Calmodulin inhibitory activity of the malbrancheamides and various analogs

Recent asymmetric synthesis of natural products bearing an α-tertiary amine moiety via temporary chirality induction strategies

Covering: 2013 to 2023The α-tertiary amine moiety is a common structural motif in natural alkaloids and is frequently associated with intriguing biological activities and inherent synthetic challenges. A major hurdle in the total synthesis of these alkaloids is the asymmetric construction of the α-tertiary amine moiety. Temporary chirality inductions have been effective strategies employed to address this issue, particularly in natural product synthesis. The temporary chirality induction strategies in α-tertiary amine synthesis can be broadly classified into three categories based on the types of temporary chirality involved: Seebach's self-regeneration of stereocenters (SRS), C-to-N-to-C chirality transfer, and memory of chirality (MOC). This review highlights the recent advancements in temporary chirality induction strategies for the total synthesis of α-tertiary amine-containing natural products between 2013 and 2023.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Halogenation in Fungi: What Do We Know and What Remains to Be Discovered?

In nature, living organisms produce a wide variety of specialized metabolites to perform many biological functions. Among these specialized metabolites, some carry halogen atoms on their structure, which can modify their chemical characteristics. Research into this type of molecule has focused on how organisms incorporate these atoms into specialized metabolites. Several families of enzymes have been described gathering metalloenzymes, flavoproteins, or S-adenosyl-L-methionine (SAM) enzymes that can incorporate these atoms into different types of chemical structures. However, even though the first halogenation enzyme was discovered in a fungus, this clade is still lagging behind other clades such as bacteria, where many enzymes have been discovered. This review will therefore focus on all halogenation enzymes that have been described in fungi and their associated metabolites by searching for proteins available in databases, but also by using all the available fungal genomes. In the second part of the review, the chemical diversity of halogenated molecules found in fungi will be discussed. This will allow the highlighting of halogenation mechanisms that are still unknown today, therefore, highlighting potentially new unknown halogenation enzymes.

Structural and stereochemical diversity in prenylated indole alkaloids containing the bicyclo[2.2.2]diazaoctane ring system from marine and terrestrial fungi

Covering: up to February 2017 Various fungi of the genera Aspergillus, Penicillium, and Malbranchea produce prenylated indole alkaloids possessing a bicyclo[2.2.2]diazaoctane ring system. After the discovery of distinct enantiomers of the natural alkaloids stephacidin A and notoamide B, from A. protuberus MF297-2 and A. amoenus NRRL 35660, another fungi, A. taichungensis, was found to produce their diastereomers, 6-epi-stephacidin A and versicolamide B, as major metabolites. Distinct enantiomers of stephacidin A and 6-epi-stephacidin A may be derived from a common precursor, notoamide S, by enzymes that form a bicyclo[2.2.2]diazaoctane core via a putative intramolecular hetero-Diels-Alder cycloaddition. This review provides our current understanding of the structural and stereochemical homologies and disparities of these alkaloids. Through the deployment of biomimetic syntheses, whole-genome sequencing, and biochemical studies, a unified biogenesis of both the dioxopiperazine and the monooxopiperazine families of prenylated indole alkaloids constituted of bicyclo[2.2.2]diazaoctane ring systems is presented.

Insights on the vasorelaxant mode of action of malbrancheamide

Objectives

This study was conducted to evaluate the vasorelaxant effect of the fungal alkaloids malbrancheamides on pre-contracted rat aorta rings. Also, we explored the probable mode of action using experimental and theoretical docking studies.

Methods

The vasorelaxant effect was assessed on rat aorta rings pre-contracted with noradrenaline (0.1 μm). The mechanism of action was evaluated using different inhibitors of the pathways involved in the vasorelaxation process, such as l-NAME, indomethacin, tetraethylammonium and atropine. The docking analyses were carried out with AutoDock 4.2 software using the crystallized structure of the cyclooxygenase domain of eNOS.

Key findings

Malbrancheamides (1–3) induced a significant vasorelaxant activity in a concentration- and endothelium-intact model in rat aorta rings, and a lesser effect in an endothelium-denuded model. Malbrancheamide-induced vasorelaxation was significantly weakened by pretreatment of endothelium-intact aortic rings with L-NAME (10 μm), indicating a nitrergic relaxant mechanism. Docking analysis predicted that 1–3 could activate eNOS throughout an allosteric fashion at C1 and C2 pockets.

Conclusions

Experimental evidence revealed that malbrancheamides induced both endothelium-independent and endothelium-dependent relaxant effects. According to theoretical studies, it is feasible that the endothelium-independent relaxation exerted by malbrancheamide could be mediated by its calmodulin inhibitory properties throughout an interference with myosin light chain phosphorylation and a positive modulation of eNOS.

Screening calmodulin-binding ligands using intensity-fading matrix-assisted laser desorption/ionization mass spectrometry

RATIONALE

Calmodulin (CaM) plays an important role in the regulation of metabolism, cytoskeleton and cell proliferation. CaM antagonists are a class of drugs that can bind to CaM and modulate the interactions between CaM and their target biological processes. Screening new calmodulin antagonists and developing novel methods for detecting calmodulin ligands are important for developing novel anti-cancer drugs that bind specifically to CaM.

METHODS

An intensity-fading matrix-assisted laser desorption/ionization mass spectrometry (IF-MALDI-MS) method for screening calmodulin ligands was established, with the non-binding drug propranolol as the internal control. The experimental sample was prepared by mixing the positive ligand trifluoperazine (or chlorpromazine), propranolol and Ca(2+)-CaM. The control sample was treated in the same way without the addition of CaM. The experimental and control samples were both analyzed by MALDI-MS. Based on the relative intensity fading (IF) of the ligand to propranolol, the MS conditions were optimized and then used to study the binding of eight alkaloids and calmodulin. Competitive experiments were performed in a similar way by adding two drugs to compare their binding affinities with calmodulin.

RESULTS

The matrix 2,6-dihydroxyacetophenone (DHAP) was suitable for detecting calmodulin ligands. Compared with propranolol, the relative intensities of six free drugs (berbamine, tetrandrine, papaverine, reserpine, brucine and tetrahydropalmatine) clearly faded after the addition of calmodulin, indicating that they can bind with CaM. The other two alkaloids (strychnine and piperine) had no or weak interaction with the target protein. Based on the data from the competitive binding experiments, the binding affinities of five drugs to calmodulin were obtained in the order: tetrandrine > trifluoperazine > berbamine > chlorpromazine > imipramine.

CONCLUSIONS

The IF-MALDI-MS method was successfully applied to screen novel calmodulin agents at both qualitative and semiquantitative levels. The new ligands may be novel leads for CaM antagonists.

Fungal origins of the bicyclo[2.2.2]diazaoctane ring system of prenylated indole alkaloids

Over eight different families of natural products consisting of nearly 70 secondary metabolites that contain the bicyclo[2.2.2]diazaoctane ring system have been isolated from various Aspergillus, Penicillium, and Malbranchea species. Since 1968, these secondary metabolites have been the focus of numerous biogenetic, synthetic, taxonomic, and biological studies and, as such, have made a lasting impact across multiple scientific disciplines. This review covers the isolation, biosynthesis, and biological activity of these unique secondary metabolites containing the bridging bicyclo[2.2.2]diazaoctane ring system. Furthermore, the diverse fungal origin of these natural products is closely examined and, in many cases, updated to reflect the currently accepted fungal taxonomy.

Biochemical characterization of NotB as an FAD-dependent oxidase in the biosynthesis of notoamide indole alkaloids

Notoamides produced by Aspergillus spp. bearing the bicyclo[2.2.2]diazaoctane core structure with unusual structural diversity represent a compelling system to understand the biosynthesis of fungal prenylated indole alkaloids. Herein, we report the in vitro characterization of NotB, which catalyzes the indole 2,3-oxidation of notoamide E (13), leading to notoamides C (11) and D (12) through an apparent pinacol-like rearrangement. This unique enzymatic reaction with high substrate specificity, together with the information derived from precursor incorporation experiments using [(13)C](2)-[(15)N](2) quadruply labeled notoamide S (10), demonstrates 10 as a pivotal branching point in notoamide biosynthesis.

Utilizing DART mass spectrometry to pinpoint halogenated metabolites from a marine invertebrate-derived fungus

Prenylated indole alkaloids are a diverse group of fungal secondary metabolites and represent an important biosynthetic class. In this study we have identified new halogenated prenyl-indole alkaloids from an invertebrate-derived Malbranchea graminicola strain. Using direct analysis in real time (DART) mass spectrometry, these compounds were initially detected from hyphae of the fungus grown on agar plates, without the need for any organic extraction. Subsequently, the metabolites were isolated from liquid culture in artificial seawater. The structures of two novel chlorinated metabolites, named (-)-spiromalbramide and (+)-isomalbrancheamide B, provide additional insights into the assembly of the malbrancheamide compound family. Remarkably, two new brominated analogues, (+)-malbrancheamide C and (+)-isomalbrancheamide C, were produced by enriching the growth medium with bromine salts.

Studies on the Biosynthesis of the Stephacidin and Notoamide Natural Products: A Stereochemical and Genetic Conundrum

The stephacidin and notoamide natural products belong to a group of prenylated indole alkaloids containing a bicyclo[2.2.2]diazaoctane core. Biosynthetically, this bicyclic core is believed to be the product of an intermolecular Diels- Alder (IMDA) cycloaddition of an achiral azadiene. Since all of the natural products in this family have been isolated in enantiomerically pure form to date, it is believed that an elusive Diels-Alderase enzyme mediates the IMDA reaction. Adding further intrigue to this biosynthetic puzzle is the fact that several related Aspergillus fungi produce a number of metabolites with the opposite absolute configuration, implying that these fungi have evolved enantiomerically distinct Diels-Alderases. We have undertaken a program to identify every step in the biogenesis of the stephacidins and notoamides, and by combining the techniques of chemical synthesis and biochemical analysis we have been able to identify the two prenyltransferases involved in the early stages of the stephacidin and notoamide biosyntheses. This has allowed us to propose a modified biosynthesis for stephacidin A, and has brought us closer to our goal of finding evidence for, or against, the presence of a Diels-Alderase in this biosynthetic pathway.

Concise enantioselective synthesis of ent-malbrancheamide B

A concise enantioselective synthesis of the fungal metabolite ent-malbrancheamide B was accomplished through the union of a C-prenylated proline derivative and a substituted indole pyruvic acid SEM enol ether, followed by a cationic double cyclization as the key step.