Late enzymes of vindoline biosynthesis. Acetyl-CoA: 17-O-deacetylvindoline 17-O-acetyl-transferase

Biosynthesis and Modulation of Terpenoid Indole Alkaloids in Catharanthus roseus: A Review of Targeting Genes and Secondary Metabolites

The medicinal plant C. roseus synthesizes biologically active alkaloids via the terpenoid indole alkaloid (TIAs) biosynthetic pathway. Most of these alkaloids have high therapeutic value, such as vinblastine and vincristine. Plant signaling components, plant hormones, precursors, growth hormones, prenylated proteins, and transcriptomic factors regulate the complex networks of TIA biosynthesis. For many years, researchers have been evaluating the scientific value of the TIA biosynthetic pathway and its potential in commercial applications for market opportunities. Metabolic engineering has revealed the major blocks in metabolic pathways regulated at the molecular level, unknown structures, metabolites, genes, enzyme expression, and regulatory genes. Conceptually, this information is necessary to create transgenic plants and microorganisms for the commercial production of high-value dimer alkaloids, such as vinca alkaloids, vinblastine, and vincristine In this review, we present current knowledge of the regulatory mechanisms of these components in the C. roseus TIA pathway, from genes to metabolites.

Catalytic function, mechanism, and application of plant acyltransferases

Acyltransferases (ATs) are important tailoring enzymes that contribute to the diversity of natural products. They catalyze the transfer of acyl groups to the skeleton, which improves the lipid solubility, stability, and pharmacological activity of natural compounds. In recent years, a number of ATs have been isolated from plants. In this review, we have summarized 141 biochemically characterized ATs during the period July 1997 to October 2020, including their function, heterologous expression systems, and catalytic mechanisms. Their catalytic performance and application potential has been further discussed.

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.

Total synthesis of (-)- and ent-(+)-vindoline and related alkaloids

A concise 11-step total synthesis of (-)- and ent-(+)-vindoline (3) is detailed based on a unique tandem intramolecular [4 + 2]/[3 + 2] cycloaddition cascade of a 1,3,4-oxadiazole inspired by the natural product structure, in which three rings and four C-C bonds are formed central to the characteristic pentacyclic ring system setting all six stereocenters and introducing essentially all the functionality found in the natural product in a single step. As key elements of the scope and stereochemical features of the reaction were defined, a series of related natural products of increasing complexity were prepared by total synthesis including both enantiomers of minovine (4), 4-desacetoxy-6,7-dihydrovindorosine (5), 4-desacetoxyvindorosine (6), and vindorosine (7) as well as N-methylaspidospermidine (11). Subsequent extensions of the approach provided both enantiomers of 6,7-dihydrovindoline (8), 4-desacetoxyvindoline (9), and 4-desacetoxy-6,7-dihydrovindoline (10).

Purification and partial amino acid sequences of the enzyme vinorine synthase involved in a crucial step of ajmaline biosynthesis

The acetyl-CoA-dependent enzyme vinorine synthase was isolated from hybrid cell suspension cultures of Rauvolfia serpentina and Rhazya stricta. The sarpagan-type alkaloid gardneral was used as a substrate of the enzyme leading to the ajmalan-type 10-methoxyvinorine. An HPLC-based assay was developed to monitor vinorine synthase activity, which allowed establishing a five step purification procedure combining anion exchange, hydrophobic interaction, hydroxyapatite and gel filtration. Purification resulted in a yield of 0.2% and an approximately 991-fold enrichment of the acetyltransfer activity. SDS-PAGE analysis showed a Mr for the enzyme of approximately 50 kDa. The four peptide fragments generated by proteolysis of the pure enzyme with endoproteinase LysC and the N-terminal part of the enzyme were sequenced. The enzyme preparation (> 875-fold enrichment) delivering the N-terminal sequence was isolated from R. serpentina cell suspensions. Sequence alignment of the five peptides showed highest homologies in a range of 30-71% to acetyltransferases from other higher plants involved in natural plant product biosynthesis. Based on the partial sequences vinorine synthase is probably a novel member of the BAHD enzyme super family.

Acetyltransfer in natural product biosynthesis––functional cloning and molecular analysis of vinorine synthase

Vinorine synthase (EC 2.3.1.160) catalyses the acetyl-CoA- or CoA-dependent reversible formation of the alkaloids vinorine (or 11-methoxy-vinorine) and 16-epi-vellosimine (or gardneral). The forward reaction leads to vinorine, which is a direct biosynthetic precursor along the complex pathway to the monoterpenoid indole alkaloid ajmaline, an antiarrhythmic drug from the Indian medicinal plant Rauvolfia serpentina. Based on partial peptide sequences a cDNA clone was isolated and functionally expressed in Escherichia coli. The Km values of the native enzyme for gardneral and acetyl-CoA were determined to be 7.5 and 57 microM. The amino acid sequence of vinorine synthase has highest level of identity (28-31%) to that of Papaver salutaridinol acetyltransferase, Fragaria alcohol acyltransferase, and Catharanthus deacetylvindoline acetyltransferase involved in morphine, flavor, and vindoline biosynthesis, respectively. Vinorine synthase is a novel member of the BAHD superfamily of acyltransferases. Site-directed mutagenesis of 13 amino acid residues provided clear evidence that both, His160 and Asp164 of the consensus sequence HxxxD belong to the catalytic center. The mutations also showed that an amino acid triad is not characteristic of vinorine synthase. The experiments demonstrated the importance of the conserved motif SxL/I/VD near the N-terminus and the consensus sequence DFGWG near the C-terminal.

Molecular characterization of the salutaridinol 7-O-acetyltransferase involved in morphine biosynthesis in opium poppy Papaver somniferum

Salutaridinol 7-O-acetyltransferase (EC ) catalyzes the conversion of the phenanthrene alkaloid salutaridinol to salutaridinol-7-O-acetate, the immediate precursor of thebaine along the morphine biosynthetic pathway. We have isolated a cDNA clone that corresponds to the internal amino acid sequences of the native enzyme purified from a cell suspension culture of opium poppy Papaver somniferum. The recombinant enzyme acetylated the 7-hydroxyl moiety of salutaridinol in the presence of acetyl-CoA. The apparent K(m) value for salutaridinol was determined to be 9 microm and 54 microm for acetyl-CoA. The gene transcript was detected in extracts from Papaver orientale and Papaver bracteatum in addition to P. somniferum. Genomic DNA gel blot analysis indicated that there is likely a single copy of this gene in the P. somniferum genome. The amino acid sequence of salutaridinol 7-O-acetyltransferase is most similar (37% identity) to that of deacetylvindoline acetyltransferase of Catharanthus roseus. Salutaridinol 7-O-acetyltransferase is the second enzyme specific to morphine biosynthesis for which we have isolated a cDNA. Taken together with the other cDNAs cloned encoding norcoclaurine 6-O-methyltransferase, (S)-N-methylcoclaurine 3'-hydroxylase, the cytochrome P-450 reductase, and codeinone reductase, significant progress has been made toward accumulating genes of this pathway to enable the end goal of a biotechnological production of morphinan alkaloids.

ALKALOID BIOSYNTHESIS IN PLANTS: Biochemistry, Cell Biology, Molecular Regulation, and Metabolic Engineering Applications

Recent advances in the cell, developmental, and molecular biology of alkaloid biosynthesis have heightened our appreciation for the complexity and importance of plant secondary pathways. Several biosynthetic genes involved in the formation of tropane, benzylisoquinoline, and terpenoid indole alkaloids have now been isolated. The early events of signal perception, the pathways of signal transduction, and the function of gene promoters have been studied in relation to the regulation of alkaloid metabolism. Enzymes involved in alkaloid biosynthesis are associated with diverse subcellular compartments including the cytosol, vacuole, tonoplast membrane, endoplasmic reticulum, chloroplast stroma, thylakoid membranes, and perhaps unique "biosynthetic" or transport vesicles. Localization studies have shown that sequential alkaloid biosynthetic enzymes can also occur in distinct cell types, suggesting the intercellular transport of pathway intermediates. Isolated genes have also been used to genetically alter the accumulation of specific alkaloids and other plant secondary metabolites. Metabolic modifications include increased indole alkaloid levels, altered tropane alkaloid accumulation, elevated serotonin synthesis, reduced indole glucosinolate production, redirected shikimate metabolism, and increased cell wall-bound tyramine formation. This review discusses the biochemistry, cell biology, molecular regulation, and metabolic engineering of alkaloid biosynthesis in plants.

Purification of acetyl-CoA: 17-O-deacetylvindoline 17-O-acetyltransferase from Catharanthus roseus leaves

The enzyme acetyl-CoA: 17-O-deacetylvindoline 17-O-acetyltransferase which terminates vindoline biosynthesis has been isolated from Catharanthus roseus leaves, further characterized and purified to homogeneity by three step column chromatography and subsequent preparative isoelectric focusing. Kinetic properties concerning the enzyme reaction are discussed. Five multiple forms of the acetyl-transferase could be observed, each consisting of two subunits. This enzyme is now the best characterized of the enzymes involved in vindoline biosynthesis.

Alkaloid production by transformed root cultures of Catharanthus roseus

Transformed roots of Catharanthus roseus were obtained following infection of detached leaves with Agrobacterium rhizogenes. Roots would not grow in full strength Gamborg's B5 medium but would grow satisfactorily if the medium was diluted to one half strength. Little alkaloid appeared in the growth medium but root tissue contained a high level and wide variety of alkaloids. Ajmalicine, serpentine, vindolinine and catharanthine were prominent components. Vinblastine could also be detected by a combination of HPLC and radioimmunoassay, though at a level of only 0.05μg/g dry weight.

Characterization of a novel N-methyltransferase (NMT) from Catharanthus roseus plants : Detection of NMT and other enzymes of the indole alkaloid biosynthetic pathway in different cell suspension culture systems

Young leaves from Catharanthus roseus plants contain a novel N-methyltransferase which transfers the methyl group from S-adenosyl-L-methionine specifically to position 1 of (2R, 3R)-2,3-dihydro-3-hydroxytabersonine, producing the N-methylated product. The enzyme shows a high degree of specificity toward substrates containing a reduced double bond at position 2,3 of tabersonine derivatives but the more substituted N-desmethyldeacetylvindoline did not act as a substrate. The enzyme catalyses the third last step in vindorosine and vindoline biosynthesis, and is associated with chlorophyll-containing fractions in partially purified enzyme preparations. The lack of vindoline accumulation in cell suspension cultures is correlated with the lack of expression of this enzyme activity as well as that of an acetyltransferase which catalyses the last step in vindoline biosynthesis. Neither fungal elicitor treatment of cell line #615 nor transfer to alkaloid production medium resulted in expression of these two enzyme activities, nor was either enzyme activity detected in photoautotrophic or hormone autotrophic cultures. Cell lines #200, 615-767 and 916 could not be induced to produce DAT or NMT enzyme activities.

Alkaloid formation by habituated and tumorous cell suspension cultures of Catharanthus roseus

Habituated and tumorous Catharanthus roseus cells grown in the absence of hormones accumulated indole alkaloids. Total alkaloids and alkaloid pattern were the same when cells were cultured in medium without hormones or in alkaloid production medium with and without indole acetic acid. Treatment of cells with Pythium homogenate as elicitor did not increase total alkaloids or change the pattern of alkaloids produced. When either habituated or tumorous cells were grown in 1B5 medium after Gamborg et al (1968) containing 2,4-dichlorophenoxyacetic acid (2,4-D), their capacity to accumulate alkaloids decreased with time. The levels of tryptophan decarboxylase (TDC) and strictosidine synthase (SS) specific activities were constant throughout growth except when cells were exposed to 2,4-D in 1B5 medium, where enzyme activities declined in step with the decrease in alkaloid accumulation. Neither habituated nor tumorous cell suspension cultures accumulated vindoline, nor could they be induced to produce this alkaloid by any of the given treatments.