Enzymatic Formation of Raucaffricine, the Major Indole Alkaloid ofRauwolfia serpentinaCell-Suspension Cultures

Sarpagan-Ajmalan-Type Indoles: Biosynthesis, Structural Biology, and Chemo-Enzymatic Significance

The biosynthetic pathway of the monoterpenoid indole alkaloid ajmaline in the genus Rauvolfia, in particular Rauvolfia serpentina Benth. ex Kurz, is one of the few pathways that have been comprehensively uncovered. Every step in the progress of plant alkaloid biosynthesis research is due to the endeavors of several generations of scientists and the advancement of technologies. The tissue and cell suspension cultures developed in the 1970s by M.H. Zenk enabled the extraction of alkaloids and crude enzymes for use as experimental materials, thus establishing the foundation for further research on enzymatic reaction networks. In vivo NMR technology was first used in biosynthetic investigations in the 1990s following the invention of high-field cryo-NMR, which allowed the rapid and reliable detection of bioconversion processes within living plant cells. Shortly before, in 1988, a milestone was reached with the heterologous expression of the strictosidine synthase cDNA, which paved the way for the application of "reverse genetics" and "macromolecular crystallography." Both methods allowed the structural analysis of several Rauvolfia enzymes involved in ajmaline biosynthesis and expanded our knowledge of the enzyme mechanisms, substrate specificities, and structure-activity relationships. It also opened the door for rational enzyme engineering and metabolic steering. Today, the research focus of ajmaline biosynthesis is shifting from "delineation" to "utilization." The Pictet-Spenglerase strictosidine synthase, strictosidine glucosidase, together with raucaffricine glucosidase, as pioneers in this area, have become useful tools to generate "privileged structures" and "diversity oriented" syntheses, which may help to construct novel scaffolds and to set up libraries of sarpagan-ajmalan-type alkaloids in chemo-enzymatic approaches.

Deoxysarpagine hydroxylase--a novel enzyme closing a short side pathway of alkaloid biosynthesis in Rauvolfia

Microsomal preparations from cell suspension cultures of the Indian plant Rauvolfia serpentina catalyze the hydroxylation of deoxysarpagine under formation of sarpagine. The newly discovered enzyme is dependent on NADPH and oxygen. It can be inhibited by typical cytochrome P450 inhibitors such as cytochrome c, ketoconazole, metyrapone, tetcyclacis and carbon monoxide. The CO-effect is reversible with light (450 nm). The data indicate that deoxysarpagine hydroxylase is a novel cytochrome P450-dependent monooxygenase. A pH optimum of 8.0 and a temperature optimum of 35 degrees C were determined. K(m) values were 25 microM for NADPH and 7.4 microM for deoxysarpagine. Deoxysarpagine hydroxylase activity was stable in presence of 20% sucrose at -25 degrees C for >3 months. The analysis of presence of the hydroxylase in nine cell cultures of seven different families indicates a very limited taxonomic distribution of this enzyme.

Molecular cloning and functional bacterial expression of a plant glucosidase specifically involved in alkaloid biosynthesis

Monoterpenoid indole alkaloids are a vast and structurally complex group of plant secondary compounds. In contrast to other groups of plant products which produce many glycosides, indole alkaloids rarely occur as glucosides. Plants of Rauvolfia serpentina accumulate ajmaline as a major alkaloid, whereas cell suspension cultures of Rauvolfia mainly accumulate the glucoalkaloid raucaffricine at levels of 1.6 g/l. Cell cultures do contain a specific glucosidase. known as raucaffricine-O-beta-D-glucosidase (RG), which catalyzes the in vitro formation of vomilenine, a direct intermediate in ajmaline biosynthesis. Here, we describe the molecular cloning and functional expression of this enzyme in Escherichia coli. RG shows up to 60% amino acid identity with other glucosidases of plant origin and it shares several sequence motifs with family 1 glucosidases which have been characterized. The best substrate specificity for recombinant RG was raucaffricine (KM 1.3 mM, Vmax 0.5 nkat/microg protein) and only a few closely related structural derivatives were also hydrolyzed. Moreover, an early intermediate of ajmaline biosynthesis, strictosidine, is a substrate for recombinant RG (KM 1.8 mM, Vmax 2.6 pkat/microg protein) which was not observed for the low amounts of enzyme isolated from Rauvolfia cells.

The gene encoding polyneuridine aldehyde esterase of monoterpenoid indole alkaloid biosynthesis in plants is an ortholog of the alpha/betahydrolase super family

The biosynthesis of the anti-arrhythmic alkaloid ajmaline is catalysed by more than 10 specific enzymes. In this multistep process polyneuridine aldehyde esterase (PNAE) catalyses a central reaction by transforming polyneuridine aldehyde into epi-vellosimine, which is the immediate precursor for the synthesis of the ajmalane skeleton. PNAE was purified from cell suspension cultures of Rauvolfia serpentina. The N-terminal sequence and endoproteinase LysC fragments of the purified protein were used for primer design and for the amplification of specific PCR products leading to the isolation of PNAE-encoding cDNA from a R. serpentina library. The PNAE cDNA was fused with a C-terminal His-tag, expressed in Escherichia coli and purified to homogeneity using Ni-affinity chromatography. The pure enzyme shows extraordinary substrate specificity, completely different to other esterases. Sequence alignments indicate that PNAE is a new member of the alpha/beta hydrolase super family.

Purification, partial amino acid sequence and structure of the product of raucaffricine-O-beta-D-glucosidase from plant cell cultures of Rauwolfia serpentina

Plant cell suspension cultures of Rauwolfia produce within 1 week approximately 250 nkat/l of raucaffricine-O-beta-D-glucosidase. A five step procedure using anion exchange chromatography, chromatography on hydroxylapatite, gel filtration and FPLC-chromatography on Mono Q and Mono P delivered in a yield of 0.9% approximately 1200-fold enriched glucosidase. A short protocol employing DEAE sepharose, TSK 55 S gel chromatography and purification on Mono Q gave a 5% recovery of glucosidase which was 340-fold enriched. SDS-PAGE showed a Mr for the enzyme of 61 kDa. The enzyme is not glycosylated. Structural investigation of the enzyme product, vomilenine, demonstrated that the alkaloid exists in aqueous solutions in an equilibrium of 21(R)- and 21(S)-vomilenine in a ratio of 3.4:1. Proteolysis of the pure enzyme with endoproteinase Lys C revealed six peptide fragments with 6-24 amino acids which were sequenced. The two largest fragments showed sequences, of which the motif Val-Thr-Glu-Asn-Gly is typical for beta-glucosidases. Sequence alignment of these fragments demonstrated high homologies to linamarase from Manihot esculenta (81% identity) or to beta-glucosidase from Prunus avium (79% identity). Raucaffricine-O-beta-D-glucosidase seems to be a new member of the family 1 of glycosyl hydrolases.