrane-binding segments

Reconstitution of the enzymatic activities using purified microsomal cytochrome P450s (P450) requires the presence of a membrane-binding segment in the mammalian flavoprotein, NADPH--cytochrome P450 reductase (CPR), and the hemeprotein, cytochrome b(5) (b(5)). The mechanism(s) by which the membrane-binding segments of these proteins exert such a critical role in influencing the reconstitution of the NADPH-supported activity of a P450 remains undefined. In the present work we describe the construction, expression, and purification of four different types of recombinant flavocytochromes containing rat b(5) and rat CPR linked by various membrane-binding segments. The physical properties of these artificial fusion proteins have been studied to determine their ability to serve as electron transfer agents. These studies are a prelude to the subsequent study (accompanying paper) evaluating the functional roles of the hydrophobic (membrane-binding) sequences of b(5) and CPR in the reconstitution of P450 activities. The present study shows that the purified recombinant fusion proteins can serve as active electron transport carriers from NADPH to cytochrome c as well as b(5) by intramolecular as well as intermolecular reactions. It is shown here that the electron transport properties of these purified fusion proteins are influenced by high concentrations of KCl, suggesting a role for charged amino acids in protein-protein interactions. The present study illustrates the application of artificial recombinant flavocytochromes as useful proteins for the study of intramolecular electron transport reactions for comparison with intermolecular interactions.

A transgenic mouse expressing human CYP4B1 in the liver

The human CYP4B1 protein was expressed in the liver of a transgenic mouse line under the control of the promoter of the human apolipoprotein E (apo E) gene. Hepatic microsomes of transgenic mice catalyzed omega-hydroxylation of lauric acid and also activated 2-aminofluorene (2-AF), which is a typical substrate for CYP4B1, to mutagenic compounds detected by an umu gene expression assay. These activities observed in transgenic mouse were efficiently inhibited by CYP4B1 antibody. However, such inhibition was not observed in control mice. This is the first report to indicate catalytic activities of human CYP4B1. For further characterization of human CYP4B1, a fusion protein of CYP4B1 and NADPH-P450 reductase was expressed in yeast cells. It was able to activate 2-AF and was also able to catalyze omega-hydroxylation of lauric acid. This transgenic mouse line and the recombinant fusion protein provide a useful tool to study human CYP4B1 and its relation to chemical toxicity and carcinogenesis.

Creation and activity of COS-1 cells stably expressing the F2 fusion of the human cholesterol side-chain cleavage enzyme system

A fusion construct for the human cholesterol side-chain cleavage enzyme system termed F2 (H(2)N-P450scc-adrenodoxin reductase-adrenodoxin-COOH), was stably expressed in nonsteroidogenic COS-1 cells. Multiple clones were obtained and analyzed, identifying the clone COS-F2-130 as the most active in converting 22R-hydroxycholesterol (22R-OH-C) to pregnenolone. The F2 fusion construct was properly transcribed and translated in COS-F2-130 cells, indicating that these cells did not proteolytically cleave the F2 protein. Steroid analyses show that the COS-F2-130 cells do not convert appreciable quantities of pregnenolone to other steroids. Isolated COS-F2-130 mitochondria showed enhanced steroidogenesis when incubated with biosynthetic N-62 StAR protein in vitro. The cells were easily transfectable with StAR expression vectors, showing that COS-F2-130 cells exhibited both StAR-independent and StAR-dependent activity. Transient expression of either full-length or N-62 StAR stimulated steroidogenesis to approximately 45% of the maximal steroidogenic capacity, as indicated by incubation with 22R-OH-C. Single, double, and triple transfections of individual vectors expressing P450scc, adrenodoxin reductase, and adrenodoxin demonstrated that the P450 moiety of the F2 fusion protein could only receive electrons from the covalently linked adrenodoxin moiety, but that free adrenodoxin reductase could foster activity of the fusion enzyme. COS-F2-130 cells provide a useful system for studying steroidogenesis, as these are the only cells described to date that convert cholesterol to pregnenolone but lack downstream enzymes that catalyze other steroidogenic reactions.

Inhibitory effects of detergents on rat CYP1A1-dependent monooxygenase: comparison of mixed and fused systems consisting of rat CYP 1A1 and yeast NADPH-P450 reductase

Inhibitory effects of detergents Triton X-100 and Chaps on 7-ethoxycoumarin O-deethylation activity were examined in the recombinant microsomes containing both rat CYP1A1 and yeast NADPH-P450 reductase (the mixed system) and their fused enzyme (the fused system). Triton X-100 showed competitive inhibition in both mixed and fused systems with K(i) values of 24.6 and 21.5 microM, respectively. These results strongly suggest that Triton X-100 binds to the substrate-binding pocket of CYP1A1. These K(i) values are far below the critical micelle concentration of Triton X-100 (240 microM). Western blot analysis revealed no disruption of the microsomal membrane by Triton X-100 in the presence of 0-77 microM Triton X-100. On the other hand, Chaps gave distinct inhibitory effects to the mixed and fused systems. In the fused system, a mixed-type inhibition was observed with K(i) and K(i)' values of 1.2 and 5.4 mM of Chaps, respectively. However, in the mixed system, multiple inhibition modes by Chaps were observed. Western blot analysis revealed that the solubilized fused enzyme by Chaps preserved the activity whereas the solubilized CYP1A1 and NADPH-P450 reductase reductase showed no activity in the mixed system. Thus, the comparison of the mixed and fused systems appears quite useful to elucidate inhibition mechanism of detergents.

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.

Biphasic kinetic behavior of rat cytochrome P-4501A1-dependent monooxygenation in recombinant yeast microsomes

Rat cytochrome P-4501A1-dependent monooxygenase activities were examined in detail using recombinant yeast microsomes containing rat cytochrome P-4501A1 and yeast NADPH-P-450 reductase. On 7-ethoxycoumarin, which is one of the most popular substrates of P-4501A1, the relationship between the initial velocity (v) and the substrate concentration ([S]) exhibited non-linear Michaelis-Menten kinetics. Hanes-Woolf plots ([S]/v vs. [S]) clearly showed a biphasic kinetic behavior. Aminopyrine N-demethylation also showed a biphasic kinetics. The regression analyses on the basis of the two-substrate binding model proposed by Korzekwa et al. (Biochemistry 37 (1998) 4137-4147) strongly suggest the presence of the two substrate-binding sites in P-4501A1 molecules for those substrates. An Arrhenius plot with high 7-ethoxycoumarin concentration showed a breakpoint at around 28 degrees C probably due to the change of the rate-limiting step of P-4501A1-dependent 7-ethoxycoumarin O-deethylation. However, the addition of 30% glycerol to the reaction mixture prevented observation of the breakpoint. The methanol used as a solvent of 7-ethoxycoumarin was found to be a non-competitive inhibitor. Based on the inhibition kinetics, the real V(max) value in the absence of methanol was calculated. These results strongly suggest that the recombinant yeast microsomal membrane containing a single P-450 isoform and yeast NADPH-P-450 reductase is quite useful for kinetic studies on P-450-dependent monooxygenation including an exact evaluation of inhibitory effects of organic solvents.

Coexpression of genetically engineered fused enzyme between yeast NADPH-P450 reductase and human cytochrome P450 3A4 and human cytochrome b5 in yeast

Human hepatic cytochrome P450 3A4 (CYP3A4) was expressed in yeast Saccharomyces cerevisiae. While the expression level was high as compared with other human hepatic cytochrome P450s, CYP3A4 showed almost no catalytic activity toward testosterone. Coexpression of CYP3A4 with yeast NADPH-P450 reductase did not give a full activity. Low monooxygenase activity of CYP3A4 was attributed to the insufficient reduction of heme iron of CYP3A4 by NADPH-P450 reductase. To enhance the efficiency of electron transfer from NADPH-P450 reductase to CYP3A4, a fused enzyme was constructed between CYP3A4 and yeast NADPH-P450 reductase. The rapid reduction of the heme iron of the fused enzyme by NADPH was observed. The fused enzyme showed a high testosterone 6beta-hydroxylation activity with a sigmoidal velocity saturation curve. However, the coupling efficiency between NADPH utilization and testosterone 6beta-hydroxylation was only 10%. Finally, coexpression of the fused enzyme and human cytochrome b5 was examined. A significant decrease in the Km value and a remarkable increase in the coupling efficiency were observed. Substrate-induced spectra revealed that the dissociation constant of the fused enzyme for testosterone significantly decreased with coexpression of human cytochrome b5. These results strongly suggest that human cytochrome b5 directly interacts with the CYP3A4 domain of the fused enzyme and modifies the tertiary structure of substrate binding pocket, resulting in tight binding of the substrate and high coupling efficiency.

Construction and characterization of a catalytic fusion protein system: P-450(11beta)-adrenodoxin reductase-adrenodoxin

Cortisol is an important intermediate for the production of steroidal drugs and can only be synthesized chemically by rather complicated multi-step procedures. The most critical step is the 11beta-hydroxylation of 11-deoxycortisol, which is catalyzed by a mitochondrial enzyme, P-450(11beta). Various fusion constructs of P-450(11beta) with its electron transfer components, adrenodoxin and adrenodoxin reductase, were produced by cDNA manipulation and were successfully expressed in COS-1 cells from which the hydroxylation activities were assayed. It was demonstrated that the fusion protein required both adrenodoxin reductase and adrenodoxin for its activity and was not able to receive electrons from an external source. The fusion protein with all three components had less activity than P-450(11beta) alone, receiving electrons from coexpressed or internal electron transfer components. The activities of the fusion proteins were determined mainly by the fusion sequence. The fusion protein with a sequence of P-450(11beta)-adrenodoxin reductase-adrenodoxin was more active than that of P-450(11beta)-adrenodoxin-adrenodoxin reductase, 1.5- and 3-fold for bovine and human P-450(11beta), respectively. Modification of the linker region by extending the size of the linker with various peptide sequences in the bovine P-450(11beta)-adrenodoxin reductase-adrenodoxin fusion protein indicated that the linker did not have significant effect on the P-450 activity. Taken together, the fusion protein obtained here can serve as a model for the investigation of electron transfer in P-450 systems and is of potential importance for biotechnological steroid production.

Inhibitory effects of vitamin A and vitamin K on rat cytochrome P4501A1-dependent monooxygenase activity

The inhibitory effects of vitamins A and K toward P4501A1-dependent 7-ethoxycoumarin O-deethylation were examined in the reconstituted system containing the microsomal fraction prepared from the recombinant Saccharomyces cerevisiae cells producing rat P4501A1 and yeast NADPH-P450 reductase. On vitamins A, all-trans-retinol, all-trans-retinal, all-trans-retinoic acid and retinol-palmitate showed competitive inhibition with K(i) values of 0.068, 0.079, 2.6 and 2.0 microM, respectively. Judging from the K(i) values, the inhibitory effects of those vitamins A appear to have physiological significance on the basis of their contents in liver, lung and kidney. On vitamins K, vitamin K(1) showed competitive inhibition with K(i) value of 24 microM, while vitamin K(2) showed noncompetitive inhibition with K(i) value of 60 microM. Judging from these K(i) values together with the contents of these vitamins K in liver, the inhibitory effects of the vitamins K are not as significant as those of vitamins A. These results suggest that the ingestion of enough amounts of vitamins A from foods might lead to the inhibition of the activity of P4501A1 which is known to be induced by smoking, drugs such as omeprazole and lansoprazole, and environmental pollutants like dioxins.

Engineering and biochemical characterization of the rat microsomal cytochrome P4501A1 fused to ferredoxin and ferredoxin-NADP(+) reductase from plant chloroplasts

Fusion proteins of rat cytochrome P4501A1 with maize ferredoxin I (Fd) and pea ferredoxin NADP(+) reductase (FNR), the last electron transfer proteins of the photosynthetic channel in plant chloroplasts, were obtained by gene fusion in the yeast expression vector pAAH5N. The encoded fusion proteins P4501A1-Fd, P4501A1-FNR, P4501A1-Fd-FNR and P4501A1-FNR-Fd were produced in microsomes of the yeast Saccharomyces cerevisiae AH22. Enzymatic assays were carried out in vitro with the isolated microsomes. P4501A1-Fd-FNR and P4501A1-FNR-Fd were found to catalyze P450-monooxygenase activities towards 7-ethoxycoumarin and the herbicide chlortoluron. P4501A1-Fd-FNR was the most efficient enzyme as measured in vitro in ferricyanide and cytochrome c reductions, as well as P450-monooxygenase assays. Apparent K(m) and k(cat) of P4501A1-Fd-FNR were 70 microM and 7800 min(-1) for NADPH, 13.2 microM and 51.1 min(-1) for 7-ethoxycoumarin, and 21.3 microM and 23. 8 min(-1) for the herbicide chlortoluron, respectively. Fd in P4501A1-Fd-FNR fusion enzyme was found to be a limiting factor compared to P4501A1 fused to the yeast NADPH-cytochrome P450 reductase, an artificial enzyme described previously. The efficiency of electron transfer in the P4501A1 fusion proteins and a possible in vivo molecular coupling of Fd and FNR with microsomal cytochrome P4501A1 produced in plant chloroplasts are discussed.

Electrostatic interaction between cytochrome P450 and NADPH-P450 reductase: comparison of mixed and fused systems consisting of rat cytochrome P450 1A1 and yeast NADPH-P450 reductase

The electrostatic interaction between rat cytochrome P450 1A1 and yeast NADPH-P450 reductase was analyzed by using recombinant yeast microsomes containing both native enzymes or their fused enzyme. The Vmax of the 7-ethoxycoumarin O-deethylation in the recombinant microsomes containing both rat cytochrome P4501A1 and yeast NADPH-P450 reductase (the mixed system) was maximal when the ionic strength of the reaction mixture was 0.1-0.15. However, on the fused enzyme between rat cytochrome P450 1A1 and yeast NADPH-P450 reductase (the fused system), the activity was uniformly reduced with increasing ionic strength. The pH profiles of Vmax were also different between the mixed and the fused systems. Based on these results, we propose a hypothesis that cytochrome P450 and NADPH-P450 reductase have more than one binding mode. The maximal activity of the mixed system at ionic strength of 0.1-0.15 is explained by change of the binding mode. On the other hand, the fused enzyme appears to have only one binding mode due to the limited topology of cytochrome P450 and NADPH-P450 reductase domains.

Characterization of the consequence of a novel Glu-380 to Asp mutation by expression of functional P450c21 in Escherichia coli

P450c21 catalyzes an important step in steroid synthesis. Its deficiency leads to symptoms of steroid imbalance. To obtain enough P450c21 for structure and function studies, we developed a method to express P450c21 in Escherichia coli. The 5'-region of the human P450c21 cDNA was modified to ensure efficient translation and the C terminus of the protein was extended with four His residues for easy purification. Mutant proteins with substitutions at residues 172 and 281 exhibited decreased enzymatic activities similar to those found in mammalian cells. One new mutation changing Glu-380 to Asp (D380) caused 3-fold reduction in enzymatic activity. The amount of apoprotein production detected by immunoblotting and the affinity of the mutant protein towards substrate as measured by Km were normal. The defect lies in the decreased ability of the apoprotein to bind heme, which was measured by CO difference and substrate-binding spectra. The D380 mutant protein had 3-fold reduction in peak heights in both spectra. This reduced heme binding resulted in 3-fold lower enzymatic activity.

Semisynthetic flavocytochromes based on cytochrome P450 2B4: reductase and oxygenase activities

A synthetic flavocytochrome with the reductase and oxygenase activities was obtained by covalent binding of riboflavin to cytochrome P450 2B4. The reactions catalyzed by the newly synthesized flavocytochromes were studied. Formation of carbon monoxide complex with the reduced form of hemoprotein led to 60-80% inhibition of oxygenase reactions, indicating the leading role of reduced heme iron in generating active oxygen species by flavocytochromes.

Cytochrome P450 enzyme systems in fungi

The involvement of cytochrome P450 enzymes in many complex fungal bioconversion processes has been characterized in recent years. Accordingly, there is now considerable scientific interest in fungal cytochrome P450 enzyme systems. In contrast to S. cerevisiae, where surprisingly few P450 genes have been identified, biochemical data suggest that many fungi possess numerous P450 genes. This review summarizes the current information pertaining to these fungal cytochrome P450 systems, with emphasis on the molecular genetics. The use of molecular techniques to improve cytochrome P450 activities in fungi is also discussed.

Expression, purification, and characterization of a catalytically active human cytochrome P450 1A2:rat NADPH-cytochrome P450 reductase fusion protein

An enzymatically active human cytochrome P450 (P450) 1A2:rat NADPH-P450 reductase fusion protein was purified and partially characterized following heterologous expression in Escherichia coli. A cDNA was engineered to include the coding sequence for human P450 1A2 at its 5' end (up to but not including the stop codon) fused in-frame to the coding sequence for a truncated (soluble) rat NADPH-P450 reductase at its 3' end via an oligonucleotide sequence encoding the hydrophilic dipeptide Ser-Thr. This fusion plasmid was expressed in E. coli and the recombinant protein was purified from the detergent-solubilized membrane fraction via sequential DEAE, ADP-agarose, and hydroxylapatite chromatographies. The purified protein has the spectral characteristics of human P450 1A2 and cytochrome c reduction activity comparable to rabbit NADPH-P450 reductase. The fusion protein catalyzed 7-ethoxyresorufin O-deethylation and phenacetin O-deethylation to appreciable levels in the presence of NADPH and phospholipid. While these activities were comparable to those of other such P450:NADPH-P450 reductase fusion proteins, they were lower than those of the system reconstituted from its individual hemoprotein and flavoprotein components. Nevertheless, the production of a functional, catalytically self-sufficient monooxygenase in E. coli enhances the prospect of using bacterial systems for production and characterization of human P450 drug metabolites as well as for biodegradation of chemicals in the environment.

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.

Construction of a P450c27 fusion enzyme: a useful tool for analysis of vitamin D3 25-hydroxylase activity

Liver mitochondrial P450c27, encoded by the CYP27 gene, can catalyse the 25-hydroxylation of vitamin D3 and the 27-hydroxylation of sterols. To facilitate the study of this enzyme in cell culture systems, we engineered a fusion protein consisting of P450c27 coupled to its electron-transport accessory proteins, ferredoxin and ferredoxin reductase, and assessed its enzyme activity by measuring the C-25 and C-27 (side-chain) hydroxylation of 1 alpha-hydroxyvitamin D3 (1 alpha-OH-D3). When incubated with 1 alpha-OH-D3, COS-1 cells transfected with a vector expressing the fusion protein produced 1 alpha,25-(OH)2D2 and 1 alpha,27-(OH)2D3 about four times more efficiently than did cells transfected with three individual components of the fusion. When incubated with the natural substrate, vitamin D3, the efficiency of hydroxylation was lower than that for 1 alpha-OH-D3 but still 1.7-fold higher for the fusion protein than for its individual components. The fusion protein was also able to reproduce qualitatively and quantitatively the activity shown by P450c27 in liver cells in situ. The P450c27-ferredoxin reductase-ferredoxin fusion construct represents a valuable tool for establishing the substrate specificity of this liver cytochrome and for evaluating its potential for activating pro-drug analogues of vitamin D.

Optimization of the benzoate-inducible benzoate p-hydroxylase cytochrome P450 enzyme system in Aspergillus niger

Introduction in the fungus Aspergillus niger of multiple copies of the A. niger bphA gene, encoding the cytochrome P450 enzyme benzoate p-hydroxylase, did not result in increased activities of this enzyme [Gorcom RFM van, et al. Mol Gen Genet (1990) 223: 192-197] probably because of low expression levels of the gene encoding the second component of the microsomal cytochrome P450 enzyme system, cytochrome P450 reductase. For improvement of this and other cytochrome-P450-dependent reactions, A. niger strains were constructed in which the copy number of the A. niger cprA gene (encoding cytochrome-P450 reductase) or the copy numbers of both cprA and the cytochrome-P450-encoding gene were increased. Expression of both genes was controlled by their own transcription control regions. Benzoate p-hydroxylase activity of different transformants was determined in microsomal fractions using a newly developed indirect in vitro assay. In transformants containing multiple copies of both genes, benzoate p-hydroxylase activity was significantly higher than in the wild-type strain or in transformants in which the copy number of only one of the genes was increased. These results clearly indicate the importance of co-expression of cytochrome-P450 reductase for achieving maximal cytochrome P450 activities in cytochrome-P450-overproducing filamentous fungi.

Molecular engineering study on electron transfer from NADPH-P450 reductase to rat mitochondrial P450c27 in yeast microsomes

We have reported the localization on yeast microsomes for a modified P450c27 (mic-P450c27) that contains the microsomal targeting signal of bovine P450c17 in front of the mature form of rat mitochondrial P450c27 (Sakaki, T., Akiyoshi-Shibata, M., Yabusaki, Y., and Ohkawa, H. (1992) J. Biol. Chem. 267, 16497-16502). In this study, we found that mic-P450c27 could be reduced by NADPH in the yeast microsomes without supplement of its physiological redox partners, adrenodoxin and NADPH-adrenodoxin reductase. In order to elucidate the direct electron transfer from NADPH-P450 reductase to mic-P450c27, we carried out simultaneous expression of mic-P450c27 and yeast P450 reductase. The reduction rate of mic-P450c27 was increased by overproduction of yeast P450 reductase, roughly in proportion to the reductase content in the microsomes. In addition, we constructed a fused enzyme between mic-P450c27 and yeast P450 reductase. The reduction rate of heme iron in the fused enzyme was too rapid to be measured. These recombinant yeast microsomes showed a notable 27-hydroxylation activity toward 5beta-cholestane-3alpha,7alpha, 12alpha-triol in the absence of adrenodoxin and adrenodoxin reductase. Finally, we purified mic-P450c27 from the recombinant yeast microsomes and reconstituted the hydroxylation system in liposomal membranes using the purified mic-P450c27 and yeast NADPH-P450 reductase. Mic-P450c27 was reduced by NADPH and showed its monooxygenase activity on the reconstituted system. Therefore, yeast NADPH-P450 reductase alone was found to transfer two electrons from NADPH to mic-P450c27. These results clearly show that mic-P450c27 not only localizes on the microsomes but also functions as a microsomal cytochrome P450 that accepts electrons from NADPH-P450 reductase.

Probing the limits of expression levels by varying promoter strength and plasmid copy number in Saccharomyces cerevisiae

Heterologous gene expression levels were measured in yeast using the Escherichia coli gusA gene (encoding beta-D-glucuronidase) as a reporter. The influence of two major parameters, promoter activity and plasmid copy number, was studied. (1) Promoters used in this study ranged from the very weak constitutive KEX2, the regulated CYC1 and PGK and the mating type-specific MF alpha 1 to the strong constitutive TEF1 and TDH promoters. Using centromeric vectors, gusA expression levels varied within three orders of magnitude. (2) Plasmid copy number was changed by shifting from a monocopy (centromeric plasmid) over a moderate copy number (2 mu-based plasmid) to a high copy number (2 mu associated with the URA3-d selection marker). gusA expression levels increased relatively with plasmid copy number in all cases studied, but did not exceed the equivalent of 2% of total soluble yeast proteins. Coupling these variables, a 5-log range in gene expression levels was covered. Taken together, these results provide a framework which allows a comparison of existing and new promoters. This framework will be useful for expressing genes to required levels.

Putidaredoxin reductase-putidaredoxin-cytochrome p450cam triple fusion protein. Construction of a self-sufficient Escherichia coli catalytic system

Fusion proteins of cytochrome P450cam with putidaredoxin (Pd) and putidaredoxin reductase (PdR), the two proteins required to transfer electrons from NADH to P450cam, were constructed by fusing cDNAs encoding the three proteins in the expression vector pCWori+. Several fusion proteins, in which the order of the three protein domains and the linkers between them were varied, were expressed in Escherichia coli, purified, and characterized. The highest activity (kcat = 30 min-1) was obtained with a PdR-Pd-P450cam construct in which the peptides TDGTASS and PLEL were used, respectively, to link the PdR to the Pd and the Pd to the P450cam domains. Oxygen and NADH consumption is tightly coupled to substrate oxidation in the fusion proteins. The rate-limiting step in the catalytic turnover of these fusion proteins is electron transfer from Pd to P450cam. This is indicated by high rates of electron transfer from the PdR and Pd domains to exogenous electron acceptors, by an increase in the activity of the P450cam domain upon addition of exogenous Pd, and by the high activity of wild-type P450cam when incubated with a PdR-Pd fusion protein. E. coli cells expressing the PdR-Pd-P450cam fusion protein efficiently oxidize camphor to 5-exo-hydroxycamphor and 5-oxocamphor. E. coli cells expressing the triple fusion protein thus constitute the first heterologous self-sufficient catalytic system for the oxidation of camphor and other substrates by P450cam.

Cloning and expression in Escherichia coli and Saccharomyces cerevisiae of a novel tobacco cytochrome P-450-like cDNA

A cDNA library constructed from poly(A)+ RNA of tobacco BY2 cells treated with 2,4-dichlorophenoxyacetic acid was screened by using a synthetic oligonucleotide corresponding to the heme binding region of avocado CYP71A1. A cloned 2-kb cDNA designated as cTBP contained an open reading frame of 1593 bp encoding a protein of molecular size of 58916. The deduced amino acid sequence included a cysteine residue corresponding to fifth ligand of heme-Fe at 497th. The coding sequence was expressed under the control of tac promoter and rrnB terminator in Escherichia coli to yield 7 to 10 nmol P450 equivalent per litre of the culture in the presence of delta-aminolevulinic acid. The modified coding sequences in which NH2-terminal residues 2-25 were replaced by the NH2-terminal 18 amino acid residues of microsomal bovine CYP17 were also expressed under the control of ADH promoter and terminator in Saccharomyces cerevisiae to yield 29 and 30 pmol of P450 equivalent/mg protein in the microsomal fraction, respectively. On co-expression of each of the modified coding sequences and yeast NADPH-cytochrome P-450 oxidoreductase gene, the yeast microsomes exhibited 7-ethoxycoumarin O-deethylase activity. Based on these results, tobacco cTBP was found to encode a novel P450-like species with a monooxygenese activity related to xenobiotic metabolism.

Increased resistance to 14 alpha-demethylase inhibitors (DMIs) in Aspergillus niger by coexpression of the Penicillium italicum eburicol 14 alpha-demethylase (cyp51) and the A. niger cytochrome P450 reductase (cprA) genes

In this paper we describe the effects of over-expression of the Penicillium italicum gene encoding eburicol 14 alpha-demethylase (cyp51), in Aspergillus niger strains with one or multiple copies of the gene encoding cytochrome P450 reductase (cprA), on the eburicol 14 alpha-demethylase activity. Eburicol 14 alpha-demethylase activity was determined by measuring the resistance of transformants against some eburicol 14 alpha-demethylase inhibitors (DMIs). DMIs are widely used as fungicides in crop protection and human and veterinarian health care. DMI resistance in a transformant overexpressing both CPR and CYP51 was increased 5-30-fold compared to DMI resistance in the wild type strain, depending on the test compound used. Resistance in this strain was approximately 2-5-fold increased compared to DMI resistance in a transformant that was overexpressing the cyp51 gene but had only the wild type copy of the cprA gene and approximately 3-12-fold increased compared to a strain overexpressing the cprA gene (and having only the wild type copy of the cyp51 gene). These results show the importance of CPR overexpression for increasing cytochrome P450 activities in filamentous fungi.