Grisemycin, a Bridged Angucyclinone with a Methylsulfinyl Moiety from a Marine-Derived Streptomyces sp

Identification of an Activity Selector for the Nitroso-Forming Activity in Bacterial Type-III Copper Enzymes

O-aminophenol oxidases, a specialized subclass of type-III copper proteins, play a crucial role in the biosynthesis of bioactive nitrosophenols, which display antiretroviral and blood cholesterol-lowering activity. Another related subclass of type-III copper proteins, tyrosinases, closely resemble o-aminophenol oxidases structurally and enzymatically but lack their unique ability to oxidize o-aminophenols into nitrosophenols. To unpuzzle the catalytic disparities of both subclasses, highly conserved amino acid residues in the vicinity of the catalytic center were identified. Notably, the Asn43 residue in the o-aminophenol oxidase from Streptomyces griseus (SgGriF) plays a pivotal role in its nitroso-forming activity. Mutating the Asn43 residue in SgGriF to isoleucine, which is present at the homologous position Ile42 in the tyrosinase from Streptomyces sp. ZL-24 (SzTYR), resulted in the loss of the nitroso-forming activity in SgGriF. Conversely, exchanging Ile42 in SzTYR to asparagine generates nitroso-forming activity in SzTYR. The results presented herein demonstrate the feasibility of converting an o-aminophenol oxidase into a tyrosinase and vice versa through a single amino acid mutation, underscoring the potential of these findings for future applications in medicinal and material sciences.

Marine actinomycetes-derived angucyclines and angucyclinones with biosynthesis and activity--past 10 years (2014-2023)

Actinomycete bacteria derived from marine environments are a good source of natural products with diverse biological activities such as cytotoxicity, antiviral, and antimicrobial actions. This review summarizes 191 angucyclines and angucyclinones derived from marine actinomycetes reported in the literature from 2014 to 2023 and introduced the latest developments in actinomycete-silenced biosynthetic gene cluster activation, including heterologous recombination and in situ activation. The key role of redox post-modifications in the biosynthetic process of atypical angucyclines. This review provides insights into the discovery and biosynthesis of valuable angucyclines and angucyclinones from marine-associated actinomycetes and potential lead compounds for the research and development of new drugs.

The Chemistry of Angucyclines

Angucyclines and angucyclinones represent a class of natural compounds that belong to the group of aromatic polyketides. They exhibit a wide array of biological properties, such as antimicrobial, antiviral, and cytotoxic. Their considerable therapeutic potential and diverse scaffolds have attracted many synthetic chemists to devise novel strategies to construct their intricate molecular architecture. Over 300 class members have been isolated from natural sources, mainly from bacterial strains of Streptomyces species. This review highlights recent advancements in their synthesis, such as oxidative cyclization, photooxidation, and metal-catalyzed [4+2]-cycloaddition, which has facilitated the efficient and practical total syntheses of various angucycline and angucyclinone natural products.

Neogrisemycin, a Trisulfide-Bridged Angucycline, Produced upon Expressing the Thioangucycline Biosynthetic Gene Cluster in Streptomyces albus J1074

Neogrisemycin (1) was isolated from recombinant Streptomyces albus J1074 strain SB4061 expressing an engineered thioangucycline (TAC) biosynthetic gene cluster (BGC). The structure and absolute configuration of 1 were established by a combination of mass spectrometry, nuclear magnetic resonance, and single-crystal X-ray diffraction analyses. Like the TACs, 1 was also proposed to derive non-enzymatically from the common epoxide (8), the nascent product encoded by the tac BGC, mediated by endogenous hydrogen trisulfide.

A review on natural products with cage-like structure

Natural products with diverse structures and significant biological activities are essential sources of drug lead compounds, and play an important role in the research and development of innovative drugs. Cage-like compounds have various structures and are widely distributed in nature, especially caged xanthones isolated from Garcinia genus, paeoniflorin and its derivatives isolated from Paeonia lactiflora Pall, tetrodotoxin (TTX) and its derivatives, and so on. In recent years, the development and utilization of cage-like compounds have been a research hotspot in chemistry, biology and other fields due to their special structures and remarkable biological activities. In this review, we mainly summarized the cage-like compounds with various structures found and isolated from natural drugs since 1956, summarized its broad biological activities, and introduced the progress in the biosynthesis of some compounds, so as to provide a reference for the discovery of more novel compounds, and the development and application of innovative drugs.

Chemoenzymatic Cascades for the Enantioselective Synthesis of β-Hydroxysulfides Bearing a Stereocentre at the C-O or C-S Bond by Ketoreductases

Chiral β-hydroxysulfides are an important class of organic compounds which find broad application in organic and pharmaceutical chemistry. Herein we describe the development of novel biocatalytic and chemoenzymatic methods for the enantioselective synthesis of β-hydroxysulfides by exploiting ketoreductase (KRED) enzymes. Four KREDs were discovered from a pool of 384 enzymes identified and isolated through a metagenomic approach. KRED311 and KRED349 catalysed the synthesis of β-hydroxysulfides bearing a stereocentre at the C-O bond with opposite absolute configurations and excellent ee values by novel chemoenzymatic and biocatalytic-chemical-biocatalytic (bio-chem-bio) cascades starting from commercially available thiophenols/thiols and α-haloketones/alcohols. KRED253 and KRED384 catalysed the enantioselective synthesis of β-hydroxysulfides bearing a stereocentre at the C-S bond with opposite enantioselectivities by dynamic kinetic resolution (DKR) of racemic α-thioaldehydes.

Visible-Light-Induced Organophotocatalytic Difunctionallization: Open-Air Hydroxysulfurization of Aryl Alkenes with Aryl Thiols

Herein, we report a regioselective visible-light-induced organophotoredox catalytic difunctionalization method to prepare β-hydroxysulfides using aryl alkenes and aryl thiols as substrates. The reaction provides a wide substrate scope of aryl alkenes (from simple styrene to complex bioactive compounds) and aryl thiols (from diverse heteroaromatic thiols to nonheteroaromatic thiols) (total 45 examples, up to 88% yield). Based on the combined experimental and computational studies, we demonstrate that in situ generated hydroperoxyl radicals from O₂ in air react with benzylic radicals, which restrains the reaction between benzylic radicals and the acidic form of thiols in a classical thiol-ene radical reaction. We show that difunctionalization is possible due to the choice of bases, diluted substrate concentrations, increment in catalyst loading, and selection of suitable aryl thiols under aerobic conditions. Considering the biological importance of heteroaromatic thiols and the lack of methods to install them, our approach offers a platform to derive various β-hydroxysulfides that contain aromatic elements.

Cytotoxic compounds from marine actinomycetes: Sources, Structures and Bioactivity

Marine actinomycetes produce a substantial number of natural products with cytotoxic activity. The strains of actinomycetes were isolated from different sources like fishes, coral, sponges, seaweeds, mangroves, sediments etc. These cytotoxic compounds can be categorized briefly into four classes: polyketides, non-ribosomal peptides and hybrids, isoprenoids and hybrids, and others, among which majority are polyketides (146). Twenty two out of the 254 compounds showed potent cytotoxicity with IC₅₀ values at ng/mL or nM level. This review highlights the sources, structures and antitumor activity of 254 natural products isolated from marine actinomycetes, which were new when they were reported from 1989 to 2020.

Discovery of Mycothiogranaticins from Streptomyces vietnamensis GIMV4.0001 and the Regulatory Effect of Mycothiol on the Granaticin Biosynthesis

Granaticins are benzoisochromanequinone polyketides with remarkable antibacterial and anticancer activities. Three sulfur-containing granaticin congeners, mycothiogranaticins A (1), B (2) and granaticin MA (3) were discovered from a granaticin-producing strain of Streptomyces vietnamensis GIMV4.0001. Two of them were structurally determined with mycothiol or N-acetylcysteine moieties and found to be bio-actively reluctant. Disruption of the mshA gene (SVTN_RS20640) that encodes the D-inositol-3-phosphate glycosyltransferase crucial for mycothiol biosynthesis, fully abolished the production of mycothiogranaticins. The result substantiated that the newly discovered mycothiogranaticins are consequences of the combination of the granaticin and mycothiol biosynthetic pathways. The overall granaticin production of the ΔmshA mutant strain was unexpectedly decreased by at least more than 50%, while similar production level of granaticins to that of the wild type strain was observed in an mycothiol-S transferase gene (SVTN_RS22215) disruptant Δmst. These results indicated that the mycothiol deficiency was responsible for the decreased production of granaticins. Mycothiol may positively regulate the biosynthesis of granaticin possibly by maintaining the cellular redox balance. To the best of our knowledge, this is the first report that mycothiol can not only be a direct building block of polyketides but also play a regulatory role in the polyketide biosynthesis.

The intriguing chemistry and biology of sulfur-containing natural products from marine microorganisms (1987-2020)

Natural products derived from marine microorganisms have received great attention as a potential resource of new compound entities for drug discovery. The unique marine environment brings us a large group of sulfur-containing natural products with abundant biological functionality including antitumor, antibiotic, anti-inflammatory and antiviral activities. We reviewed all the 484 sulfur-containing natural products (non-sulfated) isolated from marine microorganisms, of which 59.9% are thioethers, 29.8% are thiazole/thiazoline-containing compounds and 10.3% are sulfoxides, sulfones, thioesters and many others. A selection of 133 compounds was further discussed on their structure-activity relationships, mechanisms of action, biosynthesis, and druggability. This is the first systematic review on sulfur-containing natural products from marine microorganisms conducted from January 1987, when the first one was reported, to December 2020.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00101-2.

Ring D-Modified and Highly Reduced Angucyclinones From Marine Sediment-Derived Streptomyces sp

Angucyclines and angucyclinones represent the largest family of type II PKS-engineered natural products. Chemical analysis of a marine Streptomyces sp. KCB-132 yielded three new members, actetrophenone A (1) and actetrophenols A–B (2–3). Their structures were elucidated by NMR spectroscopy, X-ray crystallography and CD calculations. Actetrophenone A (1) is the first representative of a novel-type angucyclinone bearing a nonaromatic D-ring. Actetrophenol A (2) features a highly reduced and aromatized four-ring system, which is unprecedented for natural products. While (R_a)- and (S_a)-actetrophenol B (3) bear an unprecedented N-acetyltryptamine-substituted tetraphene core skeleton, this is the first report of a pair of atropisomeric isomers in the angucyclinone family. Actetrophenol A (2) exhibits remarkable antibiotic activity, notably including potent activity to multiple resistant Staphylococcus aureus and Enterococcus faecium with MIC values of 4 μg/ml, in contrast, the positive control antimicrobial agent penicillin was inactive up to 32 μg/ml.

Cryptic Sulfur Incorporation in Thioangucycline Biosynthesis

Sulfur incorporation into natural products is a critical area of biosynthetic studies. Recently, a subset of sulfur-containing angucyclines has been discovered, and yet, the sulfur incorporation step is poorly understood. In this work, a series of thioether-bridged angucyclines were discovered, and a cryptic epoxide Michael acceptor intermediate was revealed en route to thioangucyclines (TACs) A and B. However, systematic gene deletion of the biosynthetic gene cluster (BGC) by CRISPR/Cas9 could not identify any gene responsible for the conversion of the epoxide intermediate to TACs. Instead, a series of in vitro and in vivo experiments conclusively showed that the conversion is the result of two non-enzymatic steps, possibly mediated by endogenous hydrogen sulfide. Therefore, the TACs are proposed to derive from a detoxification process. These results are expected to contribute to the study of both angucyclines and the utilization of inorganic sulfur in natural product biosynthesis.

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.

Marine Actinomycetes-derived Natural Products

Actinomycetes is an abundant resource for discovering a large number of lead compounds, which play an important role in microbial drug discovery. Compared to terrestrial microorganisms, marine actinomycetes have unique metabolic pathways because of their special living environment, which has the potential to produce a variety of bioactive substances. In this paper, secondary metabolites isolated from marine actinomycetes are reviewed (2013-2018), most of which exhibited cytotoxic, antibacterial, and antiviral biological activities.

(+)- and (−)-actinoxocine, and actinaphthorans A–B, C-ring expansion and cleavage angucyclinones from a marine-derived Streptomyces sp

A pair of enantiomeric C-ring expansion angucyclinones with an unprecedented epoxybenzo[f]naphtho[1,8-bc]oxocine carbon skeleton, and two unique C-ring cleavage analogues, were isolated from a marine-derived Streptomyces sp.

β-Hydroxy sulfides and their syntheses

Sulfur-containing natural products are ubiquitous in nature, their most abundant source being marine organisms since sulfur, in the form of the sulfate ion, is the second most abundant anion in sea water after chloride. As part of natural products, sulfur can appear in a multitude of combinations and oxidation states: thiol, sulfide (acyclic or heterocyclic), disulfide, sulfoxide, sulfonate, thioaminal, hemithioacetal, various thioesters, thiocarbamate and isothiocyanate. This review article focuses on β-hydroxy sulfides and analogs; their presence in natural products, general protocols for their synthesis, and examples of their application in target oriented synthesis.

Natural Products Research in China From 2015 to 2016

This review covers the literature published by chemists from China during the 2015-2016 on natural products (NPs), with 1,985 citations referring to 6,944 new compounds isolated from marine or terrestrial microorganisms, plants, and animals. The emphasis is on 730 new compounds with a novel skeleton or/and significant bioactivity, together with their source organism and country of origin.

Marine natural products

Covering: 2016. Previous review: Nat. Prod. Rep., 2017, 34, 235-294This review covers the literature published in 2016 for marine natural products (MNPs), with 757 citations (643 for the period January to December 2016) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1277 in 432 papers for 2016), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included.

Survey of natural products reported by Asian research groups in 2016

The new natural products reported in peer-reviewed articles in 2016 in journals with good reputations were reviewed and analyzed. The advances that Asian research groups made in the field of natural products chemistry in 2016 were summarized. Compounds with unique structural features and/or promising bioactivities originating from Asian natural sources were discussed based on structural classification.

Substrate-Assisted Catalysis in Polyketide Reduction Proceeds via a Phenolate Intermediate

SimC7 is a polyketide ketoreductase involved in biosynthesis of the angucyclinone moiety of the gyrase inhibitor simocyclinone D8 (SD8). SimC7, which belongs to the short-chain dehydrogenase/reductase (SDR) superfamily, catalyzes reduction of the C-7 carbonyl of the angucyclinone, and the resulting hydroxyl is essential for antibiotic activity. SimC7 shares little sequence similarity with characterized ketoreductases, suggesting it might have a distinct mechanism. To investigate this possibility, we determined the structures of SimC7 alone, with NADP⁺, and with NADP⁺ and the substrate 7-oxo-SD8. These structures show that SimC7 is distinct from previously characterized polyketide ketoreductases, lacking the conserved catalytic triad, including the active-site tyrosine that acts as central acid-base catalyst in canonical SDR proteins. Taken together with functional analyses of active-site mutants, our data suggest that SimC7 catalyzes a substrate-assisted, two-step reaction for reduction of the C-7 carbonyl group involving intramolecular transfer of a substrate-derived proton to generate a phenolate intermediate.

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SimC7 catalyzes reduction of the C-7 carbonyl of the angucyclinone of simocyclinone

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SimC7 lacks the conserved catalytic triad characteristic of canonical ketoreductases

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SimC7 reduces the C-7 carbonyl via a substrate-assisted, two-step reaction

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This involves intramolecular transfer of a substrate proton to generate a phenolate

Hot off the Press

A personal selection of 33 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products, such as epicochalasine A from Aspergillus flavipes.