Heterologous expression and functional characterization of the NADPH-cytochrome P450 reductase from Capsicum annuum

Metabolism of phenolic compounds catalyzed by Tomato CYP736A61

Cytochrome P450s (CYPs) regulate plant growth and stress responses by producing diverse primary and secondary metabolites. However, the function of many plant CYPs remains unknown because, despite their structural similarity, predicting the enzymatic activity of CYPs is difficult. In this study, one member of the CYP736A subfamily (CYP736A61) from tomatoes was isolated and characterized its enzymatic functions. CYP736A61 was successfully expressed in Escherichia coli through co-expression with molecular chaperones. The purified CYP736A61 showed hydroxylation activity toward 7-ethoxycoumarin, producing 7-hydroxycoumarin or 3-hydroxy 7-ethoxycoumarin. Further substrate screening revealed that dihydrochalcone and stilbene derivates (resveratrol and polydatin) are the substrates of CYP736A61. CYP736A61 also mediated the hydroxylation of resveratrol and polydatin, albeit with low activity. Importantly, CYP736A61 mediated the cleavage of resveratrol and polydatin as well as pinostilbene and pterostilbene. Interestingly, CY736A61 also converted phloretin to naringenin chalcone. These results suggest that CYP736A61 is a novel CYP enzyme with stilbene cleavage activity.

Cloning and Functional Characterization of NADPH-Cytochrome P450 Reductases in Aconitum vilmorinianum

Diterpenoid alkaloids (DAs) are major pharmacologically active ingredients of Aconitum vilmorinianum, an important medicinal plant. Cytochrome P450 monooxygenases (P450s) are involved in the DA biosynthetic pathway, and the electron transfer reaction of NADPH-cytochrome P450 reductase (CPR) with P450 is the rate-limiting step of the P450 redox reaction. Here, we identified and characterized two homologs of CPR from Aconitum vilmorinianum. The open reading frames of AvCPR1 and AvCPR2 were found to be 2103 and 2100 bp, encoding 700 and 699 amino acid residues, respectively. Phylogenetic analysis characterized both AvCPR1 and AvCPR2 as class II CPRs. Cytochrome c and ferricyanide could be reduced with the recombinant proteins of AvCPR1 and AvCPR2. Both AvCPR1 and AvCPR2 were expressed in the roots, stems, leaves, and flowers of A. vilmorinianum. The expression levels of AvCPR1 and AvCPR2 were significantly increased in response to methyl jasmonate (MeJA) treatment. The yeasts co-expressing AvCPR1/AvCPR2/SmCPR1 and CYP76AH1 all produced ferruginol, indicating that AvCPR1 and AvCPR2 can transfer electrons to CYP76AH1 in the same manner as SmCPR1. Docking analysis confirmed the experimentally deduced functional activities of AvCPR1 and AvCPR2 for FMN, FAD, and NADPH. The functional characterization of AvCPRs will be helpful in disclosing molecular mechanisms relating to the biosynthesis of diterpene alkaloids in A. vilmorinianum.

A Novel, Highly Potent NADPH-Dependent Cytochrome P450 Reductase from Waste Liza klunzingeri Liver

The use of marine enzymes as catalysts for biotechnological applications is a topical subject. Marine enzymes usually display better operational properties than their animal, plant or bacterial counterparts, enlarging the range of possible biotechnological applications. Due to the fact that cytochrome P450 enzymes can degrade many different toxic environmental compounds, these enzymes have emerged as valuable tools in bioremediation processes. The present work describes the isolation, purification and biochemical characterization of a liver NADPH-dependent cytochrome P450 reductase (CPR) from the marine fish Liza klunzingeri (LkCPR). Experimental results revealed that LkCPR is a monomer of approximately 75 kDa that is active in a wide range of pH values (6-9) and temperatures (40-60 °C), showing the highest catalytic activity at pH 8 and 50 °C. The activation energy of the enzyme reaction was 16.3 kcal mol^-1 K^-1. The K_M values for cytochrome C and NADPH were 8.83 μM and 7.26 μM, and the k_cat values were 206.79 s^-1 and 202.93 s^-1, respectively. LkCPR displayed a specific activity versus cytochrome C of 402.07 µmol min^-1 mg¹, the highest activity value described for a CPR up to date (3.2-4.7 times higher than the most active reported CPRs) and showed the highest thermostability described for a CPR. Taking into account all these remarkable catalytic features, LkCPR offers great potential to be used as a suitable biocatalyst.

Functional characterization of cinnamate 4-hydroxylase from Helianthus annuus Linn using a fusion protein method

Sunflower (Helianthus annuus L.) is an important oil crop, the secondary metabolites of it include many compounds such as flavonoids and lignin. However, the research on the biosynthesis of phenolic compounds in sunflowers is still scarce. Cinnamate 4-hydroxylase (C4H) belongs to the cytochrome P450-dependent monooxygenase family and is involved in the synthesis of many phenolic compounds, but C4H in sunflowers has not yet been cloned and functionally characterized. In this study, we screened three C4H genes from the sunflower transcriptome and genomic databases, named HaC4H1, HaC4H2, and, HaC4H3, respectively. In heterologous expression experiments, we had improved a method from previous studies by the addition of restriction sites to make it easier to express multiple C4H functions and suitable for in vitro activity verification. HaC4Hs without the N-terminal membrane anchor region was fused with a redox partner of Arabidopsis thaliana cytochrome P450 enzyme (CYP450) by the method and functionally expressed in E. coli and the results showed that these three enzymes catalyzed the formation of p-coumaric acid. To further investigate whether our fusion protein approach is applicable to other C4Hs, we used this method to explore the functions of C4H from Peucedanum praeruptorum and Angelica decursiva, and they can also convert trans-cinnamic acid to p-coumaric acid. The gene expression profile showed that all three HaC4H genes showed the highest transcription levels in the roots and might be up-regulated by MeJA. In summary, these results reveal the function of HaC4Hs in sunflower and provide a simpler way to explore C4H and even other cytochrome P450 enzymes in prokaryotic expression systems.

Promoted ABA Hydroxylation by Capsicum annuum CYP707As Overexpression Suppresses Pollen Maturation in Nicotiana tabacum

Abscisic acid (ABA) is a key signaling molecule that mediates plant response to stress. Increasing evidence indicates that ABA also regulates many aspects of plant development, such as seed germination, leaf development, and ripening. ABA metabolism, including ABA biosynthesis and degradation, is an essential aspect of ABA response in plants. In this study, we identified four cytochrome P450 genes (CaCYP707A1, 2, 3, and 4) that mediate ABA hydroxylation, which is required for ABA degradation in Capsicum annuum. We observed that CaCYP707A-mediated ABA hydroxylation promotes ABA degradation, leading to low levels of ABA and a dehydration phenotype in 35S:CaCYP707A plants. Importantly, seed formation was strongly inhibited in 35S:CaCYP707A plants, and a cross-pollination test suggested that the defect in seed formation is caused by improper pollen development. Phenotypic analysis showed that pollen maturation is suppressed in 35S:CaCYP707A1 plants. Consequently, most 35S:CaCYP707A1 pollen grains degenerated, unlike non-transgenic (NT) pollen, which developed into mature pollen grains. Together our results indicate that CaCYP707A mediates ABA hydroxylation and thereby influences pollen development, helping to elucidate the mechanism underlying ABA-regulated pollen development.

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.

Effect of a functional recombinant cytochrome P450 enzyme of Helicoverpa armigera on gossypol metabolism co-expressed with NADPH-cytochrome P450 reductase in Pichia pastoris

Gossypol is a polyphonic toxic compound that is present in cotton plants. The P450 cytochromes CYP6AE14 and CYP9A12 of Helicoverpa armigera are highly induced by gossypol and have been reported to be possibly involved in gossypol degradation. To determine whether the candidate H. armigera CYP6AE14 and CYP9A12 enzymes could metabolize gossypol in vitro, functional recombinant H. armigera CYP6AE14 and CPR (CYP9A12 and CPR) enzymes were successfully expressed in Pichia pastoris (P. pastoris). UPLC-QTOF/MS demonstrated the following results: (1) Free gossypol was spontaneously degraded to the gossypol metabolites G1 (m/z 265) and G2 (m/z 293) without the addition of any enzyme. (2) Free gossypol was observed following the addition of the endogenous or recombinant H. armigera P450 cytochrome CYP6AE14/CYP9A12 enzyme: in the first pathway, free gossypol was dehydroxylated and decarboxylated to G3 (m/z 453), and in the second pathway, the aldehyde group of gossypol and its metabolite were covalently bound with the amine products to form G4 (m/z 437) and G5 (m/z 783). (3) In addition to the gossypol binding pathways, the recombinant H. armigera CPR and CYP9A12 enzymes was found that could further decarboxylate the gossypol intermediate demethylated reduction of gossypolonic acid (m/z 294) and demethylated gossic acid (m/z 265) to G0 (m/z 209) and G0' (m/z 249) respectively.

Recombinant production of eukaryotic cytochrome P450s in microbial cell factories

Cytochrome P450s (P450s) comprise one of the largest known protein families. They occur in every kingdom of life and catalyze essential reactions, such as carbon source assimilation, synthesis of hormones and secondary metabolites, or degradation of xenobiotics. Due to their outstanding ability of specifically hydroxylating complex hydrocarbons, there is a great demand to use these enzymes for biocatalysis, including applications at an industrial scale. Thus, the recombinant production of these enzymes is intensively investigated. However, especially eukaryotic P450s are difficult to produce. Challenges are faced due to complex cofactor requirements and the availability of a redox-partner (cytochrome P450 reductase, CPR) can be a key element to get active P450s. Additionally, most eukaryotic P450s are membrane bound which complicates the recombinant production. This review describes current strategies for expression of P450s in the microbial cell factories Escherichia coli, Saccharomyces cerevisiae, and Pichia pastoris.

Determinants of thermostability in the cytochrome P450 fold

Cytochromes P450 are found throughout the biosphere in a wide range of environments, serving a multitude of physiological functions. The ubiquity of the P450 fold suggests that it has been co-opted by evolution many times, and likely presents a useful compromise between structural stability and conformational flexibility. The diversity of substrates metabolized and reactions catalyzed by P450s makes them attractive starting materials for use as biocatalysts of commercially useful reactions. However, process conditions impose different requirements on enzymes to those in which they have evolved naturally. Most natural environments are relatively mild, and therefore most P450s have not been selected in Nature for the ability to withstand temperatures above ~40°C, yet industrial processes frequently require extended incubations at much higher temperatures. Thus, there has been considerable interest and effort invested in finding or engineering thermostable P450 systems. Numerous P450s have now been identified in thermophilic organisms and analysis of their structures provides information as to mechanisms by which the P450 fold can be stabilized. In addition, protein engineering, particularly by directed or artificial evolution, has revealed mutations that serve to stabilize particular mesophilic enzymes of interest. Here we review the current understanding of thermostability as it applies to the P450 fold, gleaned from the analysis of P450s characterized from thermophilic organisms and the parallel engineering of mesophilic forms for greater thermostability. We then present a perspective on how this information might be used to design stable P450 enzymes for industrial application. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

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.

Evolution of NADPH-cytochrome P450 oxidoreductases (POR) in Apiales - POR 1 is missing

The NADPH-dependent cytochrome P450 oxidoreductase (POR) is the obligate electron donor to eukaryotic microsomal cytochromes P450 enzymes. The number of PORs within plant species is limited to one to four isoforms, with the most common being two PORs per plant. These enzymes provide electrons to a huge number of different cytochromes P450s (from 50 to several hundred within one plant). Within the eudicotyledons, PORs can be divided into two major clades, POR 1 and POR 2. Based on our own sequencing analysis and publicly available data, we have identified 45 PORs from the angiosperm order Apiales. These were subjected to a phylogenetic analysis along with 237 other publicly available (NCBI and oneKP) POR sequences found within the clade Asterids. Here, we show that the order Apiales only harbor members of the POR 2 clade, which are further divided into two distinct subclades. This is in contrast to most other eudicotyledon orders that have both POR 1 and POR 2. This suggests that through gene duplications and one gene deletion, Apiales only contain members of the POR 2 clade. Three POR 2 isoforms from Thapsia garganica L., Apiaceae, were all full-length in an Illumina root transcriptome dataset (available from the SRA at NCBI). All three genes were shown to be functional upon reconstitution into nanodiscs, confirming that none of the isoforms are pseudogenes.

Molecular Cloning, Heterologous Expression, and Functional Characterization of an NADPH-Cytochrome P450 Reductase Gene from Camptotheca acuminata, a Camptothecin-Producing Plant

Camptothecin (CAM), a complex pentacyclic pyrroloqinoline alkaloid, is the starting material for CAM-type drugs that are well-known antitumor plant drugs. Although many chemical and biological research efforts have been performed to produce CAM, a few attempts have been made to uncover the enzymatic mechanism involved in the biosynthesis of CAM. Enzyme-catalyzed oxidoreduction reactions are ubiquitously presented in living organisms, especially in the biosynthetic pathway of most secondary metabolites such as CAM. Due to a lack of its reduction partner, most catalytic oxidation steps involved in the biosynthesis of CAM have not been established. In the present study, an NADPH-cytochrome P450 reductase (CPR) encoding gene CamCPR was cloned from Camptotheca acuminata, a CAM-producing plant. The full length of CamCPR cDNA contained an open reading frame of 2127-bp nucleotides, corresponding to 708-amino acid residues. CamCPR showed 70 ~ 85% identities to other characterized plant CPRs and it was categorized to the group II of CPRs on the basis of the results of multiple sequence alignment of the N-terminal hydrophobic regions. The intact and truncate CamCPRs with N- or C-terminal His₆-tag were heterologously overexpressed in Escherichia coli. The recombinant enzymes showed NADPH-dependent reductase activity toward a chemical substrate ferricyanide and a protein substrate cytochrome c. The N-terminal His₆-tagged CamCPR showed 18- ~ 30-fold reduction activity higher than the C-terminal His₆-tagged CamCPR, which supported a reported conclusion, i.e., the last C-terminal tryptophan of CPRs plays an important role in the discrimination between NADPH and NADH. Co-expression of CamCPR and a P450 monooxygenase, CYP73A25, a cinnamate 4-hydroxylase from cotton, and the following catalytic formation of p-coumaric acid suggested that CamCPR transforms electrons from NADPH to the heme center of P450 to support its oxidation reaction. Quantitative real-time PCR analysis showed that CamCPR was expressed in the roots, stems, and leaves of C. acuminata seedlings. The relative transcript level of CamCPR in leaves was 2.2-fold higher than that of roots and the stems showed 1.5-fold transcript level higher than the roots. The functional characterization of CamCPR will be helpful to disclose the mysterious mechanisms of the biosynthesis of CAM. The present study established a platform to characterize the P450 enzymes involved in the growth, development, and metabolism of eukaryotic organisms.