Novel cytotoxic isobenzofuran derivatives from Streptomyces sp. IFM 11490

Divergence of Classical and C-Ring-Cleaved Angucyclines: Elucidation of Early Tailoring Steps in Lugdunomycin and Thioangucycline Biosynthesis

Angucyclines are an important group of microbial natural products that display tremendous chemical diversity. Classical angucyclines are composed of a tetracyclic benz[a]anthracene scaffold with one ring attached at an angular orientation. However, in atypical angucyclines, the polyaromatic aglycone is cleaved at A-, B-, or C-rings, leading to structural rearrangements and enabling further chemical variety. Here, we have elucidated the branching points in angucycline biosynthesis leading toward cleavage of the C-ring in lugdunomycin and thioangucycline biosynthesis. We showed that 12-hydroxylation and 6-ketoreduction of UWM6 are shared steps in classical and C-ring-cleaved angucycline pathways, although the bifunctional 6-ketoreductase LugOIIred harbors additional unique 1-ketoreductase activity. We identified formation of the key intermediate 8-O-methyltetrangomycin by the LugN methyltransferase as the branching point toward C-ring-cleaved angucyclines. The final common step in lugdunomycin and thioangucycline biosynthesis is quinone reduction, catalyzed by the 7-ketoreductases LugG and TacO, respectively. In turn, the committing step toward thioangucyclines is 12-ketoreduction catalyzed by TacA, for which no orthologous protein exists on the lugdunomycin pathway. Our results confirm that quinone reductions are early tailoring steps and, therefore, may be mechanistically important for subsequent C-ring cleavage. Finally, many of the tailoring enzymes harbored broad substrate promiscuity, which we utilized in combinatorial enzymatic syntheses to generate the angucyclines SM 196 A and hydranthomycin. We propose that enzyme promiscuity and the competition of many of the enzymes for the same substrates lead to a branching biosynthetic network and formation of numerous shunt products typical for angucyclines rather than a canonical linear metabolic pathway.

Racemic Total Synthesis of Elmonin and Pratenone A, from Streptomyces, Using a Common Intermediate Prepared by peri-Directed C-H Functionalization

The first total synthesis of elmonin and pratenone A, two complex rearranged angucyclinones from Streptomyces, is reported. Using peri-directed C-H functionalization, the key naphthalene fragment present in both synthetic targets was efficiently prepared. Coupling to two anisole-derived fragments gave access to the natural products, in which elmonin was prepared using a biomimetic spiro-ketalization.

Natural dihydroisobenzofuran derivatives as a template for promising radical scavengers: theoretical insights into structure–activity relationships, thermochemistry and kinetics

Dihydroisobenzofuran heterocycle is a structural motif found in a number of medications with anti-tumour, anti-diabetic, and antibacterial activities. It is also found in a slew of natural substances, most notably fungus metabolites, which have been shown to possess credible radical scavenging activity. Density functional theory studies on three different derivatives were conducted to investigate their electronic structures as well as thermochemical and kinetic behaviour against ·OOH, ·OH, and ·OCH3 in biologically relevant solvents, with the goal of elucidating structure–activity relationships and discussing the potential role of the scaffolds as a template for new semisynthetic antioxidants. The importance of resonance and inductive effects, and also hydrogen bonding, has been underlined, but most importantly, it has been demonstrated that all structures have considerable scavenging potential against all studied radicals, with reactions rates close to the diffusion limit.

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.

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.

Heliomycin and tetracinomycin D: anthraquinone derivatives with histone deacetylase inhibitory activity from marine sponge-associated Streptomyces sp. SP9

Several actinomycetes strains were isolated from different marine sponges collected from the Red Sea shore in Egypt. The efficiency of their crude extracts to inhibit histone deacetylase (HDAC) enzyme was investigated in the nuclear extract of Hela cell line. The crude extract corresponding to Streptomyces sp. SP9 isolated from the marine sponge Pseudoceratina arabica showed a promising HDAC inhibitory activity with 64 and 81% at 50 and 100 µg/ml, respectively. The strain was identified as Streptomyces sp. by phylogenetic analyses based on its 16S rRNA gene sequence. The major compounds of Streptomyces sp. SP9 were isolated and purified by different chromatographic methods. The chemical structure of the isolated compounds was identified on the basis of their spectroscopic data including mass, ¹H and ¹³C NMR, and by comparison with those of authenticated samples. Structures of compounds 1 and 2 were established as heliomycin and tetracenomycin D, respectively. These compounds exhibited HDAC inhibitory activities with IC₅₀ values of 29.8 ± 0.04 µg/ml for heliomycin (1) and 10.9 ± 0.02 µg/ml for tetracenomycin D (2). A computational docking study for compounds 1 and 2 against HDAC1, HDAC2, and HDAC3 was performed to formulate a hypothetical mechanism by which the tested compounds inhibit HDAC. Tetracenomycin D (2) showed a good binding interactions with HDAC2 (- 5.230 kcal/mol) and HDAC3 (- 6.361 kcal/mol).