High-yield expression and purification of isotopically labeled norcoclaurine synthase, a Bet v 1-homologous enzyme, from Thalictrum flavum for NMR studies

Modular assembly of an artificially concise biocatalytic cascade for the manufacture of phenethylisoquinoline alkaloids

Plant-derived alkaloids are an important class of pharmaceuticals. However, they still rely on phytoextraction to meet their diverse market demands. Since multistep biocatalytic cascades have begun to revolutionize the manufacture of natural or unnatural products, to address the synthetic challenges of alkaloids, herein we establish an artificially concise four-enzyme biocatalytic cascade with avoiding plant-derived P450 modification for synthesizing phenethylisoquinoline alkaloids (PEIAs) after enzyme discovery and enzyme engineering. Efficient biosynthesis of diverse natural and unnatural PEIAs is realized from readily available substrates. Most importantly, the scale-up preparation of the colchicine precursor (S)-autumnaline with a high titer is achieved after replacing the rate-limiting O-methylation by the plug-and-play strategy. This study not only streamlines future engineering endeavors for colchicine biosynthesis, but also provides a paradigm for constructing more artificial biocatalytic cascades for the manufacture of diverse alkaloids through synthetic biology.

Expression and purification of 15N-labeled Fra a 1, a strawberry allergen, to prepare samples for NMR measurements

Raw strawberries contain allergens that cause oral allergic syndrome. Fra a 1 is one of the major allergens in strawberries and might decrease their allergenicity by heating, likely due to structural changes in the allergen leading to decreased recognition of the allergens in the oral cavity. In the present study, to understand the relationship between allergen structure and allergenicity, the expression and purification of 15N-labeled Fra a 1 were examined and the sample was used for NMR analysis. Two isoforms, Fra a 1.01 and Fra a 1.02, were used and expressed in E. coli BL21(DE3) in M9 minimal medium. Fra a 1.02 was purified as a single protein by using the GST tag approach, whereas histidine×6-tag (his6-tag) Fra a 1.02 was obtained both as the full-length (∼20 kDa) and a truncated (∼18 kDa) form. On the other hand, his6-tag Fra a 1.01 was purified as a homogeneous protein. ¹⁵N-labeled HSQC NMR spectra suggested that Fra a 1.02 was thermally denatured at lower temperatures than Fra a 1.01, despite the high amino acid sequence homology (79.4%) of these isoforms. Furthermore, the samples in the present study allowed us to analyze ligand binding that probably affects structural stability. In conclusion, GST tag was effective for obtaining a homogeneous protein when his6-tag failed to give a single form, and the present study provided a sample that could be used for NMR studies of the details of the allergenicity and structure of Fra a 1.

Allergenicity and IgE Recognition of New Dau c 1 Allergens from Carrot

SCOPE

Carrot (Daucus carota) allergy is caused by the major carrot allergen Dau c 1, which is a mixture of several isoallergens and variants with sequence identities of >67% or >90%, respectively. However, little is known about the qualitative and quantitative composition of natural Dau c 1.

METHODS AND RESULTS

Mass spectrometry of isolated natural Dau c 1 reveals the existence of several yet unknown Dau c 1-like proteins. The study expresses four Dau c 1-like proteins in Escherichia coli. Two of the purified proteins, designated Dau c 1.0501 and 1.0601, exhibit sequence identities to Dau c 1.0101 and 1.0401 between 54% and 87%. They possess allergenic potential and are accepted as new isoallergens. One protein, designated as Dau c 1-like is >50% identical with the new isoallergens but exhibits no allergenicity. Sequence and structural comparisons of this protein with the known Dau c 1 isoallergens offer relevant clues about putative structural IgE epitopes.

CONCLUSION

Identification of new isoallergens and the identification of IgE epitopes may contribute to a more refined component resolved diagnosis and may lay ground for further epitope mapping and personalized targeted treatment approaches of carrot allergy in preclinical and clinical studies.

Enzyme catalysed Pictet-Spengler formation of chiral 1,1'-disubstituted- and spiro-tetrahydroisoquinolines

The Pictet–Spengler reaction (PSR) involves the condensation and ring closure between a β-arylethylamine and a carbonyl compound. The combination of dopamine and ketones in a PSR leads to the formation of 1,1′-disubstituted tetrahydroisoquinolines (THIQs), structures that are challenging to synthesize and yet are present in a number of bioactive natural products and synthetic pharmaceuticals. Here we have discovered that norcoclaurine synthase from Thalictrum flavum (TfNCS) can catalyse the PSR between dopamine and unactivated ketones, thus facilitating the facile biocatalytic generation of 1,1′-disubstituted THIQs. Variants of TfNCS showing improved conversions have been identified and used to synthesize novel chiral 1,1′-disubstituted and spiro-THIQs. Enzyme catalysed PSRs with unactivated ketones are unprecedented, and, furthermore, there are no equivalent stereoselective chemical methods for these transformations. This discovery advances the utility of enzymes for the generation of diverse THIQs in vitro and in vivo.

The Pictet-Spengler condensation of β-arylethylamine and carbonyl compounds is an important step in the synthesis of bioactive alkaloids. Here, the authors report a Pictet-Spengler reaction between dopamine and unactivated ketones catalysed by norcoclaurine synthase and its engineered variants.

The conformational IgE epitope profile of soya bean allergen Gly m 4

BACKGROUND

Birch pollen-related soya allergy is mediated by Gly m 4. Conformational IgE epitopes of Gly m 4 are unknown.

OBJECTIVE

To identify the IgE epitope profile of Gly m 4 in subjects with birch pollen-related soya allergy utilizing an epitope library presented by Gly m 4-type model proteins.

METHODS

Sera from patients with (n = 26) and without (n = 19) allergy to soya as determined by oral provocation tests were studied. Specific IgE (Bet v 1/Gly m 4) was determined by ImmunoCAP. A library of 59 non-allergenic Gly m 4-type model proteins harbouring individual and multiple putative epitopes for IgE was tested in IgE binding assays. Primary, secondary and tertiary protein structures were assessed by mass spectrometry, circular dichroism and nuclear magnetic resonance spectroscopy.

RESULTS

All subjects were sensitized to Gly m 4 and Bet v 1. Allergen-specific serum IgE levels ranged from 0.94 to > 100 kU_A /L. The avidities of serum IgE were 5.06 ng (allergic) and 1.8 ng (tolerant) as determined by EC₅₀ for IgE binding to Gly m 4. 96% (46/48) of the protein variants bound IgE. Model proteins had Gly m 4-type conformation and individual IgE binding clustered in six major surface areas. Gly m 4-specific IgE binding could be inhibited to up to 80% by model proteins harbouring individual IgE binding sites in an epitope-wise equimolar fashion. Receiver operating curve analysis revealed an area under fitted curve of up to 0.88 for model proteins and 0.66 for Gly m 4.

CONCLUSION AND CLINICAL RELEVANCE

Serum levels and avidity of Gly m 4-specific IgE do not correlate with clinical reactivity to soya. Six IgE-binding areas, represented by 23 amino acids, account for more than 80% of total IgE binding capacity of Gly m 4. Model proteins may be used for epitope-resolved diagnosis to differentiate birch-soya allergy from clinical tolerance.

'Dopamine-first' mechanism enables the rational engineering of the norcoclaurine synthase aldehyde activity profile

Norcoclaurine synthase (NCS) (EC 4.2.1.78) catalyzes the Pictet–Spengler condensation of dopamine and an aldehyde, forming a substituted (S)-tetrahydroisoquinoline, a pharmaceutically important moiety. This unique activity has led to NCS being used for both in vitro biocatalysis and in vivo recombinant metabolism. Future engineering of NCS activity to enable the synthesis of diverse tetrahydroisoquinolines is dependent on an understanding of the NCS mechanism and kinetics. We assess two proposed mechanisms for NCS activity: (a) one based on the holo X-ray crystal structure and (b) the ‘dopamine-first’ mechanism based on computational docking. Thalictrum flavum NCS variant activities support the dopamine-first mechanism. Suppression of the non-enzymatic background reaction reveals novel kinetic parameters for NCS, showing it to act with low catalytic efficiency. This kinetic behaviour can account for the ineffectiveness of recombinant NCS in in vivo systems, and also suggests NCS may have an in planta role as a metabolic gatekeeper. The amino acid substitution L76A, situated in the proposed aldehyde binding site, results in the alteration of the enzyme's aldehyde activity profile. This both verifies the dopamine-first mechanism and demonstrates the potential for the rational engineering of NCS activity.

Enlarging the toolbox for allergen epitope definition with an allergen-type model protein

BACKGROUND

Birch pollen-allergic subjects produce polyclonal cross-reactive IgE antibodies that mediate pollen-associated food allergies. The major allergen Bet v 1 and its homologs in plant foods bind IgE in their native protein conformation. Information on location, number and clinical relevance of IgE epitopes is limited. We addressed the use of an allergen-related protein model to identify amino acids critical for IgE binding of PR-10 allergens.

METHOD

Norcoclaurine synthase (NCS) from meadow rue is structurally homologous to Bet v 1 but does not bind Bet v 1-reactive IgE. NCS was used as the template for epitope grafting. NCS variants were tested with sera from 70 birch pollen allergic subjects and with monoclonal antibody BV16 reported to compete with IgE binding to Bet v 1.

RESULTS

We generated an NCS variant (Δ29NCSN57/I58E/D60N/V63P/D68K) harboring an IgE epitope of Bet v 1. Bet v 1-type protein folding of the NCS variant was evaluated by 1H-15N-HSQC NMR spectroscopy. BV16 bound the NCS variant and 71% (50/70 sera) of our study population showed significant IgE binding. We observed IgE and BV16 cross-reactivity to the epitope presented by the NCS variant in a subgroup of Bet v 1-related allergens. Moreover BV16 blocked IgE binding to the NCS variant. Antibody cross-reactivity depended on a defined orientation of amino acids within the Bet v 1-type conformation.

CONCLUSION

Our system allows the evaluation of patient-specific epitope profiles and will facilitate both the identification of clinically relevant epitopes as biomarkers and the monitoring of therapeutic outcomes to improve diagnosis, prognosis, and therapy of allergies caused by PR-10 proteins.

Asymmetric synthesis of tetrahydroisoquinolines by enzymatic Pictet–Spengler reaction

Norcoclaurine synthase (NCS) catalyzes the stereoselective Pictet–Spengler reaction between dopamine and 4-hydroxyphenylacetaldehyde as the first step of benzylisoquinoline alkaloid synthesis in plants. Recent studies suggested that NCS shows relatively relaxed substrate specificity toward aldehydes, and thus, the enzyme can serve as a tool to synthesize unnatural, optically active tetrahydroisoquinolines. In this study, using an N-terminally truncated NCS from Coptis japonica expressed in Escherichia coli, we examined the aldehyde substrate specificity of the enzyme. Herein, we demonstrate the versatility of the enzyme by synthesizing 6,7-dihydroxy-1-phenethyl-1,2,3,4-tetrahydroisoquinoline and 6,7-dihydroxy-1-propyl-1,2,3,4-tetrahydroisoquinoline in molar yields of 86.0 and 99.6% and in enantiomer excess of 95.3 and 98.0%, respectively. The results revealed the enzyme is a promising catalyst that functions to stereoselectively produce various 1-substituted-1,2,3,4-tetrahydroisoquinolines.

Fermentative production of plant benzylisoquinoline alkaloids in microbes

Higher plants produce diverse chemicals, including alkaloids, terpenoids, and phenolic compounds (phenylpropanoids and flavonoids) as secondary metabolites. These chemicals are widely used for human health and nutrition. Alkaloids, for example, are valued in medicine due to their high biological activities, but most of these metabolites accumulate at low levels in plant cells, resulting in poor extraction yields. Increasingly, attention is devoted to the production of plant metabolites by reconstructing plant biosynthetic pathways in microorganisms. This technology has been aided by advances in synthetic biology and metabolic engineering. Here, the review a fermentation platform for low-cost production of numerous alkaloids using bioengineered Escherichia coli and/or Saccharomyces cerevisiae.

The Pictet-Spengler reaction in nature and in organic chemistry

Alkaloids are an important class of natural products that are widely distributed in nature and produced by a large variety of organisms. They have a wide spectrum of biological activity and for many years were used in folk medicine. These days, alkaloids also have numerous applications in medicine as therapeutic agents. The importance of these natural products in inspiring drug discovery programs is proven and, therefore, their continued synthesis is of significant interest. The condensation discovered by Pictet and Spengler is the most important method for the synthesis of alkaloid scaffolds. The power of this synthesis method has been convincingly proven in the construction of stereochemicaly and structurally complex alkaloids.

Conformation, catalytic site, and enzymatic mechanism of the PR10 allergen-related enzyme norcoclaurine synthase

The enzyme NCS [(S)-norcoclaurine synthase; EC 4.2.1.78] found in the common meadow rue, Thalictrum flavum, and other plant species, is involved in the biosynthesis of BIAs (benzylisoquinoline alkaloids). This group of plant secondary metabolites comprises pharmacologically-active compounds such as morphine and codeine. NCS catalyses the condensation of 4-HPAA (4-hydroxyphenylacetaldehyde) and dopamine to (S)-norcoclaurine, the common precursor of all plant BIAs. Although enzymatic properties of NCS and mechanistic aspects of the reaction have been studied in detail, no structural information on NCS was available so far. The enzyme shows significant sequence homology to members of the PR10 proteins (class 10 of pathogenesis-related proteins) such as the major birch pollen allergen Bet v 1. Our CD and NMR spectroscopic data indicated high similarity of the NCS and the Bet v 1 fold and allowed us to model NCS using Bet v 1 as a template. Virtually complete backbone assignment of the NCS sequence was used to study substrate binding by NMR titration experiments. Although binding of 4-HPAA seems to induce side-chain rearrangements in an extensive part of the protein, the putative distinct interaction site for dopamine could be clearly identified. The oligomerization state of NCS that reportedly plays an important role in enzyme functionality was determined to be concentration-dependent by SEC (size-exclusion chromatography) as well as NMR relaxation measurements, and the enzyme was found to be predominantly a monomer at the low micromolar concentrations used for activity assays.

Expression and purification of cysteine mutation isoforms of rat lipocalin-type prostaglandin D synthase for nuclear magnetic resonance study

Lipocalin-type prostaglandin (PG) D synthase (L-PGDS) is the only member of the lipocalin superfamily that displays enzymatic activity. It binds lipophilic ligands with high affinity and also can catalyze PGH2 to produce PGD2. Three cysteine residues, Cys65, Cys89, and Cys186 in L-PGDS, are conserved among all species, of which Cys89 and Cys186 residues form a disulfide bridge. In this study, we clarified the effects of thiol groups on the structure of the protein and investigated the structural significance of Cys residues of rat L-PGDS by site-directed mutagenesis. Four mutants were constructed by substituting Cys residues with alanine to identify the correct formation of disulfide bonds among these three residues. The effects of thiol groups on the structure of rat L-PGDS were also identified by these mutants. Analysis of HSQC experiments indicated that these enzymes were all properly folded with well defined tertiary structures. As the first step towards the 3-D nuclear magnetic resonance solution structure, we optimized expression of recombinant rat L-PGDS in Escherichia coli and established an efficient and economic purification protocol yielding large amounts of pure isotopically labeled rat L-PGDS. The results of assignments indicated that the wild-type rat L-PGDS obtained using this expression system was suitable for determination of 3-D nuclear magnetic resonance solution structure.

Microbial production of plant benzylisoquinoline alkaloids

Benzylisoquinoline alkaloids, such as the analgesic compounds morphine and codeine, and the antibacterial agents berberine, palmatine, and magnoflorine, are synthesized from tyrosine in the Papaveraceae, Berberidaceae, Ranunculaceae, Magnoliaceae, and many other plant families. It is difficult to produce alkaloids on a large scale under the strict control of secondary metabolism in plants, and they are too complex for cost-effective chemical synthesis. By using a system that combines microbial and plant enzymes to produce desired benzylisoquinoline alkaloids, we synthesized (S)-reticuline, the key intermediate in benzylisoquinoline alkaloid biosynthesis, from dopamine by crude enzymes from transgenic Escherichia coli. The final yield of (S)-reticuline was 55 mg/liter within 1 h. Furthermore, we synthesized an aporphine alkaloid, magnoflorine, or a protoberberine alkaloid, scoulerine, from dopamine via reticuline by using different combination cultures of transgenic E. coli and Saccharomyces cerevisiae cells. The final yields of magnoflorine and scoulerine were 7.2 and 8.3 mg/liter culture medium. These results indicate that microbial systems that incorporate plant genes cannot only enable the mass production of scarce benzylisoquinoline alkaloids but may also open up pathways for the production of novel benzylisoquinoline alkaloids.

Expression, refolding, and purification of a truncated human Delta-like1, a ligand of Notch receptors

The Notch signaling pathway plays a pivotal role in proliferation, apoptosis, and cell fate specification in both embryonic and postnatal development, and is a potential therapeutic target for human diseases such as cancer. To express in Escherichia coli and purify soluble fragment of human Delta-like1 (hDll1), we cloned two extracellular fragments of hDll1 [hDll1 (127-225) and hDll1 (26-225)]. The hDll1 (127-225) fragment was successfully expressed in E. coli as a GST fusion protein (GST-hDll1). The GST-hDll1 protein, which was expressed as inclusion bodies after induction by IPTG, was refolded and purified simultaneously using affinity chromatography and size exclusion chromatography. The purified GST-hDll1 was of more than 95% purity, and had a molecular weight of 39kDa. Reporter assay showed that GST-hDll1 could activate a reporter gene that is dependent on Notch activation. Therefore, using the E. coli expression system and different chromatography systems, we successfully expressed, refolded, and purified a biologically active GST-hDll1, which might be potentially useful for therapy and studying the Notch pathway.