Cryptic Sulfur Incorporation in Thioangucycline Biosynthesis

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.

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.

The many facets of sulfur incorporation in natural product biosynthesis

Sulfur-containing natural products (S-containing NPs) exhibit diverse chemical structures and biosynthetic machineries. Unraveling the intricate chemistry of S-incorporation requires innovative and multidisciplinary approaches. In this review, we surveyed the landscape of S-containing NP biosynthetic machineries, classified the S-incorporation chemistry into four distinct classes, and highlighted each of the four classes with representative examples from recent studies. All highlighted chemistry has been correlated to the genes encoding the biosynthetic machineries of the S-containing NPs, which open new opportunities to discover S-containing NPs through genome mining. These examples should inspire the community to explore uncharted territories in NP research, promoting further advancements in both novel S-containing NP discovery and S-incorporation chemistry.

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.

Atypical Angucyclinones with Ring Expansion and Cleavage from a Marine Streptomyces sp

A unique ring C-expanded angucyclinone, oxemycin A (1), and seven new ring-cleavage derivatives (2-5 and 9-11) were isolated from the marine actinomycete Streptomyces pratensis KCB-132, together with eight known analogues (6-8 and 12-16). Their structures were elucidated by spectroscopic analyses, single-crystal X-ray diffractions, and NMR and ECD calculations. Among these atypical angucyclinones, compound 1 represented the first seven-membered ketoester in the angucyclinone family, which sheds light on the origin of fragmented angucyclinones with C-ring cleavage at C-12/C-12a in the Baeyer-Villiger hypothesis, such as 2-4, while the related "nonoxidized" analogues 5-8 seem to originate from a diverse pathway within the Grob fragmentation hypothesis. Additionally, we have succeeded in the challenging separation of elmenols E and F (12) into their four stereoisomers, which remained stable in aprotic solvents but rapidly racemized under protic conditions. Furthermore, the absolute configurations of LS1924 and its isomers (14 and 15) were assigned by ECD calculations for the first time. Surprisingly, these two bicyclic acetals are susceptible to hydrolysis in solution, resulting in fragmented derivatives 17 and 18 with C-ring cleavage between C-6a and C-7. Compared with ring C-modified angucyclinones, ring A-cleaved 11 was more active to multiple resistant "ESKAPE" pathogens with MIC values ranging from 4.7 to 37.5 μg/mL.

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.

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.

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.