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.

Using Strictosidine Synthase to Prepare Novel Alkaloids.

The Pictet-Spenglerasestrictosidine synthase (STR) has been characterized as the central enzyme in the biosynthesis of around 2000 monoterpenoid indole alkaloids in plants. In the light of a high therapeutic value and huge scaffold diversity these alkaloids represent, STR as an enzyme has attracted great attentions in recent years, intending to be utilized in the formation of new interesting alkaloids with unusual substitution pattern or even with novel scaffolds. For outlining the application potential that STR possesses, together with insight into the reaction mechanism catalyzed by STR, strategies and methods for exploring the applicability of STR have been updated in this article by taking R. serpertina STR(RS-STR) and C. roseus.STR (CR-STR) as representative models, followed by introducing the latest released complex structures of RS-STR with new substrates. Examples provided here, including substrate scaffold tailoring, X-ray crystal complex structure comparison, protein engineering and biosynthetic pathway reprogramming, pave the way to finally construct novel alkaloids libraries by chemo-enzymatic approaches.

Ligand structures of synthetic deoxa-pyranosylamines with raucaffricine and strictosidine glucosidases provide structural insights into their binding and inhibitory behaviours.

Insight into the structure and inhibition mechanism of O-ß-d-glucosidases by deoxa-pyranosylamine type inhibitors is provided by X-ray analysis of complexes between raucaffricine and strictosidine glucosidases and N-(cyclohexylmethyl)-, N-(cyclohexyl)- and N-(bromobenzyl)-ß-d-gluco-1,5-deoxa-pyranosylamine. All inhibitors anchored exclusively in the catalytic active site by competition with appropriate enzyme substrates. Thus facilitated prospective elucidation of the binding networks with residues located at <3.9 Å distance will enable the development of potent inhibitors suitable for the production of valuable alkaloid glucosides, raucaffricine and strictosidine, by means of synthesis in Rauvolfia serpentina cell suspension cultures.

Crystal structure of perakine reductase, founding member of a novel aldo-keto reductase (AKR) subfamily that undergoes unique conformational changes during NADPH binding.

Perakine reductase (PR) catalyzes the NADPH-dependent reduction of the aldehyde perakine to yield the alcohol raucaffrinoline in the biosynthetic pathway of ajmaline in Rauvolfia, a key step in indole alkaloid biosynthesis. Sequence alignment shows that PR is the founder of the new AKR13D subfamily and is designated AKR13D1. The x-ray structure of methylated His(6)-PR was solved to 2.31 Å. However, the active site of PR was blocked by the connected parts of the neighbor symmetric molecule in the crystal. To break the interactions and obtain the enzyme-ligand complexes, the A213W mutant was generated. The atomic structure of His(6)-PR-A213W complex with NADPH was determined at 1.77 Å. Overall, PR folds in an unusual α(8)/ß(6) barrel that has not been observed in any other AKR protein to date. NADPH binds in an extended pocket, but the nicotinamide riboside moiety is disordered. Upon NADPH binding, dramatic conformational changes and movements were observed: two additional ß-strands in the C terminus become ordered to form one α-helix, and a movement of up to 24 Å occurs. This conformational change creates a large space that allows the binding of substrates of variable size for PR and enhances the enzyme activity; as a result cooperative kinetics are observed as NADPH is varied. As the founding member of the new AKR13D subfamily, PR also provides a structural template and model of cofactor binding for the AKR13 family.

Scaffold tailoring by a newly detected Pictet-Spenglerase activity of strictosidine synthase: from the common tryptoline skeleton to the rare piperazino-indole framework.

The Pictet-Spenglerase strictosidine synthase (STR1) has been recognized as a key enzyme in the biosynthesis of some 2000 indole alkaloids in plants, some with high therapeutic value. In this study, a novel function of STR1 has been detected which allows for the first time a simple enzymatic synthesis of the strictosidine analogue 3 harboring the piperazino[1,2-a]indole (PI) scaffold and to switch from the common tryptoline (hydrogenated carboline) to the rare PI skeleton. Insight into the reaction is provided by X-ray crystal analysis and modeling of STR1 ligand complexes. STR1 presently provides exclusively access to 3 and can act as a source to generate by chemoenzymatic approaches libraries of this novel class of alkaloids which may have new biological activities. Synthetic or natural monoterpenoid alkaloids with the PI core have not been reported before.

Structures of alkaloid biosynthetic glucosidases decode substrate specificity.

Two similar enzymes with different biosynthetic function in one species have evolved to catalyze two distinct reactions. X-ray structures of both enzymes help reveal their most important differences. The Rauvolfia alkaloid biosynthetic network harbors two O-glucosidases: raucaffricine glucosidase (RG), which hydrolyses raucaffricine to an intermediate downstream in the ajmaline pathway, and strictosidine glucosidase (SG), which operates upstream. RG converts strictosidine, the substrate of SG, but SG does not accept raucaffricine. Now elucidation of crystal structures of RG, inactive RG-E186Q mutant, and its complexes with ligands dihydro-raucaffricine and secologanin reveals that it is the "wider gate" of RG that allows strictosidine to enter the catalytic site, whereas the "slot-like" entrance of SG prohibits access by raucaffricine. Trp392 in RG and Trp388 in SG control the gate shape and acceptance of substrates. Ser390 directs the conformation of Trp392. 3D structures, supported by site-directed mutations and kinetic data of RG and SG, provide a structural and catalytic explanation of substrate specificity and deeper insights into O-glucosidase chemistry.

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.

Chemotherapeutic effects of bioassay-guided extracts of the American cockroach, Periplaneta americana.

The organic extract of Periplaneta americana L. (Dictyoptera; Blattidae) has been traditionally used in southwestern China as an alternative medicine against disorders such as hepatitis, trauma, gastric ulcers, burns, and heart disease. The present study describes bioassay-guided purification and chemotherapeutic evaluation of the 60% ethanolic fraction of P americana organic extracts (PAE60). The most effective cytotoxic fraction was determined by way of repeated in vitro screenings against 12 distinct cultured human carcinoma cell lines: Eca 109, BGC823, HO8910, LS174T, CNE, HeLa, K562, PC-3, A549, BEL 7404, HL-60, and KB, followed by in vivo antitumor assays of the lead fraction (PAE60). The complexity of enriched active fraction was qualitatively evaluated using thin layer chromatography. Reconstituted PAE60 was effective at inhibiting HL-60, KB, CNE, and BGC823 cell growth with IC(50) values <20 µg mL-(1). PAE60 reduced tumor growth in S180-bearing immunocompetent mice by 72.62% after 10 days following oral doses of 500 mg kg d-(1) compared with 78.75% inhibition following 40 mg kg d-(1) of cyclophosphamide (CTX). Thymus and spleen indices of S180-bearing mice treated with PAE60 were significantly greater (P < .05) than CTX treatment groups, suggesting potential immunomodulation of antitumor host defenses by PAE60. Antiviral activity was also investigated and PAE60 inhibited herpes simplex type-2 replication (IC(50) = 4.11 ± 0.64 µg mL-(1)) with a selectivity index (CC(50) to IC(50) ratio) of 64.84 in Vero cells but was less effective on type-1 virus (IC(50) of 25.6 ± 3.16 µg mL-(1)). These results support future clinical trials on P. americana as an alternative or complementary medicinal agent.

"One-pot" multicomponent approach to indolizines and pyrido[1,2-a]indoles.

A new synthetic protocol for efficient and regiospecifc assembly of indolizines and pyrido[1,2-a]indoles by coupling of substituted methyl bromides and alkynes with corresponding pyrrole-2-carboxaldehyde and 1H-indole-2-carboxaldehyde has been developed. Additionally, a possible mechanism for the reaction is proposed.

Synthesis and structure-activity relationship studies of cytotoxic cinnamic alcohol derivatives.

Three series of di- and trisubstituted derivatives of cinnamic alcohol and its conjugated dienol analogues were designed and synthesised. The derivatives were screened for cytotoxicity against nine tumour cell lines: KB, A549, Hela, CNE, PC-3, BEL-7404, HL-60, BGC823 and P388D1. Most of the cinnamic alcohol derivatives showed cytotoxic activity. The compound 7-(4',5'-dichlorobenzyloxy)-6,8-dihydroxycinnamic alcohol (55) exhibited significant cytotoxicity to seven human tumour cell lines on a micromolar range, especially with regard to the KB and P388D1 cell lines, showing IC(50) values of 0.4 and 0.5 µM, respectively. The structure-activity relationships of the derivatives are discussed.

A facile chemoenzymatic approach: one-step syntheses of monoterpenoid indole alkaloids.

Facile chemoenzymatic syntheses of cytotoxic monoterpenoid indole alkaloids with novel skeletons and multiple chiral centers are described. Synthesis of these alkaloids was achieved by a simple one-step reaction using strictosidine and 12-aza-strictosidine as the key intermediates. Strictosidines were prepared by coupling of secologanin with tryptamine and 7-aza-tryptamine, respectively, using the immobilized recombinant Rauvolfia strictosidine synthase. A detailed stereochemical analysis is presented herein. The results provide an opportunity for a chemoenzymatic approach that leads to an increased diversification of complex alkaloids with improved structures and activities.

Synthesis, biological evaluation, and structure-activity relationship study of novel cytotoxic aza-caffeic acid derivatives.

Three series of aza-caffeic acid derivatives with different linkers were designed and synthesized. Each of the synthesized derivatives was then used in cytotoxicity screening on either 8 or 12 human cancer cell lines. The structure-activity relationships on three structural regions A, B, and C are analyzed in detail, indicating that a nine bond linker B, containing a piperazine unit, is the most favorable linker leading to the generation of molecules with potent cytotoxicities. Compound (E)-1-(4-(3,4-dichlorobenzyl)piperazin-1-yl)-3-(4-(4-ethoxybenzyloxy)-3,5-dimethoxyphenyl)prop-2-en-1-one (80) exhibited the most significant and selective cytotoxicity to KB, BEL7404, K562, and Eca109 cell lines, with IC(50) values of 0.2, 2.0, 1.7, and 1.1 microM, respectively, stronger than that seen for caffeic acid phenethyl ester (CAPE) and cisplatin (CDDP). Flow cytometric and western blot analysis indicate that compound 80 plays a role in mitochondria-dependent apoptosis activity by suppressing K562 cell proliferation in a concentration- and time-dependent manner.

Construction and expression of a dual vector for chemo-enzymatic synthesis of plant indole alkaloids in Escherichia coli.

A dual vector (pQE-70-STR1-SG) containing coding regions of strictosidine synthase (STR1, EC 4.3.3.2) and strictosidine glucosidase (SG, EC 3.2.1.105) from the Indian medicinal plant Rauvolfia serpentina was constructed. Functional expression of the vector in Escherichia coli cells (M15 strain) was proven by isolation of prepurified enzyme extracts, which show both STR1 and SG activities. Incubation of the enzyme in the presence of tryptamine and secologanin delivered the indole alkaloid cathenamine, demonstrating functional co-expression of both STR1- and SG-cDNAs. Cathenamine reduction by sodium borohydride leading to tetrahydroalstonine revealed the chemo-enzymatic indole alkaloid synthesis.

Improved expression of His(6)-tagged strictosidine synthase cDNA for chemo-enzymatic alkaloid diversification.

Strictosidine synthase (STR1) catalyzes the stereoselective formation of 3alpha(S)-strictosidine from tryptamine and secologanin. Strictosidine is the key intermediate in the biosynthesis of 2,000 plant monoterpenoid indole alkaloids, and it is a key precursor of enzyme-mediated synthesis of alkaloids. An improved expression system is described which leads to optimized His(6)-STR1 synthesis in Escherichia coli. Optimal production of STR1 was achieved by determining the impact of co-expression of chaperones pG-Tf2 and pG-LJE8. The amount and activity of STR1 was doubled in the presence of chaperone pG-Tf2 alone. His(6)-STR1 immobilized on Ni-NTA can be used for enzymatic synthesis of strictosidines on a preparative scale. With the newly co-expressed His(6)-STR1, novel 3alpha(S)-12-azastrictosidine was obtained by enzymatic catalysis of 7-azatryptamine and secologanin. The results obtained are of significant importance for application to chemo-enzymatic approaches leading to diversification of alkaloids with novel improved structures.

Enzymatic and chemo-enzymatic approaches towards natural and non-natural alkaloids: indoles, isoquinolines, and others.

The multi-step enzyme catalysed biosyntheses of monoterpenoid indole and isoquinoline alkaloids are described. Special emphasis is placed on those pathways leading to alkaloids of pharmacological and medicinal significance which have been fully elucidated at the enzyme level. The successful identification and cloning of cDNAs of single enzymes and their application provides great opportunities to develop novel strategies for both in vitro and in vivo alkaloid production in whole plants or tissue cultures, as well as in microbial systems such as Escherichia coli and yeast. Enzyme crystallisation, 3D analyses and site-directed mutation allowed rational engineering of enzyme substrate acceptance, which in turn can be used for reprogramming in vivo alkaloid biosynthesis and for the design of biomimetic alkaloid syntheses. These strategies broaden structural diversity and allow the creation of large libraries of unnatural alkaloid with expected optimised or novel biological activities. The chemo-enzymatic syntheses of the above-mentioned alkaloid groups and their precursors (in addition to selected examples of other alkaloid families) provides an overview of how enzyme reactions are integrated into the development of total chemical syntheses.

Preparation of C-23 esterified silybin derivatives and evaluation of their lipid peroxidation inhibitory and DNA protective properties.

A diverse series of C-23 esterified silybin derivatives (1a-n) were designed and synthesized. The antioxidative properties of these compounds were evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH) and superoxide anion radical scavenging, ferrous ion chelation, and inhibition of rat liver homogenate lipid peroxidation. Their protective effects on the prevention of hydrogen peroxide induced DNA damage were also investigated. Most of the synthesized compounds exhibited more effective antioxidant activities than silybin. The esterified silybin analogues displayed satisfactory performance especially on iron chelation and antiperoxidative activity. Compound 1n in particular exhibited remarkable antiperoxidative effect with an IC(50) value of 0.2+/-0.1 microM, which was stronger than that of quercetin (IC(50)=1.8+/-0.6 microM). Compounds 1c, 1e, 1g, 1h and 1k displayed potent, dose-dependent protective properties against DNA cleavage. The results of the bioassays support the antioxidative and DNA protective effects of these synthesized silybin derivatives.

Phytochemical investigation and cytotoxic evaluation of the components of the medicinal plant Ligularia atroviolacea.

A phytochemical investigation of the roots of Ligularia atroviolacea resulted in the isolation of 24 compounds including seven new eremophilanoids named eremophila-3,7(11),8-triene-12,8;14,6alpha-diolide (1), 3beta-(angeloyloxy)eremophil-7(11)-en-12,8beta-olid-14-oic acid (2), 1alpha-chloro-10beta-hydroxy-6beta-(2-methylpropanoyloxy)-9-oxo-7,8-furoeremophilane (3), (10betaH)-8-oxoeremophila-3(4),6(7)-diene-12,14-dioic acid (4), (10alphaH)-8-oxoeremophila-3(4),6(7)-diene-12,14-dioic acid (5), 8beta-[eremophila-3',7'(11')-diene-12',8'alpha;14',6'alpha-diolide]eremophila-3,7(11)-diene-12,8alpha;14,6alpha-diolide (6), and ligulatrovine A (7), eleven known eremophilanoids, 8-18, four steroids, one glucose derivative, and one fatty acid. The structures of these compounds were elucidated by spectroscopic methods including 2D-NMR experiments. The structure of 3 was also established by an X-ray diffraction study. The in vitro cytotoxicity evaluation of selected compounds was performed on seven cultured tumor cell lines, i.e., KB, BEL-7404, A549, HL-60, HeLa, CNE, and P-388D1. The preliminary taxonomy of this species was also discussed, and the possible biogenesis of a dimer possessing a new noreremophilanoid type skeleton, 7, is presented in a preliminary form.

Structural basis and enzymatic mechanism of the biosynthesis of C9- from C10-monoterpenoid indole alkaloids.

Cutting carbons: The three-dimensional structure of polyneuridine aldehyde esterase (PNAE) gives insight into the enzymatic mechanism of the biosynthesis of C(9)- from C(10)-monoterpenoid indole alkaloids (see scheme). PNAE is a very substrate-specific serine esterase. It harbors the catalytic triad S87-D216-H244, and is a new member of the alpha/beta-fold hydrolase superfamily. Its novel function leads to the diversification of alkaloid structures.

Preparation of two sets of 5,6,7-trioxygenated dihydroflavonol derivatives as free radical scavengers and neuronal cell protectors to oxidative damage.

An unusual class of 5,6,7-trioxygenated dihydroflavonols (3a-e and 4a-j) were designed and prepared. Their antioxidative properties were assessed by examining their capacities in several in vitro models, including superoxide anion and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging, rat liver homogenate lipid peroxidation inhibition, PC12 cells protection from oxidative damage, and xanthine oxidase inhibition. These dihydroflavonols displayed positive quenching abilities towards O(2)(-) and DPPH free radicals, in which the majority exhibited superior antioxidant properties to Vitamin C. cis-Configurated compound (+/-)-3e demonstrated remarkable inhibition to LPO with an IC(50) value of 1.9+/-0.3 microM, which was apparently stronger than that of quercetin (IC(50)=6.0+/-0.4 microM). trans-Configurated dihydroflavonol (+/-)-4h exhibited significant protective effect on PC12 cells against oxidative damage with an EC(50) value of 41.5+/-5.3 microM, more effective compared to that of quercetin (EC(50)=81.8+/-8.7 microM). The 6-OH-5,7-dimethoxy analogue (+/-)-3d showed significant inhibition of xanthine oxidase with an IC(50) value of 16.0+/-0.8 microM, which is superior to that of allopurinol (IC(50)=23.5+/-2.0 microM). In addition to the hypothesized action mechanism of the bio-active compounds, 3D modeling was used to analyze the relationship between the minimized-energy structures and antioxidant activities.