Purification and characterization of deacetylipecoside synthase from Alangium lamarckii Thw

The scaffold-forming steps of plant alkaloid biosynthesis

Alkaloids from plants are characterised by structural diversity and bioactivity, and maintain a privileged position in both modern and traditional medicines. In recent years, there have been significant advances in elucidating the biosynthetic origins of plant alkaloids. In this review, I will describe the progress made in determining the metabolic origins of the so-called true alkaloids, specialised metabolites derived from amino acids containing a nitrogen heterocycle. By identifying key biosynthetic steps that feature in the majority of pathways, I highlight the key roles played by modifications to primary metabolism, iminium reactivity and spontaneous reactions in the molecular and evolutionary origins of these pathways.

Pictet-Spenglerases in alkaloid biosynthesis: Future applications in biocatalysis

Pictet-Spenglerases provide a key role in the biosynthesis of many biologically active alkaloids. There is increasing use of these biocatalysts as an alternative to traditional organic synthetic methods as they provide stereoselective and regioselective control under mild conditions. Products from these enzymes also contain privileged drug scaffolds (such as tetrahydroisoquinoline or β-carboline moieties), so there is interest in the characterization and use of these enzymes as versatile biocatalysts to synthesize analogs of the corresponding natural products for drug discovery. This review discusses all known Pictet-Spenglerase enzymes and their applications as biocatalysts.

Biosynthesis of plant tetrahydroisoquinoline alkaloids through an imine reductase route

Herein, we report a biocatalytic approach to synthesize plant tetrahydroisoquinoline alkaloids (THIQAs) from dihydroisoquinoline (DHIQ) precursors using imine reductases and N-methyltransferase (NMT). The imine reductase IR45 was engineered to significantly expand its substrate specificity, enabling efficient and stereoselective conversion of 1-phenyl and 1-benzyl 6,7-dimethoxy-DHIQs into the corresponding (S)-tetrahydroisoquinolines (S-THIQs). Coclaurine N-methyltransferase (CNMT) was able to further efficiently convert these (S)-THIQ intermediates into (S)-THIQAs. By assembling IRED, CNMT, and glucose dehydrogenase (GDH) in one reaction, we effectively constituted two artificial biosynthetic pathways in Escherichia coli and successfully applied them to the production of five (S)-THIQAs. This highly efficient (100% yield from DHIQs) and easily tailorable (adding other genes) biosynthetic approach will be useful for producing a variety of plant THIQAs.

Inverted Binding of Non-natural Substrates in Strictosidine Synthase Leads to a Switch of Stereochemical Outcome in Enzyme-Catalyzed Pictet-Spengler Reactions

The Pictet–Spengler reaction is a valuable route to 1,2,3,4-tetrahydro-β-carboline (THBC) and isoquinoline scaffolds found in many important pharmaceuticals. Strictosidine synthase (STR) catalyzes the Pictet–Spengler condensation of tryptamine and the aldehyde secologanin to give (S)-strictosidine as a key intermediate in indole alkaloid biosynthesis. STRs also accept short-chain aliphatic aldehydes to give enantioenriched alkaloid products with up to 99% ee STRs are thus valuable asymmetric organocatalysts for applications in organic synthesis. The STR catalysis of reactions of small aldehydes gives an unexpected switch in stereopreference, leading to formation of the (R)-products. Here we report a rationale for the formation of the (R)-configured products by the STR enzyme from Ophiorrhiza pumila (OpSTR) using a combination of X-ray crystallography, mutational, and molecular dynamics (MD) studies. We discovered that short-chain aldehydes bind in an inverted fashion compared to secologanin leading to the inverted stereopreference for the observed (R)-product in those cases. The study demonstrates that the same catalyst can have two different productive binding modes for one substrate but give different absolute configuration of the products by binding the aldehyde substrate differently. These results will guide future engineering of STRs and related enzymes for biocatalytic applications.

Characterization of a new microbial Pictet-Spenglerase NscbB affording the β-carboline skeletons from Nocardiopsis synnemataformans DSM 44143

The enzymatic Pictet-Spengler (PS) reaction, catalyzed by Pictet-Spenglerase (PSase), is the feature of β-carboline (βC) alkaloid biosynthesis. NscbB is a rare microbial PSase discovered from a cryptic β-carboline alkaloid biosynthetic gene cluster (BGC) in the Nocardiopsis synnemataformans DSM 44143 (kidney transplant patient derived) by homologous alignment with its well-characterized counterpart McbB. The biochemical analysis showed that NscbB could catalyze l-tryptophan (K_M = 89.64 ± 8.69 μM) and methylglyoxal (K_M = 147.70 ± 16.38 μM), without cofactors, to form the two βC skeletons 1-acetyl-3-carboxy-β-carboline and 1-acetyl-β-carboline in vitro. Additionally, the heterologous expression of nscbB in E. coli BL21 (DE3) revealed the efficient bioproductivity of NscbB to bioproduce two βC skeletons, 1-acetyl-3-carboxy-β-carboline (5.5 mg/L) and 1-acetyl-β-carboline (3.1 mg/L), within 16 h fermentation. These results demonstrate NscbB is a novel and practical microbial PSase, which is useful for future bioactive βC alkaloid exploitation and development.

Isolation and Sequencing of Salsolinol Synthase, an Enzyme Catalyzing Salsolinol Biosynthesis

Salsolinol (1-methyl-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline), a derivate of dopamine, is suspected to be the most probable neurotoxin in the degeneration of dopaminergic neurons. Numerous hypotheses regarding its pathophysiological roles have been raised, especially related to Parkinson's disease and alcohol addiction. In the mammalian brain, salsolinol may be enzymatically synthesized by salsolinol synthase from dopamine and acetaldehyde. However, the direct evidence of its biosynthesis was still missing. In this study, we purified salsolinol synthase from rat brain by a systematical procedure involving acid precipitation, ultrafiltration, and hydrophilic interaction chromatography. The molecular weight of salsolinol synthase determined by MALDI-TOF MS is 8622.29 Da, comprising 77 amino acids (MQIFVKTLTG KTITLEVEPS DTIKNVKAKI QDKEGIPPDQ QRLIFAGKQL EDGRTLSDYN IQKKSTLHLV LRLRVDY). Homology analysis showed that the enzyme is a ubiquitin-like protein, with a difference of four amino acids, which suggests it is a novel protein. After it was overexpressed in eukaryotic cells, the production of salsolinol was significantly increased as compared with control, confirming the catalytic function of this enzyme. To our knowledge, it is the first systematic purification and sequencing of salsolinol synthase. Together, this work reveals a formerly anonymous protein and urges further exploration of its possible prognostic value and implications in Parkinson's disease and other related disorders.

[Diversity of Secondary Metabolites from Some Medicinal Plants and Cultivated Lichen Mycobionts]

 Studies on the structural determination, biosynthesis, and biological activities of secondary metabolites from natural sources are significant in the field of natural products chemistry. This review focuses on diverse secondary metabolites isolated from medicinal plants and cultivated mycobionts of lichens in our laboratory. Monoterpene-tetrahydroisoquinoline glycosides and alkaloids isolated from Cephaelis acuminata and Alangium lamarckii gave important information on the biosynthesis of ipecac alkaloids. A variety of glycosides linked with a secologanin unit and indole alkaloids were obtained from medicinal plants belonging to the families of Rubiaceae, Apocynaceae, and Loganiaceae. Plant species of the four genera Fraxinus, Syringa, Jasminum, and Ligustrum of the family Oleaceae were chemically investigated to provide several types of secoiridoid and iridoid glucosides. The biosynthetic pathway leading from protopine to benzophenanthridine alkaloids in suspension cell cultures of Eschscholtzia californica was elucidated. The structures and biological activities of the bisbenzylisoquinoline alkaloids of Stephania cepharantha and Nelumbo nucifera were also investigated. In addition, the mycobionts of lichens were cultivated to afford various types of metabolites that differ from the lichen substances of intact lichens but are structurally similar to fungal metabolites. The biosynthetic origins of some metabolites were also studied. These findings suggest that cultures of lichen mycobionts could be sources of new bioactive compounds and good systems for investigating secondary metabolism in lichens.

The Enzymology of Organic Transformations: A Survey of Name Reactions in Biological Systems

Chemical reactions that are named in honor of their true, or at least perceived, discoverers are known as "name reactions". This Review is a collection of biological representatives of named chemical reactions. Emphasis is placed on reaction types and catalytic mechanisms that showcase both the chemical diversity in natural product biosynthesis as well as the parallels with synthetic organic chemistry. An attempt has been made, whenever possible, to describe the enzymatic mechanisms of catalysis within the context of their synthetic counterparts and to discuss the mechanistic hypotheses for those reactions that are currently active areas of investigation. This Review has been categorized by reaction type, for example condensation, nucleophilic addition, reduction and oxidation, substitution, carboxylation, radical-mediated, and rearrangements, which are subdivided by name reactions.

Function and application of a non-ester-hydrolyzing carboxylesterase discovered in tulip

Plants have evolved secondary metabolite biosynthetic pathways of immense rich diversity. The genes encoding enzymes for secondary metabolite biosynthesis have evolved through gene duplication followed by neofunctionalization, thereby generating functional diversity. Emerging evidence demonstrates that some of those enzymes catalyze reactions entirely different from those usually catalyzed by other members of the same family; e.g. transacylation catalyzed by an enzyme similar to a hydrolytic enzyme. Tuliposide-converting enzyme (TCE), which we recently discovered from tulip, catalyzes the conversion of major defensive secondary metabolites, tuliposides, to antimicrobial tulipalins. The TCEs belong to the carboxylesterase family in the α/β-hydrolase fold superfamily, and specifically catalyze intramolecular transesterification, but not hydrolysis. This non-ester-hydrolyzing carboxylesterase is an example of an enzyme showing catalytic properties that are unpredictable from its primary structure. This review describes the biochemical and physiological aspects of tulipalin biogenesis, and the diverse functions of plant carboxylesterases in the α/β-hydrolase fold superfamily.

Benzylisoquinoline alkaloid metabolism: a century of discovery and a brave new world

Benzylisoquinoline alkaloids (BIAs) are a structurally diverse group of plant specialized metabolites with a long history of investigation. Although the ecophysiological functions of most BIAs are unknown, the medicinal properties of many compounds have been exploited for centuries. These include the narcotic analgesics codeine and morphine, the antimicrobial agents sanguinarine and berberine, and the antitussive and anticancer drug noscapine. BIA biosynthesis involves a restricted number of enzyme types that catalyze landmark coupling reactions and subsequent functional group modifications. A pathogenesis-related (PR)10/Bet v1 'Pictet-Spenglerase', several O-methyl-, N-methyl- and O-acetyltransferases, cytochromes P450, FAD-dependent oxidases, non-heme dioxygenases and NADPH-dependent reductases have been implicated in the multistep pathways leading to structurally diverse alkaloids. A small number of plant species, including opium poppy (Papaver somniferum) and other members of the Ranunculales, have emerged as model systems to study BIA metabolism. The expansion of resources to include a wider range of plant species is creating an opportunity to investigate previously uncharacterized BIA pathways. Contemporary knowledge of BIA metabolism reflects over a century of research coupled with the development of key innovations such as radioactive tracing, enzyme isolation and molecular cloning, and functional genomics approaches such as virus-induced gene silencing. Recently, the emergence of transcriptomics, proteomics and metabolomics has expedited the discovery of new BIA biosynthetic genes. The growing repository of BIA biosynthetic genes is providing the parts required to apply emerging synthetic biology platforms to the development of production systems in microbes as an alternative to plants as a commecial source of valuable BIAs.

Biocatalytic production of tetrahydroisoquinolines

The promiscuity of the enzyme norcoclaurine synthase is described. This biocatalyst yielded a diverse array of substituted tetrahydroisoquinolines by cyclizing dopamine with various acetaldehydes in a Pictet-Spengler reaction. This enzymatic reaction may provide a biocatalytic route to a range of tetrahydroisoquinoline alkaloids.

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.

Three new O-methyltransferases are sufficient for all O-methylation reactions of ipecac alkaloid biosynthesis in root culture of Psychotria ipecacuanha

The medicinal plant Psychotria ipecacuanha produces ipecac alkaloids, a series of monoterpenoid-isoquinoline alkaloids such as emetine and cephaeline, whose biosynthesis derives from condensation of dopamine and secologanin. Here, we identified three cDNAs, IpeOMT1-IpeOMT3, encoding ipecac alkaloid O-methyltransferases (OMTs) from P. ipecacuanha. They were coordinately transcribed with the recently identified ipecac alkaloid beta-glucosidase Ipeglu1. Their amino acid sequences were closely related to each other and rather to the flavonoid OMTs than to the OMTs involved in benzylisoquinoline alkaloid biosynthesis. Characterization of the recombinant IpeOMT enzymes with integration of the enzymatic properties of the IpeGlu1 revealed that emetine biosynthesis branches off from N-deacetylisoipecoside through its 6-O-methylation by IpeOMT1, with a minor contribution by IpeOMT2, followed by deglucosylation by IpeGlu1. The 7-hydroxy group of the isoquinoline skeleton of the aglycon is methylated by IpeOMT3 prior to the formation of protoemetine that is condensed with a second dopamine molecule, followed by sequential O-methylations by IpeOMT2 and IpeOMT1 to form cephaeline and emetine, respectively. In addition to this central pathway of ipecac alkaloid biosynthesis, formation of all methyl derivatives of ipecac alkaloids in P. ipecacuanha could be explained by the enzymatic activities of IpeOMT1-IpeOMT3, indicating that they are sufficient for all O-methylation reactions of ipecac alkaloid biosynthesis.

The new beta-D-glucosidase in terpenoid-isoquinoline alkaloid biosynthesis in Psychotria ipecacuanha

Ipecac alkaloids produced in the medicinal plant Psychotria ipecacuanha such as emetine and cephaeline possess a monoterpenoid-tetrahydroisoquinoline skeleton, which is formed by condensation of dopamine and secologanin. Deglucosylation of one of the condensed products N-deacetylisoipecoside (1 alpha(S)-epimer) is considered to be a part of the reactions for emetine biosynthesis, whereas its 1 beta(R)-epimer N-deacetylipecoside is converted to ipecoside in P. ipecacuanha. Here, we isolated a cDNA clone Ipeglu1 encoding Ipecac alkaloid beta-D-glucosidase from P. ipecacuanha. The deduced protein showed 54 and 48% identities to raucaffricine beta-glucosidase and strictosidine beta-glucosidase, respectively. Recombinant IpeGlu1 enzyme preferentially hydrolyzed glucosidic Ipecac alkaloids except for their lactams, but showed poor or no activity toward other substrates, including terpenoid-indole alkaloid glucosides. Liquid chromatography-tandem mass spectrometry analysis of deglucosylated products of N-deacetylisoipecoside revealed spontaneous transitions of the highly reactive aglycons, one of which was supposed to be the intermediate for emetine biosynthesis. IpeGlu1 activity was extremely poor toward 7-O-methyl and 6,7-O,O-dimethyl derivatives. However, 6-O-methyl derivatives were hydrolyzed as efficiently as non-methylated substrates, suggesting the possibility of 6-O-methylation prior to deglucosylation by IpeGlu1. In contrast to the strictosidine beta-glucosidase that stereospecifically hydrolyzes 3 alpha(S)-epimer in terpenoid-indole alkaloid biosynthesis, IpeGlu1 lacked stereospecificity for its substrates where 1 beta(R)-epimers were preferred to 1 alpha(S)-epimers, although ipecoside (1 beta(R)) is a major alkaloidal glucoside in P. ipecacuanha, suggesting the compartmentalization of IpeGlu1 from ipecoside. These facts have significant implications for distinct physiological roles of 1 alpha(S)- and 1 beta(R)-epimers and for the involvement of IpeGlu1 in the metabolic fate of both of them.

Purification, cloning, functional expression and characterization of perakine reductase: the first example from the AKR enzyme family, extending the alkaloidal network of the plant Rauvolfia

Perakine reductase (PR) catalyzes an NADPH-dependent step in a side-branch of the 10-step biosynthetic pathway of the alkaloid ajmaline. The enzyme was cloned by a "reverse-genetic" approach from cell suspension cultures of the plant Rauvolfia serpentina (Apocynaceae) and functionally expressed in Escherichia coli as the N-terminal His(6)-tagged protein. PR displays a broad substrate acceptance, converting 16 out of 28 tested compounds with reducible carbonyl function which belong to three substrate groups: benzaldehyde, cinnamic aldehyde derivatives and monoterpenoid indole alkaloids. The enzyme has an extraordinary selectivity in the group of alkaloids. Sequence alignments define PR as a new member of the aldo-keto reductase (AKR) super family, exhibiting the conserved catalytic tetrad Asp52, Tyr57, Lys84, His126. Site-directed mutagenesis of each of these functional residues to an alanine residue results in >97.8% loss of enzyme activity, in compounds of each substrate group. PR represents the first example of the large AKR-family which is involved in the biosynthesis of plant monoterpenoid indole alkaloids. In addition to a new esterase, PR significantly extends the Rauvolfia alkaloid network to the novel group of peraksine alkaloids.

3D-Structure and function of strictosidine synthase--the key enzyme of monoterpenoid indole alkaloid biosynthesis

Strictosidine synthase (STR; EC 4.3.3.2) plays a key role in the biosynthesis of monoterpenoid indole alkaloids by catalyzing the Pictet-Spengler reaction between tryptamine and secologanin, leading exclusively to 3alpha-(S)-strictosidine. The structure of the native enzyme from the Indian medicinal plant Rauvolfia serpentina represents the first example of a six-bladed four-stranded beta-propeller fold from the plant kingdom. Moreover, the architecture of the enzyme-substrate and enzyme-product complexes reveals deep insight into the active centre and mechanism of the synthase highlighting the importance of Glu309 as the catalytic residue. The present review describes the 3D-structure and function of R. serpentina strictosidine synthase and provides a summary of the strictosidine synthase substrate specificity studies carried out in different organisms to date. Based on the enzyme-product complex, this paper goes on to describe a rational, structure-based redesign of the enzyme, which offers the opportunity to produce novel strictosidine derivatives which can be used to generate alkaloid libraries of the N-analogues heteroyohimbine type. Finally, alignment studies of functionally expressed strictosidine synthases are presented and the evolutionary aspects of sequence- and structure-related beta-propeller folds are discussed.

Strictosidine synthase: mechanism of a Pictet-Spengler catalyzing enzyme

The Pictet-Spengler reaction, which yields either a beta-carboline or a tetrahydroquinoline product from an aromatic amine and an aldehyde, is widely utilized in plant alkaloid biosynthesis. Here we deconvolute the role that the biosynthetic enzyme strictosidine synthase plays in catalyzing the stereoselective synthesis of a beta-carboline product. Notably, the rate-controlling step of the enzyme mechanism, as identified by the appearance of a primary kinetic isotope effect (KIE), is the rearomatization of a positively charged intermediate. The KIE of a nonenzymatic Pictet-Spengler reaction indicates that rearomatization is also rate-controlling in solution, suggesting that the enzyme does not significantly change the mechanism of the reaction. Additionally, the pH dependence of the solution and enzymatic reactions provides evidence for a sequence of acid-base catalysis steps that catalyze the Pictet-Spengler reaction. An additional acid-catalyzed step, most likely protonation of a carbinolamine intermediate, is also significantly rate controlling. We propose that this step is efficiently catalyzed by the enzyme. Structural analysis of a bisubstrate inhibitor bound to the enzyme suggests that the active site is exquisitely tuned to correctly orient the iminium intermediate for productive cyclization to form the diastereoselective product. Furthermore, ab initio calculations suggest the structures of possible productive transition states involved in the mechanism. Importantly, these calculations suggest that a spiroindolenine intermediate, often invoked in the Pictet-Spengler mechanism, does not occur. A detailed mechanism for enzymatic catalysis of the beta-carboline product is proposed from these data.

Structural biology in plant natural product biosynthesis--architecture of enzymes from monoterpenoid indole and tropane alkaloid biosynthesis

Several cDNAs of enzymes catalyzing biosynthetic pathways of plant-derived alkaloids have recently been heterologously expressed, and the production of appropriate enzymes from ajmaline and tropane alkaloid biosynthesis in bacteria allows their crystallization. This review describes the architecture of these enzymes with and without their ligands.

Molecular architecture of strictosidine glucosidase: the gateway to the biosynthesis of the monoterpenoid indole alkaloid family

Strictosidine beta-D-glucosidase (SG) follows strictosidine synthase (STR1) in the production of the reactive intermediate required for the formation of the large family of monoterpenoid indole alkaloids in plants. This family is composed of approximately 2000 structurally diverse compounds. SG plays an important role in the plant cell by activating the glucoside strictosidine and allowing it to enter the multiple indole alkaloid pathways. Here, we report detailed three-dimensional information describing both native SG and the complex of its inactive mutant Glu207Gln with the substrate strictosidine, thus providing a structural characterization of substrate binding and identifying the amino acids that occupy the active site surface of the enzyme. Structural analysis and site-directed mutagenesis experiments demonstrate the essential role of Glu-207, Glu-416, His-161, and Trp-388 in catalysis. Comparison of the catalytic pocket of SG with that of other plant glucosidases demonstrates the structural importance of Trp-388. Compared with all other glucosidases of plant, bacterial, and archaeal origin, SG's residue Trp-388 is present in a unique structural conformation that is specific to the SG enzyme. In addition to STR1 and vinorine synthase, SG represents the third structural example of enzymes participating in the biosynthetic pathway of the Rauvolfia alkaloid ajmaline. The data presented here will contribute to deciphering the structure and reaction mechanism of other higher plant glucosidases.

The structure of Rauvolfia serpentina strictosidine synthase is a novel six-bladed beta-propeller fold in plant proteins

The enzyme strictosidine synthase (STR1) from the Indian medicinal plant Rauvolfia serpentina is of primary importance for the biosynthetic pathway of the indole alkaloid ajmaline. Moreover, STR1 initiates all biosynthetic pathways leading to the entire monoterpenoid indole alkaloid family representing an enormous structural variety of approximately 2000 compounds in higher plants. The crystal structures of STR1 in complex with its natural substrates tryptamine and secologanin provide structural understanding of the observed substrate preference and identify residues lining the active site surface that contact the substrates. STR1 catalyzes a Pictet-Spengler-type reaction and represents a novel six-bladed beta-propeller fold in plant proteins. Structure-based sequence alignment revealed a common repetitive sequence motif (three hydrophobic residues are followed by a small residue and a hydrophilic residue), indicating a possible evolutionary relationship between STR1 and several sequence-unrelated six-bladed beta-propeller structures. Structural analysis and site-directed mutagenesis experiments demonstrate the essential role of Glu-309 in catalysis. The data will aid in deciphering the details of the reaction mechanism of STR1 as well as other members of this enzyme family.

Purification and characterization of norcoclaurine synthase. The first committed enzyme in benzylisoquinoline alkaloid biosynthesis in plants

Norcoclaurine synthase (NCS; EC ) catalyzes the condensation of dopamine and 4-hydroxyphenylacetaldehyde (4-HPAA) as the first committed step in benzylisoquinoline alkaloid biosynthesis in plants. NCS was purified 1590-fold to homogeneity from cell suspension cultures of meadow rue (Thalictrum flavum ssp. glaucum). The purification procedure, which resulted in a 4.2% yield, involved hydrophobic interaction, anion exchange, hydroxyapatite, and gel filtration chromatography. Purified NCS displayed native and denatured molecular masses of approximately 28 and 15 kDa, respectively, suggesting that the enzyme is composed of two subunits. Two-dimensional polyacrylamide gel electrophoresis revealed two major and two minor isoforms with pI values between 5.5 and 6.2. NCS activity was maximal at pH 6.5 to 7.0 and temperatures between 42 and 55 degrees C and was not affected by divalent cations. The enzyme showed hyperbolic saturation kinetics for 4-HPAA (K(m) = 335 microm) but sigmoidal saturation kinetics for dopamine (Hill coefficient = 1.8) suggesting cooperativity between the dopamine binding sites on each subunit; thus, NCS might play a regulatory, or rate-limiting, role in controlling the rate of pathway flux in benzylisoquinoline alkaloid biosynthesis. Product inhibition kinetics performed at saturating levels of one substrate and with norlaudanosoline as the inhibitor showed that NCS follows an iso-ordered bi-uni mechanism with 4-HPAA binding before dopamine. NCS activity was highest in soluble protein extracts from roots followed by stems, leaves, and flower buds.

Heterologous expression of a Rauvolfia cDNA encoding strictosidine glucosidase, a biosynthetic key to over 2000 monoterpenoid indole alkaloids

Strictosidine glucosidase (SG) is an enzyme that catalyses the second step in the biosynthesis of various classes of monoterpenoid indole alkaloids. Based on the comparison of cDNA sequences of SG from Catharanthus roseus and raucaffricine glucosidase (RG) from Rauvolfia serpentina, primers for RT-PCR were designed and the cDNA encoding SG was cloned from R. serpentina cell suspension cultures. The active enzyme was expressed in Escherichia coli and purified to homogeneity. Analysis of its deduced amino-acid sequence assigned the SG from R. serpentina to family 1 of glycosyl hydrolases. In contrast to the SG from C. roseus, the enzyme from R. serpentina is predicted to lack an uncleavable N-terminal signal sequence, which is believed to direct proteins to the endoplasmic reticulum. The temperature and pH optimum, enzyme kinetic parameters and substrate specificity of the heterologously expressed SG were studied and compared to those of the C. roseus enzyme, revealing some differences between the two glucosidases. In vitro deglucosylation of strictosidine by R. serpentina SG proceeds by the same mechanism as has been shown for the C. roseus enzyme preparation. The reaction gives rise to the end product cathenamine and involves 4,21-dehydrocorynantheine aldehyde as an intermediate. The enzymatic hydrolysis of dolichantoside (Nbeta-methylstrictosidine) leads to several products. One of them was identified as a new compound, 3-isocorreantine A. From the data it can be concluded that the divergence of the biosynthetic pathways leading to different classes of indole alkaloids formed in R. serpentina and C. roseus cell suspension cultures occurs at a later stage than strictosidine deglucosylation.

Why are the terpenoid indole alkaloids of type I homochiral?

In the presence of the enzyme strictosidine synthase, the coupling reaction of secologanin and tryptamine is completely stereoselective and affords strictosidine with 3S configuration, exclusively. The stereoselectivity is transferred and retained in most indole alkaloids of type I in which C-3 is not involved in subsequent reactions. By using results of model reactions, the stereoselectivity was interpreted by the bulkiness of the enzyme temporarily attached to the N-4 atom in the formation of the indolenine intermediate. 3S configuration is kept in the subsequent 1,2-rearrangement into the beta-carboline structure. In the formation of the oxindole derivatives, the 3S configuration is preferred, but not necessarily complete.