Angucycline group antibiotics

Asymmetric Total Synthesis of 4-Hydroxy-8-O-methyltetrangomycin, 4-Hydroxytetrangomycin, and 4-Keto-8-O-methyltetrangomycin

Herein, we report the first asymmetric total synthesis of 4-hydroxy-8-O-methyltetrangomycin (1), 4-hydroxytetrangomycin (2), and 4-keto-8-O-methyltetrangomycin (3), angucyclinones featuring a highly oxidized nonaromatic A ring. A sequential enyne metathesis/Diels-Alder approach was utilized successfully to construct the tetracyclic skeleton of the angucyclinones. Late-stage acetonide deprotection challenges were overcome by A ring functional group manipulation, yielding a dihydroxy intermediate prior to the benzylic photo-oxidation, facilitating the total syntheses of angucyclinones 1-3. The key stereocenter was established through a known Sharpless asymmetric epoxidation/regioselective epoxide opening reaction.

Deciphering the Glycosylation Steps in the Biosynthesis of P-1894B and Grincamycin Isolated from Marine-Derived Streptomyces lusitanus SCSIO LR32

Recently, we re-isolated the glycosylated angucycline antibiotics P-1894B (1) and grincamycin (1') from the marine-derived Streptomyces lusitanus SCSIO LR32 as potent antitumor agents and identified their biosynthesis gene cluster gcn. Both P-1894B (1) and grincamycin (1') possess a trisaccharide and a disaccharide moiety comprised of five deoxysugars. In this work, three genes encoding glycosyltransferases (GcnG1, GcnG2, and GcnG3) responsible for the assembly of deoxysugars into angucycline aglycone were identified from the biosynthesis gene cluster gcn. Gene inactivations of gcnG1, gcnG2, gcnG3, and gcnG1G2 by lambda-RED-mediated gene replacements led to the construction of four mutants, in which the glycosyltransferase genes were disrupted, respectively. The metabolites from the mutants were purified and identified, including two new analogues designated as grincamycin U (3a) and V (3'). The sequential glycosylation steps in the biosynthesis of P-1894B (1) and grincamycin (1') catalyzed by GcnG3, GcnG1, and GcnG2 were elucidated.

Angucyclinones with IDO and TDO inhibitory activities isolated from the actinomycetes Umezawaea beigongshangensis

Four previously undescribed angucyclinones umezawaones A-D (1-4) were isolated from the liquid cultures of Umezawaea beigongshangensis. Their structures were determined by spectroscopic analyses, single crystal X-ray diffraction, quantum chemical 13C NMR and electronic circular dichroism calculations. All compounds displayed strong inhibitory activities against indoleamine 2,3-dioxygenase and tryptophan-2,3-dioxygenase in enzymatic assay, especially compound 2.

Unravelling key enzymatic steps in C-ring cleavage during angucycline biosynthesis

Angucyclines are type II polyketide natural products, often characterized by unusual structural rearrangements through B- or C-ring cleavage of their tetracyclic backbone. While the enzymes involved in B-ring cleavage have been extensively studied, little is known of the enzymes leading to C-ring cleavage. Here, we unravel the function of the oxygenases involved in the biosynthesis of lugdunomycin, a highly rearranged C-ring cleaved angucycline derivative. Targeted deletion of the oxygenase genes, in combination with molecular networking and structural elucidation, showed that LugOI is essential for C12 oxidation and maintaining a keto group at C6 that is reduced by LugOII, resulting in a key intermediate towards C-ring cleavage. An epoxide group is then inserted by LugOIII, and stabilized by the novel enzyme LugOV for the subsequent cleavage. Thus, for the first time we describe the oxidative enzymatic steps that form the basis for a wide range of rearranged angucycline natural products.

Targeted Isolation of N-Acetylcysteine-Containing Angucycline Derivatives from Streptomyces sp. MC16 and Their Antiproliferative Effects

Liquid chromatography-mass spectrometry (LC-MS/MS)-based molecular networking analysis was applied to Streptomyces sp. MC16. The automatic classification of the MolNetEnhancer module revealed that its major constituent was an angucycline derivative. By targeted isolation of unique clusters in the molecular network, which showed different patterns from typical angucycline compounds, two new N-acetylcysteine-attached angucycline derivatives (1 and 2) were isolated. The structures were elucidated based on intensive NMR analysis and high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). All isolated compounds (1-4) were tested for their inhibitory effects on the proliferation of A431, A549, and HeLa cell lines. Antibiotics 100-1 (3) and vineomycinone B2 (4) showed moderate inhibitory effects on these three cell lines with IC50 values ranging from 18.5 to 59.0 μM, while compounds 1 and 2 with an additional N-acetylcysteine residue showed weak inhibitory effects only on the HeLa cell line with IC50 values of 54.7 and 65.2 μM, respectively.

Overexpression of Global Regulator SCrp Leads to the Discovery of New Angucyclines in Streptomyces sp. XS-16

Six angucyclines including three unreported compounds (1-3) were isolated from Streptomyces sp. XS-16 by overexpressing the native global regulator of SCrp (cyclic AMP receptor). The structures were characterized based on nuclear magnetic resonance (NMR) and spectrometry analysis and assisted by electronic circular dichroism (ECD) calculations. All compounds were tested for their antitumor and antimicrobial activities, and compound 1 showed different inhibitory activities against various tumor cell lines with IC₅₀ values ranging from 0.32 to 5.33 μM.

Sharpless Asymmetric Dihydroxylation: An Impressive Gadget for the Synthesis of Natural Products: A Review

Sharpless asymmetric dihydroxylation is an important reaction in the enantioselective synthesis of chiral vicinal diols that involves the treatment of alkene with osmium tetroxide along with optically active quinine ligand. Sharpless introduced this methodology after considering the importance of enantioselectivity in the total synthesis of medicinally important compounds. Vicinal diols, produced as a result of this reaction, act as intermediates in the synthesis of different naturally occurring compounds. Hence, Sharpless asymmetric dihydroxylation plays an important role in synthetic organic chemistry due to its undeniable contribution to the synthesis of biologically active organic compounds. This review emphasizes the significance of Sharpless asymmetric dihydroxylation in the total synthesis of various natural products, published since 2020.

Metabolic adjustments in response to ATP spilling by the small DX protein in a Streptomyces strain

ATP wasting is recognized as an efficient strategy to enhance metabolic activity and productivity of specific metabolites in several microorganisms. However, such strategy has been rarely implemented in Streptomyces species whereas antibiotic production by members of this genus is known to be triggered in condition of phosphate limitation that is correlated with a low ATP content. In consequence, to assess the effects of ATP spilling on the primary and specialized metabolisms of Streptomyces, the gene encoding the small synthetic protein DX, that has high affinity for ATP and dephosphorylates ATP into ADP, was cloned in the integrative vector pOSV10 under the control of the strong ErmE promoter. This construct and the empty vector were introduced into the species Streptomyces albogriseolus/viridodiastaticus yielding A37 and A36, respectively. A37 yielded higher biomass than A36 indicating that the DX-mediated ATP degradation resulted into a stimulation of A37 metabolism, consistently with what was reported in other microorganisms. The comparative analysis of the metabolomes of A36 and A37 revealed that A37 had a lower content in glycolytic and Tricarboxylic Acid Cycle intermediates as well as in amino acids than A36, these metabolites being consumed for biomass generation in A37. In contrast, the abundance of other molecules indicative either of energetic stress (ADP, AMP, UMP, ornithine and thymine), of activation (NAD and threonic acid) or inhibition (citramalic acid, fatty acids, TAG and L-alanine) of the oxidative metabolism, was higher in A37 than in A36. Furthermore, hydroxyl-pyrimidine derivatives and polycyclic aromatic polyketide antibiotics belonging to the angucycline class and thought to have a negative impact on respiration were also more abundantly produced by A37 than by A36. This comparative analysis thus revealed the occurrence in A37 of antagonistic metabolic strategies, namely, activation or slowing down of oxidative metabolism and respiration, to maintain the cellular energetic balance. This study thus demonstrated that DX constitutes an efficient biotechnological tool to enhance the expression of the specialized metabolic pathways present in the Streptomyces genomes that may include cryptic pathways. Its use thus might lead to the discovery of novel bioactive molecules potentially useful to human health.

New metal-free one-pot synthesis of α-2-deoxy-Ulosides by microwave-assisted double Michael Addition of β-enamino ketones

New metal-free one-pot synthesis of α-2-deoxy-ulosides in moderate to good yields by microwave-assisted double Michael addition of various O-nucleophiles to β-enamino ketones in the presence of 12 N HCl. These glycosyl additions occurred with high α-stereoselectivity and were complete in 10-25 min in 51-93% yield. In addition, high α-stereoselectivity was also observed when S-nucleophiles were examined.

Characterization of the Biosynthetic Gene Cluster and Shunt Products Yields Insights into the Biosynthesis of Balmoralmycin

Angucyclines are a family of structurally diverse, aromatic polyketides with some members that exhibit potent bioactivity. Angucyclines have also attracted considerable attention due to the intriguing biosynthetic origins that underlie their structural complexity and diversity. Balmoralmycin (compound 1) represents a unique group of angucyclines that contain an angular benz[α]anthracene tetracyclic system, a characteristic C-glycosidic bond-linked deoxy-sugar (d-olivose), and an unsaturated fatty acid chain. In this study, we identified a Streptomyces strain that produces balmoralmycin and seven biosynthetically related coproducts (compounds 2-8). Four of the coproducts (compounds 5-8) are novel compounds that feature a highly oxygenated or fragmented lactone ring, and three of them (compounds 3-5) exhibited cytotoxicity against the human pancreatic cancer cell line MIA PaCa-2 with IC₅₀ values ranging from 0.9 to 1.2 μg/mL. Genome sequencing and CRISPR/dCas9-assisted gene knockdown led to the identification of the ~43 kb balmoralmycin biosynthetic gene cluster (bal BGC). The bal BGC encodes a type II polyketide synthase (PKS) system for assembling the angucycline aglycone, six enzymes for generating the deoxysugar d-olivose, and a hybrid type II/III PKS system for synthesizing the 2,4-decadienoic acid chain. Based on the genetic and chemical information, we propose a mechanism for the biosynthesis of balmoralmycin and the shunt products. The chemical and genetic studies yielded insights into the biosynthetic origin of the structural diversity of angucyclines. IMPORTANCE Angucyclines are structurally diverse aromatic polyketides that have attracted considerable attention due to their potent bioactivity and intriguing biosynthetic origin. Balmoralmycin is a representative of a small family of angucyclines with unique structural features and an unknown biosynthetic origin. We report a newly isolated Streptomyces strain that produces balmoralmycin in a high fermentation titer as well as several structurally related shunt products. Based on the chemical and genetic information, a biosynthetic pathway that involves a type II polyketide synthase (PKS) system, cyclases/aromatases, oxidoreductases, and other ancillary enzymes was established. The elucidation of the balmoralmycin pathway enriches our understanding of how structural diversity is generated in angucyclines and opens the door for the production of balmoralmycin derivatives via pathway engineering.

New Angucycline Glycosides from a Marine-Derived Bacterium Streptomyces ardesiacus

Chemical investigation of the ethyl acetate extract from the culture broth of the marine-derived actinobacterium Streptomyces ardesiacus 156VN-095 led to the isolation of three hitherto undescribed angucycline glycosides, including urdamycins W and X (1 and 2) and grincamycin U (9), as well as their seven known congeners. The structures of the new compounds were elucidated by means of spectroscopic methods (HRESIMS, 1D and 2 D NMR) and comparison of their experimental data with literature values. Compounds 1-3 and 9 were evaluated for their anti-Gram-positive bacterial effect and cytotoxicity against six cancer cell lines. Compound 1 displayed significant cytotoxicity against all the tested cell lines with GI₅₀ values of 0.019-0.104 µM. Collectively, these findings highlight the potential of angucycline glycosides as leading structures for the development of new anti-cancer drugs.

Saquayamycin B1 Suppresses Proliferation, Invasion, and Migration by Inhibiting PI3K/AKT Signaling Pathway in Human Colorectal Cancer Cells

Moromycin B (Mor B), saquayamycin B₁ (Saq B₁), saquayamycin B (Saq B), and landomycin N (Lan N), four angucyclines produced by the marine-derived actinomycete Streptomyces sp., are a class of polyketone compounds containing benzanthracene. Here, the structure–activity relationship of these four compounds was analyzed in human colorectal cancer (CRC) cells. Saq B₁, which showed the strongest cytotoxicity with an IC₅₀ of 0.18–0.84 µM for CRC cells in MTT assays, was employed to test underlying mechanisms of action in SW480 and SW620 cells (two invasive CRC cell lines). Our results showed that Saq B₁ inhibited CRC cell proliferation in a dose- and time-dependent manner. Notably, lower cytotoxicity was measured in normal human hepatocyte cells (QSG-7701). Furthermore, we observed proapoptosis, antimigration, and anti-invasion activities of Saq B₁ in CRC cells. At the same time, the protein and mRNA expression of important markers related to the epithelial–mesenchymal transition (EMT) and apoptosis changed, including N-cadherin, E-cadherin, and Bcl-2, in Saq B₁-treated CRC cells. Surprisingly, the PI3K/AKT signaling pathway was shown to be involved in Saq B₁-induced apoptosis, and in inhibiting invasion and migration. Computer docking models also suggested that Saq B₁ might bind to PI3Kα. Collectively, these results indicate that Saq B₁ effectively inhibited growth and decreased the motor ability of CRC cells by regulating the PI3K/AKT signaling pathway, which provides more possibilities for the development of drugs in the treatment of CRC.

Micromonospora: A Prolific Source of Bioactive Secondary Metabolites with Therapeutic Potential

Micromonospora, one of the most important actinomycetes genera, is well-known as the treasure trove of bioactive secondary metabolites (SMs). Herein, together with an in-depth genomic analysis of the reported Micromonospora strains, all SMs from this genus are comprehensively summarized, containing structural features, bioactive properties, and mode of actions as well as their biosynthetic and chemical synthesis pathways. The perspective enables a detailed view of Micromonospora-derived SMs, which will enrich the chemical diversity of natural products and inspire new drug discovery in the pharmaceutical industry.

Integrated Metabolomic, Molecular Networking, and Genome Mining Analyses Uncover Novel Angucyclines From Streptomyces sp. RO-S4 Strain Isolated From Bejaia Bay, Algeria

Multi-omic approaches have recently made big strides toward the effective exploration of microorganisms, accelerating the discovery of new bioactive compounds. We combined metabolomic, molecular networking, and genomic-based approaches to investigate the metabolic potential of the Streptomyces sp. RO-S4 strain isolated from the polluted waters of Bejaia Bay in Algeria. Antagonistic assays against methicillin-resistant Staphylococcus aureus with RO-S4 organic extracts showed an inhibition zone of 20 mm by using the agar diffusion method, and its minimum inhibitory concentration was 16 μg/ml. A molecular network was created using GNPS and annotated through the comparison of MS/MS spectra against several databases. The predominant compounds in the RO-S4 extract belonged to the angucycline family. Three compounds were annotated as known metabolites, while all the others were putatively new to Science. Notably, all compounds had fridamycin-like aglycones, and several of them had a lactonized D ring analogous to that of urdamycin L. The whole genome of Streptomyces RO-S4 was sequenced to identify the biosynthetic gene cluster (BGC) linked to these angucyclines, which yielded a draft genome of 7,497,846 bp with 72.4% G+C content. Subsequently, a genome mining analysis revealed 19 putative biosynthetic gene clusters, including a grincamycin-like BGC with high similarity to that of Streptomyces sp. CZN-748, that was previously reported to also produce mostly open fridamycin-like aglycones. As the ring-opening process leading to these compounds is still not defined, we performed a comparative analysis with other angucycline BGCs and advanced some hypotheses to explain the ring-opening and lactonization, possibly linked to the uncoupling between the activity of GcnE and GcnM homologs in the RO-S4 strain. The combination of metabolomic and genomic approaches greatly improved the interpretation of the metabolic potential of the RO-S4 strain.

Kumemicinones A-G, Cytotoxic Angucyclinones from a Deep Sea-Derived Actinomycete of the Genus Actinomadura

Chemical investigation of the fermentation products of a deep sea water-derived actinomycete, Actinomadura sp. KD439, identified seven new angucyclinones, designated as kumemicinones A-G (1-7), together with the known SF2315B and miaosporone E. NMR and MS spectroscopic analyses, combined with X-ray crystallography and quantum chemical calculations of NMR chemical shifts and electronic circular dichroism (ECD) spectra, uncovered the structures of new angucyclinones as regioisomers of SF2315B at the allyl alcohol unit (1 and 2), an epoxy ring-opened γ-hydroxy enone isomer (3), a B/C-ring-rearranged product (4), or dimers with a new mode of bridging (5-7), adding new structural variation to this antibiotic group. The absolute configuration of SF2315B was also determined by comparison of ECD spectra with those of 1 and 2. All the angucyclinones exhibited cytotoxicity against P388 murine leukemia cells, with IC₅₀ values ranging from 1.8 to 53 μM.

Purification and characterization of anti-phytopathogenic fungi angucyclinone from soil-derived Streptomyces cellulosae

Actinomycete strain YIM PH20352, isolated from the rhizosphere soil sample of Panax notoginseng collected in WenShang, Yunnan Province, China, exhibited antifungal activity against some phytopathogenic fungi. The structures of bioactive molecules, isolated from the ethyl acetate extract of the fermentation broth of the strain, were identified as rabelomycin (1) and dehydrorabelomycin (2) based on extensive spectroscopic analyses. Compound 1 exhibited antifungal activity against four tested root-rot pathogens of the Panax notoginseng including Plectosphaerella cucumerina, Alternaria panax, Fusarium oxysporum, and Fusarium solani with the MIC values at 32, 64, 128, and 128 μg/mL, respectively. Compound 2 exhibited antifungal activity against F. oxysporum, P. cucumerina, F. solani, and A. panax with the MIC values at 64, 64, 128, and 128 μg/mL, respectively. Based on the phylogenetic analyses, the closest phylogenetic relative of strain YIM PH20352 is Streptomyces cellulosae NBRC 13027 ^T (AB184265) (99.88%), so strain YIM PH20352 was identified as Streptomyces cellulosae. To the best of our knowledge, this is the first report of rabelomycin and rabelomycin-type antibiotics from Streptomyces cellulosae and their antifungal activity against root-rot pathogens of the Panax notoginseng.

Dimeric Tetracenomycin Derivatives from a Taklamakan Desert-Derived Streptomyces sp. HDN154193

Bitetracenomycin A (1) and its diastereomers [(±)-bitetracenomycin B, (±)-2] were discovered from the cultures of Streptomyces sp. HDN154193. Compounds 1 and (±)-2 were the first tetracenomycin dimers obtained from a natural source with sp³ methine protons at the bridge positions (C-12/12'), which also exhibited broad-spectrum antibacterial activity. The racemate (±)-2 was semisynthesized and separated into enantiomers (+)-2 and (-)-2, and the absolute configurations were determined by specific rotation and ECD data. These metabolites exhibited potent antibacterial activity especially against drug-resistant strains (MRSA and MRCNS) with MIC values ranging from 1.0 to 1.9 μg/mL.

Sterically Strained Brønsted Pair Catalysis by Bulky Pyridinium Salts: Direct Stereoselective Synthesis of 2-Deoxy and 2,6-Dideoxy-β-thioglycosides from Glycals

A sterically strained ionic Brønsted pair complex obtained from a sterically bulky base 2,4,6-tri-tert-butylpyridine and hydrochloric acid imbues unusual reactivity to the anionic chloride. The complete shielding of the cationic [N-H]⁺ by the bulky ortho-tert-butyl groups weakens the possible hydrogen-bonding interactions with the chloride anion, and the [N-H]⁺···Cl^- distance is unusually longer (3.10 Å). This results in strained/frustrated electrostatic interactions between the ion-pair, thus infusing an increased reactivity in both of the ions, which results in the activation of a third molecule like thiol via hydrogen-bonding. This intriguing weak interaction-based reactivity has been utilized to develop an organocatalytic synthesis of 2-deoxy-β-thioglycosides from glycals. While the ¹H NMR studies showcase the diamagnetic activation of thiols in the presence of the catalyst, the electron paramagnetic resonance (EPR) studies reveal the generation of a radical species that suggests a possible frustrated radical pair catalysis. Besides, IR spectroscopic studies explain the intriguing influence of size/charge density of the anion on the solvation-insusceptible, cationic [TTBPyH]⁺ and on the observed reactivity.

Genome-Guided Discovery of Highly Oxygenated Aromatic Polyketides, Saccharothrixins D-M, from the Rare Marine Actinomycete Saccharothrix sp. D09

Angucyclines and angucyclinones are aromatic polyketides with intriguing structures and therapeutic value. Genome mining of the rare marine actinomycete Saccharothrix sp. D09 led to the identification of a type II polyketide synthase biosynthetic gene cluster, sxn, which encodes several distinct subclasses of oxidoreductases, implying that this strain has the potential to produce novel polycyclic aromatic polyketides with unusual redox modifications. The "one strain-many compounds" (OSMAC) strategy and comparative metabolite analysis facilitated the discovery of 20 angucycline derivatives from the D09 strain, including six new highly oxygenated saccharothrixins D-I (1-6), four new glycosylated saccharothrixins J-M (7-10), and 10 known analogues (11-20). Their structures were elucidated based on detailed HRESIMS, NMR spectroscopic, and X-ray crystallographic analysis. With the help of gene disruption and heterologous expression, we proposed their plausible biosynthetic pathways. In addition, compounds 3, 4, and 8 showed antibacterial activity against Helicobacter pylori with MIC values ranging from 16 to 32 μg/mL. Compound 3 also revealed anti-inflammatory activity by inhibiting the production of NO with an IC₅₀ value of 28 μM.

Landomycins as glutathione-depleting agents and natural fluorescent probes for cellular Michael adduct-dependent quinone metabolism

Landomycins are angucyclines with promising antineoplastic activity produced by Streptomyces bacteria. The aglycone landomycinone is the distinctive core, while the oligosaccharide chain differs within derivatives. Herein, we report that landomycins spontaneously form Michael adducts with biothiols, including reduced cysteine and glutathione, both cell-free or intracellularly involving the benz[a]anthraquinone moiety of landomycinone. While landomycins generally do not display emissive properties, the respective Michael adducts exerted intense blue fluorescence in a glycosidic chain-dependent manner. This allowed label-free tracking of the short-lived nature of the mono-SH-adduct followed by oxygen-dependent evolution with addition of another SH-group. Accordingly, hypoxia distinctly stabilized the fluorescent mono-adduct. While extracellular adduct formation completely blocked the cytotoxic activity of landomycins, intracellularly it led to massively decreased reduced glutathione levels. Accordingly, landomycin E strongly synergized with glutathione-depleting agents like menadione but exerted reduced activity under hypoxia. Summarizing, landomycins represent natural glutathione-depleting agents and fluorescence probes for intracellular anthraquinone-based angucycline metabolism.

Genome mining of novel rubiginones from Streptomyces sp. CB02414 and characterization of the post-PKS modification steps in rubiginone biosynthesis

Background

Rubiginones belong to the angucycline family of aromatic polyketides, and they have been shown to potentiate the vincristine (VCR)-induced cytotoxicity against VCR-resistant cancer cell lines. However, the biosynthetic gene clusters (BGCs) and biosynthetic pathways for rubiginones have not been reported yet.

Results

In this study, based on bioinformatics analysis of the genome of Streptomyces sp. CB02414, we predicted the functions of the two type II polyketide synthases (PKSs) BGCs. The rub gene cluster was predicted to encode metabolites of the angucycline family. Scale-up fermentation of the CB02414 wild-type strain led to the discovery of eight rubiginones, including five new ones (rubiginones J, K, L, M, and N). Rubiginone J was proposed to be the final product of the rub gene cluster, which features extensive oxidation on the A-ring of the angucycline skeleton. Based on the production profiles of the CB02414 wild-type and the mutant strains, we proposed a biosynthetic pathway for the rubiginones in CB02414.

Conclusions

A genome mining strategy enabled the efficient discovery of new rubiginones from Streptomyces sp. CB02414. Based on the isolated biosynthetic intermediates, a plausible biosynthetic pathway for the rubiginones was proposed. Our research lays the foundation for further studies on the mechanism of the cytochrome P450-catalyzed oxidation of angucyclines and for the generation of novel angucyclines using combinatorial biosynthesis strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01681-5.

6,9-Dihydroxytetrangulol, a novel angucyclinone antibiotic accumulated in kiqO gene disruptant in the biosynthesis of kinanthraquinone

A novel angucyclinone, 6,9-dihydroxytetrangulol, was isolated from Streptomyces lividans TK23 transformed with a kinanthraquinone biosynthetic gene cluster in which the kiqO gene was disrupted. The chemical structure was elucidated by spectroscopic analyses. It showed significant antibacterial activities with an IC₅₀ value of 1.9 μM against Staphylococcus aureus and moderate anticancer activities against HL-60 cells.

Himalaquinones A-G, Angucyclinone-Derived Metabolites Produced by the Himalayan Isolate Streptomyces sp. PU-MM59

Himalaquinones A-G, seven new anthraquinone-derived metabolites, were obtained from the Himalayan-based Streptomyces sp. PU-MM59. The chemical structures of the new compounds were identified based on cumulative analyses of HRESIMS and NMR spectra. Himalaquinones A-F were determined to be unique anthraquinones that contained unusual C-4a 3-methylbut-3-enoic acid aromatic substitutions, while himalaquinone G was identified as a new 5,6-dihydrodiol-bearing angucyclinone. Comparative bioactivity assessment (antimicrobial, cancer cell line cytotoxicity, impact on 4E-BP1 phosphorylation, and effect on axolotl embryo tail regeneration) revealed cytotoxic landomycin and saquayamycin analogues to inhibit 4E-BP1p and inhibit regeneration. In contrast, himalaquinone G, while also cytotoxic and a regeneration inhibitor, did not affect 4E-BP1p status at the doses tested. As such, this work implicates a unique mechanism for himalaquinone G and possibly other 5,6-dihydrodiol-bearing angucyclinones.

The inverting mechanism of the metal ion-independent LanGT2: the first step to understand the glycosylation of natural product antibiotic precursors through QM/MM simulations

Glycosyltransferases (GTs) from the GT1 family are responsible for the glycosylation of various important organic structures such as terpenes, steroids and peptide antibiotics, making it one of the most intensely studied families of GTs. The target of our study, LanGT2, is a member of the GT1 family that uses an inverting mechanism for transferring olivose from TDP-olivose, the donor substrate, to the natural product tetrangulol (Tet), the precursor of the antibiotic landomycin A. X-ray crystallography in conjunction with mutagenesis experiments has revealed the catalytic significance of 3 amino acids (Ser10, Ser219 and Asp137), suggesting Asp137 as the base catalyst. In the absence of X-ray structures that include the acceptor substrate Tet, in silico experiments and MD simulations that have modeled ternary complexes propose that Asp137 could recruit a water molecule to facilitate the nucleophilic activation of Tet, since the distance between Asp137 and the nucleophile is too long to directly deprotonate the nucleophilic moiety. So far, there is no computational evidence regarding the precise mechanism by which LanGT2 catalyzes the transfer of olivose, which raises questions such as: is a water-assisted mechanism possible? and how does this metal ion-independent GT stabilize the growing negative charge of the diphosphate leaving group? In this work, the QM/MM approach was used to unravel the catalytic mechanism of LanGT2, and to identify the role of crucial catalytic amino acids at a molecular level. Our calculations show that the minimum energy path (MEP) describes an SN2-like mechanism, identifying an oxocarbenium ion-like TS in which the olivosyl moiety adopts a 4H3 conformation. Interactions established between the diphosphate group of TDP and Ser10, Ser219, Arg220 and His283 are key to stabilize the development of charge on the leaving group. Our work also suggests that a water-mediated proton transfer mechanism is feasible, in which the water molecule is key to stabilize the phenolate ion-like nucleophile in the TS. This is the first computational insight into the inverting mechanism of an antibiotic natural product GT, and its implications may serve to guide the design of new biocatalysts for natural product glycodiversification.

Angucyclines containing β-ᴅ-glucuronic acid from Streptomyces sp. KCB15JA151

Two angucyclines, pseudonocardones D (1) and E (2), were isolated from Streptomyces sp. KCB15JA151. The planar structure was elucidated by comprehensive spectroscopic analysis. The absolute configuration of the sugar unit was determined based on the basis of coupling constants, ROESY, chemical derivatization and HPLC analysis. The biological activities of compounds 1 and 2 were examined by performing a computational target prediction, which led to tests of the antiestrogenic activity. The result suggested that compound 1 might be an ERα antagonist.

Marine Anthraquinones: Pharmacological and Toxicological Issues

The marine ecosystem, populated by a myriad of animals, plants, and microorganisms, is an inexhaustible reservoir of pharmacologically active molecules. Among the multiple secondary metabolites produced by marine sources, there are anthraquinones and their derivatives. Besides being mainly known to be produced by terrestrial species, even marine organisms and the uncountable kingdom of marine microorganisms biosynthesize anthraquinones. Anthraquinones possess many different biological activities, including a remarkable antitumor activity. However, due to their peculiar chemical structures, anthraquinones are often associated with toxicological issues, even relevant, such as genotoxicity and mutagenicity. The aim of this review is to critically describe the anticancer potential of anthraquinones derived from marine sources and their genotoxic and mutagenic potential. Marine-derived anthraquinones show a promising anticancer potential, although clinical studies are missing. Additionally, an in-depth investigation of their toxicological profile is needed before advocating anthraquinones as a therapeutic armamentarium in the oncological area.

Identification and characterization of isocitrate dehydrogenase 1 (IDH1) as a functional target of marine natural product grincamycin B

Grincamycins (GCNs) are a class of angucycline glycosides isolated from actinomycete Streptomyces strains that have potent antitumor activities, but their antitumor mechanisms remain unknown. In this study, we tried to identify the cellular target of grincamycin B (GCN B), one of most dominant and active secondary metabolites, using a combined strategy. We showed that GCN B-selective-induced apoptosis of human acute promyelocytic leukemia (APL) cell line NB4 through increase of ER stress and intracellular reactive oxygen species (ROS) accumulation. Using a strategy of combining phenotype, transcriptomics and protein microarray approaches, we identified that isocitrate dehydrogenase 1(IDH1) was the putative target of GCN B, and confirmed that GCNs were a subset of selective inhibitors targeting both wild-type and mutant IDH1 in vitro. It is well-known that IDH1 converts isocitrate to 2-oxoglutarate (2-OG), maintaining intracellular 2-OG homeostasis. IDH1 and its mutant as the target of GCN B were validated in NB4 cells and zebrafish model. Knockdown of IDH1 in NB4 cells caused the similar phenotype as GCN B treatment, and supplementation of N-acetylcysteine partially rescued the apoptosis caused by IDH1 interference in NB4 cells. In zebrafish model, GCN B effectively restored myeloid abnormality caused by overexpression of mutant IDH1(R132C). Taken together, we demonstrate that IDH1 is one of the antitumor targets of GCNs, suggesting wild-type IDH1 may be a potential target for hematological malignancies intervention in the future.

Investigation of angucycline compounds as potential drug candidates against SARS Cov-2 main protease using docking and molecular dynamic approaches

Abstract

The emerged Coronavirus disease (COVID-19) causes severe or even fatal respiratory tract infection, and to date there is no FDA-approved therapeutics or effective treatment available to effectively combat this viral infection. This urgent situation is an attractive research area in the field of drug design and development. One of the most important targets of SARS-coronavirus-2 (SARS Cov-2) is the main protease (3CLpro). Actinomycetes are important resources for drug discovery. The angucylines that are mainly produced by Streptomyces genus of actinomycetes exhibit a broad range of biological activities such as anticancer, antibacterial and antiviral. This study aims to investigate the binding affinity and molecular interactions of 157 available angucycline compounds with 3CLpro using docking and molecular dynamics simulations. MM-PBSA calculations showed that moromycin A has a better binding energy (− 30.42 kcal mol⁻¹) compared with other ligands (in a range of − 18.66 to − 22.89 kcal mol⁻¹) including saquayamycin K4 (− 21.27 kcal mol⁻¹) except the co-crystallized ligand N3. However, in vitro and in vivo studies are essential to assess the effectiveness of angucycline compounds against coronavirus.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s11030-021-10219-1.

Disclosing biosynthetic connections and functions of atypical angucyclinones with a fragmented C-ring

This review on atypical angucyclinones possessing an aromatic cleavage of the C-ring covers literature between 1995 and early 2020.The unusual framework of the middle C-ring, "broken" as a result of biotransformations and oxidations in vivo and bearing an sp³-C connection, is of interest for biosynthetic investigations. The reported 39 natural compounds (55 including stereoisomers) have been analyzed and arranged into three structural groups. The biosynthetic origin of all these compounds has been thoroughly reviewed and revised, based on the found connections with oxidized angucyclinone structures. The data on biological activities has been summarized. Careful consideration of the origin of the structure allowed us to outline a hypothesis on the biological function as well as prospective applications of such atypical angucyclinones.

Asymmetric Total Synthesis of PD-116740

A new approach was developed to achieve the asymmetric total synthesis of (+)-PD-116740, an angucyclinone from the actinomycete isolate (WP 4669). A sequence of asymmetric dihydroxylation followed by oxidative cyclization was applied to stereoselectively construct the core trans-9,10-dihydrophenanthrene-9,10-diol B-C-D ring. A new Cu salt Cu(OH)OTf·NMI₂ was found to be the best oxidant to induce the oxidative coupling and phenol oxidation.

Unusual bridged angucyclinones and potent anticancer compounds from Streptomyces bulli GJA1

Two new unique angucyclinones (1 and 2) with unprecedented ether bridges connecting carbons 5 and 7 were isolated from the cultures of Streptomyces bulli GJA1, an endophyte of Gardenia jasminoides, together with two known ones (3 and 4). The MS2-based molecular networking system facilitated the isolation of compounds with target functionalities. The stereochemistry of 1 was completely established by ROESY and ECD experiments. Compound 3 showed antivirulence activities by inhibiting the peptide toxin (PSMs) production and the biofilm formation of MRSA. Compounds 3 and 4 showed very potent anti-proliferative effects against OV1 and ES2 ovarian cancer cells.

Emerging Organometallic Methods for the Synthesis of C-Branched (Hetero)aryl, Alkenyl, and Alkyl Glycosides: C-H Functionalization and Dual Photoredox Approaches

Transition-metal-catalyzed C-H functionalization and photoredox nickel dual catalysis have emerged as innovative and powerful avenues for the synthesis of C-branched glycosides. These two concepts have been recently established and provide efficient and mild methods for accessing a series of valuable complex C-branched glycosides of great interest. Herein, recent developments in the synthesis of C-branched aryl/alkenyl/alkyl glycosides through these two approaches are highlighted.

Development of Routes for the Stereoselective Preparation of β-Aryl-C-glycosides via C-1 Aryl Enones

A wide range of enones derived from d-glucal, d-galactal, l-rhamnal, d-rhamnal, and l-arabinal underwent Heck-coupling with various arylboronic acids bearing electron-donating and -withdrawing groups in the presence of palladium acetate and 1,10-phenanthroline. These reactions provided synthetically useful C-1 aryl enones in good yields. Many sensitive functional groups as well as protecting groups present in arylboronic acids and enones, respectively, remained intact under optimized conditions. The stereoselective hydrogenation of C-1 aryl enones with Pd-C/H₂ provides the β-isomer of 2-deoxy-aryl-C-glycosides in excellent yield. The C-1 aryl enones were also used as precursors for the synthesis of 2-hydroxy-β-aryl-C-glycosides. Regioselective C-2 halogenations and vinylations of C-1 aryl enones were achieved in excellent yields.

Monacycliones G-K and ent-Gephyromycin A, Angucycline Derivatives from the Marine-Derived Streptomyces sp. HDN15129

Six new angucycline derivatives, named monacycliones G-K (1-5) and ent-gephyromycin A (6), as well as three known ones (7-9) were discovered from the marine sediment-derived actinomycete Streptomyces sp. HDN15129 guided by Global Natural Products Social (GNPS) molecular networking. Structures including absolute configurations were elucidated by extensive NMR, MS, and ECD analyses. Among them, monacyclione G (1) possesses a unique scaffold featuring a xanthone core linked to the aminodeoxysugar ossamine, and monacycliones H-J (2-4) are rare examples of natural angucyclines with an S-methyl group. Monacycliones I and J (3 and 4) showed cytotoxic activity against multiple human cancer cell lines, with IC₅₀ values ranging from 3.5 to 10 μM.

Marine Sediment-Derived Streptomyces Strain Produces Angucycline Antibiotics against Multidrug-Resistant Staphylococcus aureus Harboring SCCmec Type 1 Gene

The Philippine archipelago is geographically positioned in the tropics with rich areas of marine biodiversity. Its marine sediments harbor actinomycetes that exhibit antibacterial activity. Screening of actinomycetes isolated from marine sediments collected near the coast of Islas de Gigantes, Iloilo showed one isolate that exhibited high activity against the multidrug-resistant Staphylococcus aureus (MRSA) strain carrying the Staphylococcal Cassette Chromosome mec (SCCmec) type 1 gene, a biomarker for drug resistance. The isolate was identified as Streptomyces sp. strain DSD011 based on its 16s rRNA and protein-coding genes (atpD, recA, rpoB, and trpB) sequences, and was found to be a new species of salt-tolerant marine Streptomyces. Further, the strain harbors both non-ribosomal peptide synthetase (NRPS) and type II polyketide synthase (PKS) in its genome. The targeted chromatographic isolation and chemical investigations by Liquid Chromatography Mass Spectrometry-Time of Flight (LCMS-TOF), tandem mass spectrometry (MS/MS), and Global Natural Product Social molecular networking (GNPS) of the antibiotics produced by the strain afforded the two polycyclic aromatic polyketide angucycline glycosides, fridamycin A (1) and fridamycin D (2), which are products of type II PKS biosynthesis. Compounds 1 and 2 displayed antibacterial activity against MRSA with minimum inhibitory concentration (MIC) of 500 μg/mL and 62.5 μg/mL, respectively. These results suggest that the underexplored marine sediments near the coast of Islas de Gigantes, Iloilo offer access to undiscovered Streptomyces species that are invaluable sources of antibiotic leads.

Functional and Structural Insights into a Novel Promiscuous Ketoreductase of the Lugdunomycin Biosynthetic Pathway

Angucyclines are a structurally diverse class of actinobacterial natural products defined by their varied polycyclic ring systems, which display a wide range of biological activities. We recently discovered lugdunomycin (1), a highly rearranged polyketide antibiotic derived from the angucycline backbone that is synthesized via several yet unexplained enzymatic reactions. Here, we show via in vivo, in vitro, and structural analysis that the promiscuous reductase LugOII catalyzes both a C6 and an unprecedented C1 ketoreduction. This then sets the stage for the subsequent C-ring cleavage that is key to the rearranged scaffolds of 1. The 1.1 Å structures of LugOII in complex with either ligand 8-O-Methylrabelomycin (4) or 8-O-Methyltetrangomycin (5) and of apoenzyme were resolved, which revealed a canonical Rossman fold and a remarkable conformational change during substrate capture and release. Mutational analysis uncovered key residues for substrate access, position, and catalysis as well as specific determinants that control its dual functionality. The insights obtained in this work hold promise for the discovery and engineering of other promiscuous reductases that may be harnessed for the generation of novel biocatalysts for chemoenzymatic applications.

Antibacterial and Cytotoxic Bridged and Ring Cleavage Angucyclinones From a Marine Streptomyces sp

Chemical investigation of a marine-derived Streptomyces sp. KCB-132, cultivated in liquid ISP2 medium, had led to the discovery of three C-ring cleavage angucyclinone N-heterocycles, pratensilins A–C, with a novel spiro indolinone-naphthofuran skeleton. Addition of 50 μM LaCl₃ to the same medium and subsequent chemical analysis of this strain returned a new member of this rare class, pratensilin D (1), along with two new angucyclinone derivatives, featuring ether-bridged (2) and A-ring cleavage (3) structural properties. Their structures and absolute configurations were assigned by spectroscopic analysis, single-crystal X-ray diffractions, and equivalent circulating density (ECD) calculations. (+)- and (–)-1, a pair of enantiomeric nitrogen-containing angucyclinones, exhibited different strengths of antibacterial and cytotoxic activities.

Biosynthesis, synthetic studies, and biological activities of the jadomycin alkaloids and related analogues

The jadomycins are an expanding class of compounds produced from Streptomyces venezuelae, by diverting the normal biosynthesis which provides the antibiotic chloramphenicol. In the presence of amino acids, and either by heat shock, supplementation with ethanol, or when phage SV1 is added to the culture, the formation of substituted jadomycins and benzo[b]phenanthridines can be achieved. The first part of this review provides details of intermediates involved in the biosynthesis of the jadomycins and the related benzo[b]phenanthridines. Both the jadomycins and the benzo[b]phenanthridines share biosynthetic pathways with a large class of naturally occurring compounds known as the angucyclines. The biosynthetic pathways diverge when it is postulated that an intermediate quinone, such as 3-(2-formyl-6-hydroxy-4-methylphenyl)-8-hydroxy-1,4-naphthoquinone-2-carboxylic acid is formed. The quinone then undergoes reactions with amino acids and derivatives in the culture medium to ultimately afford a library of jadomycins and a few benzo[b]phenanthridines. The second part of the review initially details synthetic efforts toward the synthesis of the naturally occurring benzo[b]phenanthridine, phenanthroviridin, and then outlines methods that have been used to assemble a selection of jadomycins. Total syntheses of jadomycin A and B, derived from l-isoleucine, are described. In addition, the synthesis of the aglycon of jadomycins M, W, S, and T is outlined. These four jadomycins were derived from l-methionine, l-tryptophan, l-serine and l-threonine respectively. As a result of these synthetic efforts, the structures of jadomycin S and T have been revised. The third part of the review describes the reported antibacterial and anticancer activities of both the jadomycins and some naturally occurring benzo[b]phenanthridines.

Catalytic C-C Cleavage/Alkyne-Carbonyl Metathesis Sequence of Cyclobutanones

A ring-opening/alkyne-carbonyl metathesis sequence of alkyne-tethered cyclobutanones catalyzed by AgSbF₆ is realized for the first time to furnish multisubstituted naphthyl ketones under mild conditions. A range of substrates decorated with various substituents at different positions were all well accommodated. Preliminary mechanistic studies show that silver salt acted as a Lewis acid to facilitate both C-C cleavage of the cyclobutanone moiety and the subsequent metathesis between C═O and C≡C bonds.

Baikalomycins A-C, New Aquayamycin-Type Angucyclines Isolated from Lake Baikal Derived Streptomyces sp. IB201691-2A

Natural products produced by bacteria found in unusual and poorly studied ecosystems, such as Lake Baikal, represent a promising source of new valuable drug leads. Here we report the isolation of a new Streptomyces sp. strain IB201691-2A from the Lake Baikal endemic mollusk Benedictia baicalensis. In the course of an activity guided screening three new angucyclines, named baikalomycins A–C, were isolated and characterized, highlighting the potential of poorly investigated ecological niches. Besides that, the strain was found to accumulate large quantities of rabelomycin and 5-hydroxy-rabelomycin, known shunt products in angucyclines biosynthesis. Baikalomycins A–C demonstrated varying degrees of anticancer activity. Rabelomycin and 5-hydroxy-rabelomycin further demonstrated antiproliferative activities. The structure elucidation showed that baikalomycin A is a modified aquayamycin with β-d-amicetose and two additional hydroxyl groups at unusual positions (6a and 12a) of aglycone. Baikalomycins B and C have alternating second sugars attached, α-l-amicetose and α-l-aculose, respectively. The gene cluster for baikalomycins biosynthesis was identified by genome mining, cloned using a transformation-associated recombination technique and successfully expressed in S. albus J1074. It contains a typical set of genes responsible for an angucycline core assembly, all necessary genes for the deoxy sugars biosynthesis, and three genes coding for the glycosyltransferase enzymes. Heterologous expression and deletion experiments allowed to assign the function of glycosyltransferases involved in the decoration of baikalomycins aglycone.