Biosynthesis of the diterpene verrucosan-2beta-ol in the phototrophic eubacterium Chloroflexus aurantiacus. A retrobiosynthetic NMR study

Subrutilane-A Hexacyclic Sesterterpene from Streptomyces subrutilus

Mining of a terpene synthase from Streptomyces subrutilus resulted in the identification of the hexacyclic sesterterpene subrutilane, besides eight pentacyclic side products. Subrutilane represents the first case of a saturated sesterterpene hydrocarbon. Its structure, including the absolute configuration, was unambiguously determined through X-ray crystallographic analysis and stereoselective deuteration. The cyclisation mechanism to subrutilane and its side products was investigated in all detail by isotopic labelling experiments and DFT calculations. The subrutilane synthase (SrS) also converted (2Z)-GFPP into one major product. Additional compounds were obtained from the substrate analogues (7R)-6,7-dihydro-GFPP and (2Z,7R)-6,7-dihydro-GFPP with blocked reactivity at the C6-C7 bond. Interestingly, the early steps of the cyclisation cascade with (2Z)-GFPP and the saturated substrate analogues were analogous to those of GFPP, but then deviations from the natural cyclisation mode occur.

DFT Study on the Biosynthesis of Asperterpenol and Preasperterpenoid Sesterterpenoids: Exclusion of Secondary Carbocation Intermediates and Origin of Structural Diversification

The biosynthetic pathway to asperterpenol, a sesterterpenoid featuring a 6/6/8/5 tetracyclic ring system, was proposed to involve three secondary (2°) carbocation intermediates (B, D, and I), but it remains controversial whether or not these are viable. Further, the proposed 11/6/5 tricyclic intermediate C has the same "ChemDraw" structure as an intermediate in the biosynthesis of preasperterpenoid, which has a very different 5/7/(3)6/5 pentacyclic skeleton. Here, we present a detailed scrutiny of the asperterpenol/preasperterpenoid biosynthetic pathways based on comprehensive DFT calculations.

DFT Study on the Biosynthesis of Verrucosane Diterpenoids and Mangicol Sesterterpenoids: Involvement of Secondary-Carbocation-Free Reaction Cascades

Some experimental observations indicate that a sequential formation of secondary (2°) carbocations might be involved in some biosynthetic pathways, including those of verrucosane-type diterpenoids and mangicol-type sesterterpenoids, but it remains controversial whether or not such 2° cations are viable intermediates. Here, we performed comprehensive density functional theory calculations of these biosynthetic pathways. The results do not support previously proposed pathways/mechanisms: in particular, we find that none of the putative 2° carbocation intermediates is involved in either of the biosynthetic pathways. In verrucosane biosynthesis, the proposed 2° carbocations (II and IV) in the early stage are bypassed by the formation of the adjacent 3° carbocations and by unusual skeletal rearrangement reactions, and in the later stage, the putative 2° carbocation intermediates (VI, VII, and VIII) are not present as the proposed forms but as nonclassical structures between homoallyl and cyclopropylcarbinyl cations. In the mangicol biosynthesis, one of the two proposed 2° carbocations (X) is bypassed by a C-C bond-breaking reaction to generate a 3° carbocation with a C=C bond, while the other (XI) is bypassed by a strong hyperconjugative interaction leading to a nonclassical carbocation. We propose new biosynthetic pathways/mechanisms for the verrucosane-type diterpenoids and mangicol-type sesterterpenoids. These pathways are in good agreement with the findings of previous biosynthetic studies, including isotope-labeling experiments and byproducts analysis, and moreover can account for the biosynthesis of related terpenes.

Terpene synthases in disguise: enzymology, structure, and opportunities of non-canonical terpene synthases

Covering: up to July 2019 Terpene synthases (TSs) are responsible for generating much of the structural diversity found in the superfamily of terpenoid natural products. These elegant enzymes mediate complex carbocation-based cyclization and rearrangement cascades with a variety of electron-rich linear and cyclic substrates. For decades, two main classes of TSs, divided by how they generate the reaction-triggering initial carbocation, have dominated the field of terpene enzymology. Recently, several novel and unconventional TSs that perform TS-like reactions but do not resemble canonical TSs in sequence or structure have been discovered. In this review, we identify 12 families of non-canonical TSs and examine their sequences, structures, functions, and proposed mechanisms. Nature provides a wide diversity of enzymes, including prenyltransferases, methyltransferases, P450s, and NAD+-dependent dehydrogenases, as well as completely new enzymes, that utilize distinctive reaction mechanisms for TS chemistry. These unique non-canonical TSs provide immense opportunities to understand how nature evolved different tools for terpene biosynthesis by structural and mechanistic characterization while affording new probes for the discovery of novel terpenoid natural products and gene clusters via genome mining. With every new discovery, the dualistic paradigm of TSs is contradicted and the field of terpene chemistry and enzymology continues to expand.

The Diterpene Sclareol Vascular Effect in Normotensive and Hypertensive Rats

BACKGROUND:

The diterpene Sclareol has antimicrobial action, cytotoxic and cytostatic effects and anti-tumor activities. However, researches on the cardiovascular system are scarce.

OBJECTIVE:

This study was designed to investigate the mechanisms involved in the Sclareol cardiovascular effect in normotensive and hypertensive rats.

METHODS:

The arterial hypertension was promoted using 2-kidneys 1-clip model in rats. The effect of sclareol on blood pressure was performed by using three dose (10, 20 and 40 mg/kg). Cumulative dose-response curves for Sclareol were determined for endothelium-intact and endothelium-denuded aortic rings in presence or absence of L-NAME and ODQ. The NOx levels were measure in the plasma sample.

RESULTS:

The Sclareol administration in vivo caused a significant reduction in blood pressure in both groups. In vitro the sclareol promoted relaxation in aorta, with endothelium, pre-contracted to Phe. The inhibitors of the nitric oxide synthase and soluble guanylate cyclase were as efficient as the removal of endothelium, in inhibiting the Sclareol-induced relaxation. Otherwise, it was no change of NOx. Also, for unknown reasons, the Sclareol is not selective for hypertensive animals.

CONCLUSION:

The diterpene Sclareol showed in vivo hypotensive and in-vitro vasodilator effects; The chemiluminescence plasmatic NO analysis showed no significant difference between groups and The Sclareol exhibit better effect on normotensive than hypertensive animals to reduce blood pressure. It is concluded that the diterpenes metabolites would be a promising source prototype for the development of new agents in the cardiovascular therapy.

FUNDAMENTO:

O diterpeno Esclareol tem ação antimicrobiana, efeitos citotóxicos e citostáticos e atividades antitumorais. No entanto, pesquisas sobre o sistema cardiovascular são escassas.

OBJETIVO:

Este estudo foi desenvolvido para investigar os mecanismos envolvidos no efeito cardiovascular de Esclareol em ratos normotensos e hipertensos.

MÉTODOS:

A hipertensão arterial foi promovida utilizando modelo de 2 clones de 1-clipe em ratos. O efeito do esclareol sobre a pressão arterial foi realizado utilizando três doses (10, 20 e 40 mg/kg). As curvas dose-resposta cumulativas para Esclareol foram determinadas para anéis aórticos endotélio-intactos e desprovidos de endotélio na presença ou ausência de L-NAME e ODQ. Os níveis de NOx foram medidos na amostra de plasma.

RESULTADOS:

A administração de Esclareol in vivo causou uma redução significativa na pressão sanguínea em ambos os grupos. In vitro o esclareol promoveu relaxamento na aorta, com endotélio, pré-contraído a Phe. Os inibidores da óxido nítrico sintase e da guanilato ciclase solúvel foram tão eficientes quanto a remoção do endotélio, na inibição do relaxamento induzido por Esclareol. Por outra parte, não houve mudança de NOx. Além disso, por razões desconhecidas, o Sclareol não é seletivo para animais hipertensos.

CONCLUSÃO:

O diterpeno Esclareol apresentou efeitos hipotensores in vivo e vasodilatadores in vitro; A análise de NO plasmático por quimioluminescência não mostrou diferença significativa entre os grupos e O Esclareol exibe melhor efeito sobre os animais normotensos do que hipertensos para reduzir a pressão arterial. Conclui-se que os metabólitos de diterpenos seriam um protótipo de fonte promissora para o desenvolvimento de novos agentes na terapia cardiovascular.

The biochemical basis for structural diversity in the carotenoids of chlorophototrophic bacteria

Ongoing work has led to the identification of most of the biochemical steps in carotenoid biosynthesis in chlorophototrophic bacteria. In carotenogenesis, a relatively small number of modifications leads to a great diversity of carotenoid structures. This review examines the individual steps in the pathway, discusses how each contributes to structural diversity among carotenoids, and summarizes recent progress in elucidating the biosynthetic pathways for carotenoids in chlorophototrophs.

New neoverrucosane diterpenoids produced by the marine gliding bacterium Saprospira grandis

Chemical examination of the culture broth extracts of the marine gliding bacterium Saprospira grandis (ATCC 23116) has resulted in the isolation of four new diterpenoids of the neoverrucosane class. The structures of the new diterpenoids, compounds 1-4, were assigned by combined spectroscopic methods emphasizing 2D NMR experiments. The relative stereochemistry of 1 was determined by 2D ROESY NMR methods, while the absolute stereochemistry was assigned by application of the modified Mosher method. This study adds to the rare observation of terpene production by prokaryotic microorganisms and suggests that marine gliding bacteria may be a significant source for new terpenoid secondary metabolites.

Biosynthesis of chloroplastidic and extrachloroplastidic terpenoids in liverwort cultured cells: 13C serine as a probe of terpene biosynthesis via mevalonate and non-mevalonate pathways

Two terpenoid biosynthetic pathways, the mevalonate and non-mevalonate (glyceraldehyde phosphate-pyruvate) routes, were examined by feeding (13)C-labeled serines ([1-(13)C]- and [3-(13)C]-) to the cultured cells of the liverwort, Heteroscyphus planus. The labeling patterns observed in the isoprenoid unit of the biosynthetically (13)C-labeled stigmasterol corresponded to those expected from the mevalonate pathway, while those of the phytyl side chain corresponded to those from the non-mevalonate pathway. Thus, serine is a potential probe to determine the origin of terpenoid biosynthesis, in either the mevalonate or non-mevalonate pathway.

Biosynthetic controls on the 13C contents of organic components in the photoautotrophic bacterium Chloroflexus aurantiacus

To assess the effects related to known and proposed biosynthetic pathways on the (13)C content of lipids and storage products of the photoautotrophic bacterium Chloroflexus aurantiacus, the isotopic compositions of bulk cell material, alkyl and isoprenoid lipids, and storage products such as glycogen and polyhydroxyalkanoic acids have been investigated. The bulk cell material was 13 per thousand depleted in (13)C relative to the dissolved inorganic carbon. Evidently, inorganic carbon fixation by the main carboxylating enzymes used by C. aurantiacus, which are assumed to use bicarbonate rather than CO(2), results in a relatively small carbon isotopic fractionation compared with CO(2) fixation by the Calvin cycle. Even carbon numbered fatty acids, odd carbon numbered fatty acids, and isoprenoid lipids were 14, 15, and 17-18 per thousand depleted in (13)C relative to the carbon source, respectively. Based on the (13)C contents of alkyl and isoprenoid lipids, a 40 per thousand difference in (13)C content between the carboxyl and methyl carbon from acetyl-coenzyme A has been calculated. Both sugars and polyhydroxyalkanoic acid were enriched in (13)C relative to the alkyl and isoprenoid lipids. To the best of our knowledge this is the first report in which the stable carbon isotopic composition of a large range of biosynthetic products in a photoautotrophic organism has been investigated and interpreted based on previously proposed inorganic carbon fixation and biosynthetic pathways. Our results indicate that compound-specific stable carbon isotope analysis may provide a rapid screening tool for carbon fixation pathways.

qNMR--a versatile concept for the validation of natural product reference compounds

The validation of reference compounds for natural products is a domain of the same physico-chemical methods that are used for their isolation, especially those techniques involving coupled high-resolution chromatography. Acknowledging the great problem of co-eluting impurities contained in the 'biogenetic cocktail' of a plant extract, there is a strong demand for non-chromatographic alternatives in the quality assessment of reference compounds. Because of this, the concept of qNMR is introduced as a versatile tool based on qualitative and quantitative 1H-NMR allowing the precise and simultaneous determination of both the compound content as well as the amount and nature of the impurities. As opposed to measuring carbons, 1H-NMR benefits from much higher sensitivity and is far more versatile for routine analysis with respect to time and cost. Since quantification of impurities is reliant upon their identification and, therefore, limited by knowledge about their structure, the concept emphasizes the high demand for qualitative reference dossiers including quality NMR data for profiling potential impurities which may be analogues, isomers, or degradation or oxidation products of the reference compounds. The qNMR concept is developed with focus on its potential in the certification and quality control of reference compounds. Taking into account published work in the field of quantitative NMR, selected natural products are analysed in order to elaborate suitable experimental parameters and to obtain preliminary validation data. The method is discussed with respect to sensitivity, precision and selectivity. Typical relative errors are found to be below 2% for the quantification of both the major analyte and the minor impurities even when the latter are contained at the 1% level only. Documentation of the conformity of signal integration and precision is based on measurements of a certified reference standard. Determination of the natural 13C isotope is suggested as an elegant method of validation because the content values could be reproduced with errors below 1%. The qNMR concept offers a rapid and efficient way to assess the purity of natural products in a single analytical step without the need of performing multiple analyses, while still offering the option to retain the substance. Thus, qNMR pays tribute to the increasing demands in reference compound certification, but also holds out the prospect of easy access to the valid characterisation of natural products tested in vitro or in vivo for their biological and pharmacological effects.

Metabolic engineering of isoprenoids

The metabolic engineering of natural products has begun to prosper in the past few years due to genomic research and the discovery of biosynthetic genes. While the biosynthetic pathways and genes for some isoprenoids have been known for many years, new pathways have been found and known pathways have been further investigated. In this article, we review the recent advances in metabolic engineering of isoprenoids, focusing on the molecular genetics that affects pathway engineering the most. Examples in mono- sequi-, and diterpenoid synthesis as well as carotenoid production are discussed.

Isoprenoid biosynthesis: the evolution of two ancient and distinct pathways across genomes

Isopentenyl diphosphate (IPP) is the central intermediate in the biosynthesis of isoprenoids, the most ancient and diverse class of natural products. Two distinct routes of IPP biosynthesis occur in nature: the mevalonate pathway and the recently discovered deoxyxylulose 5-phosphate (DXP) pathway. The evolutionary history of the enzymes involved in both routes and the phylogenetic distribution of their genes across genomes suggest that the mevalonate pathway is germane to archaebacteria, that the DXP pathway is germane to eubacteria, and that eukaryotes have inherited their genes for IPP biosynthesis from prokaryotes. The occurrence of genes specific to the DXP pathway is restricted to plastid-bearing eukaryotes, indicating that these genes were acquired from the cyanobacterial ancestor of plastids. However, the individual phylogenies of these genes, with only one exception, do not provide evidence for a specific affinity between the plant genes and their cyanobacterial homologues. The results suggest that lateral gene transfer between eubacteria subsequent to the origin of plastids has played a major role in the evolution of this pathway.

The role of lateral gene transfer in the evolution of isoprenoid biosynthesis pathways

Lateral gene transfer (LGT) is a major force in microbial genome evolution. Here, we present an overview of lateral transfers affecting genes involved in isopentenyl diphosphate (IPP) synthesis. Two alternative metabolic pathways can synthesize this universal precursor of isoprenoids, the 1-deoxy-D-xylulose 5-phosphate (DOXP) pathway and the mevalonate (MVA) pathway. We have surveyed recent genomic data and the biochemical literature to determine the distribution of the genes composing these pathways within the bacterial domain. The scattered distribution observed is incompatible with a simple scheme of vertical transmission. LGT (among and between bacteria, archaea and eukaryotes) more parsimoniously explains many features of this pattern. This alternative scenario is supported by phylogenetic analyses, which unambiguously confirm several cases of lateral transfer. Available biochemical data allow the formulation of hypotheses about selective pressures favouring transfer. The phylogenetic diversity of the organisms involved and the range of possible causes and effects of these transfer events make the IPP biosynthetic pathways an ideal system for studying the evolutionary role of LGT.

Biosynthesis of a neo-epi-verrucosane diterpene in the liverwort Fossombronia alaskana. A retrobiosynthetic NMR study

The biosynthesis of the diterpene 8alpha-acetoxy-13alpha-hydroxy-5-oxo-13-epi- neoverrucosane in the arctic liverwort Fossombronia alaskana was studied by incorporation experiments using [1-(13)C]- and [U-(13)C(6)]glucose as precursors. The (13)C-labeling patterns of acetyl-CoA, pyruvate, and phosphoenolpyruvate in intermediary metabolism were reconstructed from the (13)C NMR data of biosynthetic amino acids (leucine, alanine, phenylalanine) and were used to predict hypothetical labeling patterns for isopentenyl pyrophosphate formed via the mevalonate pathway and the deoxyxylulose pathway. The labeling patterns observed for the neoverrucosane diterpene were consistent with the intermediate formation of geranyllinaloyl pyrophosphate assembled from dimethylallyl pyrophosphate and three molecules of isopentenyl pyrophosphate generated predominantly or entirely via 1-deoxyxylulose 5-phosphate. The experimental data can be integrated into a detailed biosynthetic scheme involving a 1,5-hydride shift. The postulated involvement of the 1,5-hydride shift was confirmed by an incorporation experiment with [6,6-(2)H(2)]glucose.

The deoxyxylulose phosphate pathway of terpenoid biosynthesis in plants and microorganisms

Recent studies have uncovered the existence of an alternative, non-mevalonate pathway for the formation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate, the two building blocks of terpene biosynthesis.