Biocatalytic organic synthesis of optically pure (S)-scoulerine and berbine and benzylisoquinoline alkaloids

Metabolomics and transcriptomics reveal the mechanism of alkaloid synthesis in Corydalis yanhusuo bulbs

Corydalis yanhusuo W.T. Wang is a traditional herb. Benzylisoquinoline alkaloids (BIAs) are the main pharmacological active ingredients that play an important role in sedation, relieving pain, promoting blood circulation, and inhibiting cancer cells. However, there are few studies on the biosynthetic pathway of benzylisoquinoline alkaloids in Corydalis yanhusuo, especially on some specific components, such as tetrahydropalmatine. We carried out widely targeted metabolome and transcriptomic analyses to construct the biosynthetic pathway of benzylisoquinoline alkaloids and identified candidate genes. In this study, 702 metabolites were detected, including 216 alkaloids. Protoberberine-type and aporphine-type alkaloids are the main chemical components in C. yanhusuo bulbs. Key genes for benzylisoquinoline alkaloids biosynthesis, including 6-OMT, CNMT, NMCH, BBE, SOMT1, CFS, SPS, STOX, MSH, TNMT and P6H, were successfully identified. There was no significant difference in the content of benzylisoquinoline alkaloids and the expression level of genes between the two suborgans (mother-bulb and son-bulb). The expression levels of BIA genes in the expansion stage (MB-A and SB-A) were significantly higher than those in the maturity stage (MB-C and SB-C), and the content of benzylisoquinoline alkaloids was consistent with the pattern of gene regulation. Five complete single genes were likely to encode the functional enzyme of CoOMT, which participated in tetrahydropalmatine biosynthesis in C. yanhusuo bulbs. These studies provide a strong theoretical basis for the subsequent development of metabolic engineering of benzylisoquinoline alkaloids (especially tetrahydropalmatine) of C. yanhusuo.

Recent advances in total synthesis of protoberberine and chiral tetrahydroberberine alkaloids

Covering: Up to 2024Due to the widespread distribution of protoberberine alkaloids (PBs) and tetrahydroberberine alkaloids (THPBs) in nature, coupled with their myriad unique physiological activities, they have garnered considerable attention from medical practitioners. Over the past few decades, synthetic chemists have devised various total synthesis methods to attain these structures, continually expanding reaction pathways to achieve more efficient synthetic strategies. Simultaneously, the chiral construction of THPBs has become a focal point. In this comprehensive review, we categorically summarized the developmental trajectory of the total synthesis of these alkaloids based on the core closure strategies of protoberberine and tetrahydroberberine.

Concise synthesis of (R)-reticuline and (+)-salutaridine by combining early-stage organic synthesis and late-stage biocatalysis

Efficient access to the morphinan scaffold remains a major challenge in both synthetic chemistry and biotechnology. Here, a biomimetic chemo-enzymatic strategy to synthesize the natural promorphinan intermediate (+)-salutaridine is demonstrated. By combining early-stage organic synthesis with enzymatic asymmetric key step transformations, the prochiral natural intermediate 1,2-dehydroreticuline was prepared and subsequently stereoselectively reduced by the enzyme 1,2-dehydroreticuline reductase obtaining (R)-reticuline in high ee and yield (>99% ee, up to quant. conversion, 92% isol. yield). In the final step, membrane-bound salutaridine synthase was used to perform the selective ortho-para phenol coupling to give (+)-salutaridine. The synthetic route shows the potential of combining early-stage advanced organic chemistry to minimize protecting group techniques with late-stage multi-step biocatalysis to provide an unprecedented access to the medicinally important compound class of promorphinans.

Scoulerine promotes cytotoxicity and attenuates stemness in ovarian cancer by targeting PI3K/AKT/mTOR axis

In women, ovarian cancer is a common gynecological cancer associated with poor prognosis, reoccurrence and chemoresistance. Scoulerine, a benzylisoquinoline alkaloid, has been reported effective against several carcinomas. Thus, we investigated the impact of scoulerine on ovarian cancer cells (OVCAR3). Cell viability was assessed by MTT assay, migration was determined by Boyden Chamber assay, while the invasion was monitored by Boyden Chamber assay using the matrigel. The stemness properties of OVCAR3 cells were observed by tumorsphere assay. Epithelial to mesenchymal transition (EMT) and stemness-related protein markers were monitored by real-time PCR analysis and immunoblotting. Scoulerine inhibits the viability of OVCAR3 cells with the IC 50 observed at 10 µmol L-1 after 48 h treatment. Scoulerine inhibited the colony-forming ability, migration and invasiveness of OVCAR3 cells in a dose-dependent fashion. Scoulerine treatment also drastically reduced the spheroid-forming ability of OVCAR3 cells. The mesenchymal and stemness--related markers like N-cadherin, vimentin, CD-44, Oct-4, Sox-2 and Aldh1A1 were downregulated, whereas the epithelial markers like E-cadherin and CD-24 were upregulated in scoulerine-treated cells. The upstream PI3K/Akt/mTOR-axis was downregulated in scoulerine-treated cells. We concluded that scoulerine successfully perturbs the cancerous properties of OVCAR3 cells by targeting the PI3K/Akt/mTOR axis. In vivo studies revealed a substantial decrease in tumor mass and volume after scoulerine treatment. Furthermore, scoulerine treatment was found to decrease oxidative stress factors in ovarian cancer mice model. Scoulerine is a potential anticancer agent against ovarian cancer and can be considered as a lead molecule for this malignancy, provided further investigations are performed.

Alkaloids of Dicranostigma franchetianum (Papaveraceae) and Berberine Derivatives as a New Class of Antimycobacterial Agents

Tuberculosis (TB) is a widespread infectious disease caused by Mycobacterium tuberculosis. The increasing incidence of multidrug-resistant (MDR) and extensively drug-resistant (XDR) strains has created a need for new antiTB agents with new chemical scaffolds to combat the disease. Thus, the key question is: how to search for new antiTB and where to look for them? One of the possibilities is to search among natural products (NPs). In order to search for new antiTB drugs, the detailed phytochemical study of the whole Dicranostigma franchetianum plant was performed isolating wide spectrum of isoquinoline alkaloids (IAs). The chemical structures of the isolated alkaloids were determined by a combination of MS, HRMS, 1D, and 2D NMR techniques, and by comparison with literature data. Alkaloids were screened against Mycobacterium tuberculosis H37Ra and four other mycobacterial strains (M. aurum, M. avium, M. kansasii, and M. smegmatis). Alkaloids 3 and 5 showed moderate antimycobacterial activity against all tested strains (MICs 15.625–31.25 µg/mL). Furthermore, ten semisynthetic berberine (16a–16k) derivatives were developed and tested for antimycobacterial activity. In general, the derivatization of berberine was connected with a significant increase in antimycobacterial activity against all tested strains (MICs 0.39–7.81 μg/mL). Two derivatives (16e, 16k) were identified as compounds with micromolar MICs against M. tuberculosis H37Ra (MIC 2.96 and 2.78 µM). All compounds were also evaluated for their in vitro hepatotoxicity on a hepatocellular carcinoma cell line (HepG2), exerting lower cytotoxicity profile than their MIC values, thereby potentially reaching an effective concentration without revealing toxic side effects.

The Phytochemical Scoulerine Inhibits Aurora Kinase Activity to Induce Mitotic and Cytokinetic Defects

To identify novel bioactive compounds, an image-based, cell culture screening of natural product extracts was conducted. Specifically, our screen was designed to identify phytochemicals that might phenocopy inhibition of the chromosomal passenger protein complex in eliciting mitotic and cytokinetic defects. A known alkaloid, scoulerine, was identified from the rhizomes of the plant Corydalis decumbens as being able to elicit a transient mitotic arrest followed by either apoptosis induction or polyploidy. In examining the mitotic abnormality further, we observed that scoulerine could elicit supernumerary centrosomes during mitosis, but not earlier in the cell cycle. The localization of NUMA1 at spindle poles was also inhibited, suggesting diminished potential for microtubule recruitment and spindle-pole focusing. Polyploid cells emerged subsequent to cytokinetic failure. The concentration required for scoulerine to elicit all its cell division phenotypes was similar, and an examination of related compounds highlighted the requirement for proper positioning of a hydroxyl and a methoxy group about an aromatic ring for activity. Mechanistically, scoulerine inhibited AURKB activity at concentrations that elicited supernumerary centrosomes and polyploidy. AURKA was only inhibited at higher concentrations, so AURKB inhibition is the likely mechanism by which scoulerine elicited division defects. AURKB inhibition was never complete, so scoulerine may be a suboptimal AURK inhibitor or work upstream of the chromosomal passenger protein complex to reduce AURKB activity. Scoulerine inhibited the viability of a variety of human cancer cell lines. Collectively, these findings uncover a previously unknown activity of scoulerine that could facilitate targeting human cancers. Scoulerine, or a next-generation analogue, may be useful as a nontoxic component of combination therapies where inhibiting the chromosomal passenger protein complex is desired.

Gene editing and synthetically accessible inhibitors reveal role for TPC2 in HCC cell proliferation and tumor growth

The role of two-pore channel 2 (TPC2), one of the few cation channels localized on endolysosomal membranes, in cancer remains poorly understood. Here, we report that TPC2 knockout reduces proliferation of cancer cells in vitro, affects their energy metabolism, and successfully abrogates tumor growth in vivo. Concurrently, we have developed simplified analogs of the alkaloid tetrandrine as potent TPC2 inhibitors by screening a library of synthesized benzyltetrahydroisoquinoline derivatives. Removal of dispensable substructures of the lead molecule tetrandrine increases antiproliferative properties against cancer cells and impairs proangiogenic signaling of endothelial cells to a greater extent than tetrandrine. Simultaneously, toxic effects on non-cancerous cells are reduced, allowing in vivo administration and revealing a TPC2 inhibitor with antitumor efficacy in mice. Hence, our study unveils TPC2 as valid target for cancer therapy and provides easily accessible tetrandrine analogs as a promising option for effective pharmacological interference.

Identification of Novel 1-O-Substituted Aporphine Analogues as Potent 5-HT2C Receptor Agonists

The 5-HT_2C receptor has emerged as a promising target in the treatment of a variety of central nervous system disorders. We have first identified aporphines as a new class of 5-HT_2C receptor agonists. Structure-activity relationship results indicate that the aporphine core may be required for 5-HT_2C receptor activity, and substitutions at its C1 position are important for 5-HT_2C receptor activity. Our efforts to optimize our hit 15781 lead to the identification of the highly potent and selective 5-HT_2C agonist 18b (MQ02-439) with an EC₅₀ value of 104 nM and weak antagonism at the 5-HT_2A and 5-HT_2B receptors. The findings may serve as good starting points for the development of more potent and selective 5-HT_2C agonists as valuable pharmacological tools or potential drug candidates.

Closing the gap: yeast electron-transferring flavoprotein links the oxidation of d-lactate and d-α-hydroxyglutarate to energy production via the respiratory chain

Electron‐transferring flavoproteins (ETFs) have been found in all kingdoms of life, mostly assisting in shuttling electrons to the respiratory chain for ATP production. While the human (h) ETF has been studied in great detail, very little is known about the biochemical properties of the homologous protein in the model organism Saccharomyces cerevisiae (yETF). In view of the absence of client dehydrogenases, for example, the acyl‐CoA dehydrogenases involved in the β‐oxidation of fatty acids, d‐lactate dehydrogenase 2 (Dld2) appeared to be the only relevant enzyme that is serviced by yETF for electron transfer to the mitochondrial electron transport chain. However, this hypothesis was never tested experimentally. Here, we report the biochemical properties of yETF and Dld2 as well as the electron transfer reaction between the two proteins. Our study revealed that Dld2 oxidizes d‐α‐hydroxyglutarate more efficiently than d‐lactate exhibiting k_catapp/K_Mapp values of 1200 ± 300 m⁻¹·s⁻¹ and 11 ± 2 m⁻¹·s⁻¹, respectively. As expected, substrate‐reduced Dld2 very slowly reacted with oxygen or the artificial electron acceptor 2,6‐dichlorophenol indophenol. However, photoreduced Dld2 was rapidly reoxidized by oxygen, suggesting that the reaction products, that is, α‐ketoglutarate and pyruvate, ‘lock’ the reduced enzyme in an unreactive state. Interestingly, however, we could demonstrate that substrate‐reduced Dld2 rapidly transfers electrons to yETF. Therefore, we conclude that the formation of a product‐reduced Dld2 complex suppresses electron transfer to dioxygen but favors the rapid reduction in yETF, thus preventing the loss of electrons and the generation of reactive oxygen species.

Construction of N-Heterocycles through Cyclization of Tertiary Amines

N-Heterocycles have been found in a large number of natural products, drug molecules, and bioactive compounds, and they thereby play a vital role in diverse research disciplines including drug discovery, organic synthesis, chemical biology, and material science. To this end, the development of new methods and strategies for the construction of N-heterocyclic frameworks is arguably one of the most dynamic and significant research areas in organic synthesis. One of these powerful approaches to the synthesis of N-heterocycles is to establish cyclization reactions based on the transformation of tertiary amines, which has emerged as an attractive research topic. In this Minireview, the significant achievements in the construction of N-heterocycles through cyclization of tertiary amines are highlighted and a comprehensive overview of the rational design, development, and application of these synthetic methods is presented.

The single berberine bridge enzyme homolog of Physcomitrella patens is a cellobiose oxidase

The berberine bridge enzyme from the California poppy Eschscholzia californica (EcBBE) catalyzes the oxidative cyclization of (S)‐reticuline to (S)‐scoulerine, that is, the formation of the berberine bridge in the biosynthesis of benzylisoquinoline alkaloids. Interestingly, a large number of BBE‐like genes have been identified in plants that lack alkaloid biosynthesis. This finding raised the question of the primordial role of BBE in the plant kingdom, which prompted us to investigate the closest relative of EcBBE in Physcomitrella patens (PpBBE1), the most basal plant harboring a BBE‐like gene. Here, we report the biochemical, structural, and in vivo characterization of PpBBE1. Our studies revealed that PpBBE1 is structurally and biochemically very similar to EcBBE. In contrast to EcBBE, we found that PpBBE1 catalyzes the oxidation of the disaccharide cellobiose to the corresponding lactone, that is, PpBBE1 is a cellobiose oxidase. The enzymatic reaction mechanism was characterized by a structure‐guided mutagenesis approach that enabled us to assign a catalytic role to amino acid residues in the active site of PpBBE1. In vivo experiments revealed the highest level of PpBBE1 expression in chloronema, the earliest stage of the plant's life cycle, where carbon metabolism is strongly upregulated. It was also shown that the enzyme is secreted to the extracellular space, where it may be involved in later steps of cellulose degradation, thereby allowing the moss to make use of cellulose for energy production. Overall, our results suggest that the primordial role of BBE‐like enzymes in plants revolved around primary metabolic reactions in carbohydrate utilization.

Database

Structural data are available in the PDB under the accession numbers 6EO4 and 6EO5.

Comparative study on alkaloids and their anti-proliferative activities from three Zanthoxylum species

Background

Alkaloids have been considered as the most promising bioactive ingredients in plant species from the genus Zanthoxylum. This study reports on the compositions and contents of the Zanthoxylum alkaloids (ZAs) from three Zanthoxylum species, and their potential anti-proliferation activities.

Methods

An HPLC-UV/ESI-MS/MS method was established and employed to analyze the alkaloids in different Zanthoxylum extracts. The common and unique peaks and their relative contents were summarized and compared to evaluate the similarity and dissimilarity of the three Zanthoxylum species. Meanwhile, inhibitory activity tests to four carcinoma cell lines, i.e., stomach tumor cells (SGC-7901), cervical tumor cells (Hela), colon tumor cells (HT-29) and Hepatic tumor cells (Hep G2), were carried out in vitro to evaluate the bioactivities of the ZAs.

Results

Seventy peaks were detected in the crude total alkaloid samples, and 58 of them were identified. As a result, 13 common peaks were found in the extracts of all the three Zanthoxylum species, while some unique peaks were also observed in specific species, with 17 peaks in Z. simulans, 15 peaks in Z. ailanthoides and 11 peaks in Z. chalybeum, respectively. The comparison of the composition and relative contents indicated that alkaloids of benzophenanthridine type commonly present in all the three Zanthoxylum species with high relative contents among the others, which are 60.52% in Z. ailanthoides, 30.52% in Z. simulans and 13.84% in Z. chalybeum, respectively. In terms of activity test, Most of the crude alkaloids extracts showed remarkable inhibitory activities against various tumor cells, and the inhibitory rates ranged from 60.71 to 93.63% at a concentration of 200 μg/mL. However, SGC-7901 cells seemed to be more sensitive to the ZAs than the other three cancer cells.

Conclusion

The alkaloid profiles detected in this work revealed significant differences in both structures and contents among Zanthoxylum species. The inhibitory rates for different cancer cells in this study indicated that the potential anti-cancer activity should be attributed to quaternary alkaloids in these three species, which will provide great guidance for further exploring this traditional medicinal resource as new healthcare products.

Laboratory-scale production of (S)-reticuline, an important intermediate of benzylisoquinoline alkaloids, using a bacterial-based method

Benzylisoquinoline alkaloids (BIAs) are a group of plant secondary metabolites that have been identified as targets for drug discovery because of their diverse pharmaceutical activities. Well-known BIAs are relatively abundant in plants and have therefore been extensively studied. However, although unknown BIAs are also thought to have valuable activities, they are difficult to obtain because the raw materials are present at low abundance in nature. We have previously reported the fermentative production of an important intermediate (S)-reticuline from dopamine using Escherichia coli. However, the yield is typically limited. Here, we improved production efficiency by combining in vivo tetrahydropapaveroline production in E. coli with in vitro enzymatic synthesis of (S)-reticuline. Finally, 593 mg of pure (S)-reticuline was obtained from 1 L of the reaction mixture. Because this bacterial-based method is simple, it could be widely used for production of (S)-reticuline and related BIAs, thereby facilitating studies of BIAs for drug discovery.

Improved simplicity and practicability in copper-catalyzed alkynylation of tetrahydroisoquinoline

ABSTRACT

Alkynylation reactions of N-protected tetrahydroisoquinolines have been performed using several different protocols of cross dehydrogenative coupling. Initially, a CuCl-catalyzed method was investigated, which worked well with three different N-protecting groups, namely phenyl, PMP, and benzyl and t-BuOOH as oxidant in acetonitrile as solvent. The peroxide could then be replaced by simple air and acetonitrile for water, leading to an overall very environmentally friendly protocol. Finally, a decarboxylative alkynylation protocol starting from alkynoic acids was also developed using again air as oxidant. This avoids the use of gaseous alkynes in the introduction of short-chained alkyne substituents.

GRAPHICAL ABSTRACT

Structure and biochemical properties of recombinant human dimethylglycine dehydrogenase and comparison to the disease-related H109R variant

The human dimethylglycine dehydrogenase (hDMGDH) is a flavin adenine dinucleotide (FAD)- and tetrahydrofolate (THF)-dependent, mitochondrial matrix enzyme taking part in choline degradation, one-carbon metabolism and electron transfer to the respiratory chain. The rare natural variant H109R causes dimethylglycine dehydrogenase deficiency leading to increased blood and urinary dimethylglycine concentrations. A detailed biochemical and structural characterization of hDMGDH was thus far hampered by insufficient heterologous expression of the protein. In the present study, we report the development of an intracellular, heterologous expression system in Komagataella phaffii (formerly known as Pichia pastoris) providing the opportunity to determine kinetic parameters, spectroscopic properties, thermostability, and the redox potential of hDMGDH. Moreover, we have successfully crystallized the wild-type enzyme and determined the structure to 3.1-Å resolution. The structure-based analysis of our biochemical data provided new insights into the kinetic properties of the enzyme in particular with respect to oxygen reactivity. A comparative study with the H109R variant demonstrated that the variant suffers from decreased protein stability, cofactor saturation, and substrate affinity.

DATABASE

Structural data are available in the PDB database under the accession number 5L46.

Asymmetric Synthesis of Isoquinoline Alkaloids: 2004-2015

In the past decade, the asymmetric synthesis of chiral nonracemic isoquinoline alkaloids, a family of natural products showing a wide range of structural diversity and biological and pharmaceutical activity, has been based either on continuation or improvement of known traditional methods or on new, recently developed, strategies. Both diastereoselective and enantioselective catalytic methods have been applied. This review describes the stereochemically modified traditional syntheses (the Pictet-Spengler, the Bischler-Napieralski, and the Pomeranz-Fritsch-Bobbitt) along with strategies based on closing of the nitrogen-containing ring B of the isoquinoline core by the formation of bonds between C₁-N₂, N₂-C₃, C₁-N₂/N₂-C₃, and C₁-N₂/C₄-C_4a atoms. Methods involving introduction of substituents at the C1 carbon of isoquinoline core along with syntheses applying various biocatalytic techniques have also been reviewed.

Building Bridges: Biocatalytic C-C-Bond Formation toward Multifunctional Products

Carbon-carbon bond formation is the key reaction for organic synthesis to construct the carbon framework of organic molecules. The review gives a selection of biocatalytic C-C-bond-forming reactions which have been investigated during the last 5 years and which have already been proven to be applicable for organic synthesis. In most cases, the reactions lead to products functionalized at the site of C-C-bond formation (e.g., α-hydroxy ketones, aminoalcohols, diols, 1,4-diketones, etc.) or allow to decorate aromatic and heteroaromatic molecules. Furthermore, examples for cyclization of (non)natural precursors leading to saturated carbocycles are given as well as the stereoselective cyclopropanation of olefins affording cyclopropanes. Although many tools are already available, recent research also makes it clear that nature provides an even broader set of enzymes to perform specific C-C coupling reactions. The possibilities are without limit; however, a big library of variants for different types of reactions is required to have the specific enzyme for a desired specific (stereoselective) reaction at hand.

Structure of a Berberine Bridge Enzyme-Like Enzyme with an Active Site Specific to the Plant Family Brassicaceae

Berberine bridge enzyme-like (BBE-like) proteins form a multigene family (pfam 08031), which is present in plants, fungi and bacteria. They adopt the vanillyl alcohol-oxidase fold and predominantly show bi-covalent tethering of the FAD cofactor to a cysteine and histidine residue, respectively. The Arabidopsis thaliana genome was recently shown to contain genes coding for 28 BBE-like proteins, while featuring four distinct active site compositions. We determined the structure of a member of the AtBBE-like protein family (termed AtBBE-like 28), which has an active site composition that has not been structurally and biochemically characterized thus far. The most salient and distinguishing features of the active site found in AtBBE-like 28 are a mono-covalent linkage of a histidine to the 8α-position of the flavin-isoalloxazine ring and the lack of a second covalent linkage to the 6-position, owing to the replacement of a cysteine with a histidine. In addition, the structure reveals the interaction of a glutamic acid (Glu426) with an aspartic acid (Asp369) at the active site, which appear to share a proton. This arrangement leads to the delocalization of a negative charge at the active site that may be exploited for catalysis. The structure also indicates a shift of the position of the isoalloxazine ring in comparison to other members of the BBE-like family. The dioxygen surrogate chloride was found near the C(4a) position of the isoalloxazine ring in the oxygen pocket, pointing to a rapid reoxidation of reduced enzyme by dioxygen. A T-DNA insertional mutant line for AtBBE-like 28 results in a phenotype, that is characterized by reduced biomass and lower salt stress tolerance. Multiple sequence analysis showed that the active site composition found in AtBBE-like 28 is only present in the Brassicaceae, suggesting that it plays a specific role in the metabolism of this plant family.

Enantioselective Oxidative Aerobic Dealkylation of N-Ethyl Benzylisoquinolines by Employing the Berberine Bridge Enzyme

N-Dealkylation methods are well described for organic chemistry and the reaction is known in nature and drug metabolism; however, to our knowledge, enantioselective N-dealkylation has not been yet reported. In this study, exclusively the (S)-enantiomers of racemic N-ethyl tertiary amines (1-benzyl-N-ethyl-1,2,3,4-tetrahydroisoquinolines) were dealkylated to give the corresponding secondary (S)-amines in an enantioselective fashion at the expense of molecular oxygen. The reaction is catalyzed by the berberine bridge enzyme, which is known for CC bond formation. The dealkylation was demonstrated on a 100 mg scale and gave optically pure dealkylated products (ee>99 %).

Optimization of yeast-based production of medicinal protoberberine alkaloids

Background

Protoberberine alkaloids are bioactive molecules abundant in plant preparations for traditional medicines. Yeast engineered to express biosynthetic pathways for fermentative production of these compounds will further enable investigation of the medicinal properties of these molecules and development of alkaloid-based drugs with improved efficacy and safety. Here, we describe the optimization of a biosynthetic pathway in Saccharomyces cerevisiae for conversion of rac-norlaudanosoline to the protoberberine alkaloid (S)-canadine.

Results

This yeast strain is engineered to express seven heterologous enzymes, resulting in protoberberine alkaloid production from a simple benzylisoquinoline alkaloid precursor. The seven enzymes include three membrane-bound enzymes: the flavin-dependent oxidase berberine bridge enzyme, the cytochrome P450 canadine synthase, and a cytochrome P450 reductase. A number of strategies were implemented to improve flux through the pathway, including enzyme variant screening, genetic copy number variation, and culture optimization, that led to an over 70-fold increase in canadine titer up to 1.8 mg/L. Increased canadine titers enable extension of the pathway to produce berberine, a major constituent of several traditional medicines, for the first time in a microbial host. We also demonstrate that this strain is viable at pilot scale.

Conclusions

By applying metabolic engineering and synthetic biology strategies for increased conversion of simple benzylisoquinoline alkaloids to complex protoberberine alkaloids, this work will facilitate chemoenzymatic synthesis or de novo biosynthesis of these and other high-value compounds using a microbial cell factory.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0332-3) contains supplementary material, which is available to authorized users.

Oxidation of Monolignols by Members of the Berberine Bridge Enzyme Family Suggests a Role in Plant Cell Wall Metabolism

Plant genomes contain a large number of genes encoding for berberine bridge enzyme (BBE)-like enzymes. Despite the widespread occurrence and abundance of this protein family in the plant kingdom, the biochemical function remains largely unexplored. In this study, we have expressed two members of the BBE-like enzyme family from Arabidopsis thaliana in the host organism Komagataella pastoris. The two proteins, termed AtBBE-like 13 and AtBBE-like 15, were purified, and their catalytic properties were determined. In addition, AtBBE-like 15 was crystallized and structurally characterized by x-ray crystallography. Here, we show that the enzymes catalyze the oxidation of aromatic allylic alcohols, such as coumaryl, sinapyl, and coniferyl alcohol, to the corresponding aldehydes and that AtBBE-like 15 adopts the same fold as vanillyl alcohol oxidase as reported previously for berberine bridge enzyme and other FAD-dependent oxidoreductases. Further analysis of the substrate range identified coniferin, the glycosylated storage form of coniferyl alcohol, as a substrate of the enzymes, whereas other glycosylated monolignols were rather poor substrates. A detailed analysis of the motifs present in the active sites of the BBE-like enzymes in A. thaliana suggested that 14 out of 28 members of the family might catalyze similar reactions. Based on these findings, we propose a novel role of BBE-like enzymes in monolignol metabolism that was previously not recognized for this enzyme family.

Rationally engineered flavin-dependent oxidase reveals steric control of dioxygen reduction

The ability of flavoenzymes to reduce dioxygen varies greatly, and is controlled by the protein environment, which may cause either a rapid reaction (oxidases) or a sluggish reaction (dehydrogenases). Previously, a 'gatekeeper' amino acid residue was identified that controls the reactivity to dioxygen in proteins from the vanillyl alcohol oxidase superfamily of flavoenzymes. We have identified an alternative gatekeeper residue that similarly controls dioxygen reactivity in the grass pollen allergen Phl p 4, a member of this superfamily that has glucose dehydrogenase activity and the highest redox potential measured in a flavoenzyme. A substitution at the alternative gatekeeper site (I153V) transformed the enzyme into an efficient oxidase by increasing dioxygen reactivity by a factor of 60,000. An inverse exchange (V169I) in the structurally related berberine bridge enzyme (BBE) decreased its dioxygen reactivity by a factor of 500. Structural and biochemical characterization of these and additional variants showed that our model enzymes possess a cavity that binds an anion and resembles the 'oxyanion hole' in the proximity of the flavin ring. We showed also that steric control of access to this site is the most important parameter affecting dioxygen reactivity in BBE-like enzymes. Analysis of flavin-dependent oxidases from other superfamilies revealed similar structural features, suggesting that dioxygen reactivity may be governed by a common mechanistic principle.

DATABASE

Structural data are available in PDB database under the accession numbers 4PVE, 4PVH, 4PVJ, 4PVK, 4PWB, 4PWC and 4PZF.

Deracemization by simultaneous bio-oxidative kinetic resolution and stereoinversion

Deracemization, that is, the transformation of a racemate into a single product enantiomer with theoretically 100% conversion and 100% ee, is an appealing but also challenging option for asymmetric synthesis. Herein a novel chemo-enzymatic deracemization concept by a cascade is described: the pathway involves two enantioselective oxidation steps and one non-stereoselective reduction step, enabling stereoinversion and a simultaneous kinetic resolution. The concept was exemplified for the transformation of rac-benzylisoquinolines to optically pure (S)-berbines. The racemic substrates were transformed to optically pure products (ee>97%) with up to 98% conversion and up to 88% yield of isolated product.

Deracemisation of benzylisoquinoline alkaloids employing monoamine oxidase variants

Deracemisation of benzylisoquinoline alkaloids was performed employing a recently developed variant of monoamine oxidase from Aspergillus niger (MAO-N variant D11).

The role of biocatalysis in the asymmetric synthesis of alkaloids

Alkaloids are not only one of the most intensively studied classes of natural products, their wide spectrum of pharmacological activities also makes them indispensable drug ingredients in both traditional and modern medicine. Among the methods for their production, biotechnological approaches are gaining importance, and biocatalysis has emerged as an essential tool in this context. A number of chemo-enzymatic strategies for alkaloid synthesis have been developed over the years, in which the biotransformations nowadays take an increasingly 'central' role. This review summarises different applications of biocatalysis in the asymmetric synthesis of alkaloids and discusses how recent developments and novel enzymes render innovative and efficient chemo-enzymatic production routes possible.

Controlling stereoselectivity by enzymatic and chemical means to access enantiomerically pure (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinoline derivatives

A chemoenzymatic strategy for the synthesis of enantiomerically pure novel alkaloids (1S,3R)-1-benzyl-2,3-dimethyl-1,2,3,4-tetrahydroisoquinolines is presented. The key steps are the biocatalytic stereoselective reductive amination of substituted 1-phenylpropan-2-one derivatives to yield chiral amines employing microbial ω-transaminases, and the diastereoselective reduction of a Bischler–Napieralski imine intermediate by catalytic hydrogenation in the presence of palladium on charcoal, leading exclusively to the desired cis-isomer.

Asymmetric Preparation of prim-, sec-, and tert-Amines Employing Selected Biocatalysts

This account focuses on the application of ω-transaminases, lyases, and oxidases for the preparation of amines considering mainly work from our own lab. Examples are given to access α-chiral primary amines from the corresponding ketones as well as terminal amines from primary alcohols via a two-step biocascade. 2,6-Disubstituted piperidines, as examples for secondary amines, are prepared by biocatalytical regioselective asymmetric monoamination of designated diketones followed by spontaneous ring closure and a subsequent diastereoselective reduction step. Optically pure tert-amines such as berbines and N-methyl benzylisoquinolines are obtained by kinetic resolution via an enantioselective aerobic oxidative C-C bond formation.

Flavoprotein oxidases: classification and applications

This review provides an overview of oxidases that utilise a flavin cofactor for catalysis. This class of oxidative flavoenzymes has shown to harbour a large number of biotechnologically interesting enzymes. Applications range from their use as biocatalysts for the synthesis of pharmaceutical compounds to the integration in biosensors. Through the recent developments in genome sequencing, the number of newly discovered oxidases is steadily growing. Recent progress in the field of flavoprotein oxidase discovery and the obtained biochemical knowledge on these enzymes are reviewed. Except for a structure-based classification of known flavoprotein oxidases, also their potential in recent biotechnological applications is discussed.