Genome-Driven Discovery of Antiviral Atralabdans A-C from the Soil-Dwelling Streptomyces atratus

Heterologous expression of an atr terpenoid gene cluster derived from Streptomyces atratus Gö66 in S. albus J1074 led to the discovery of three novel labdane diterpenoids featuring an unprecedented 6/6/5-fused tricyclic skeleton, designated as atralabdans A-C (1-3), along with a known compound, labdanmycin A. Compounds 1-3 were identified through extensive spectroscopic analysis, including NMR calculations with DP4+ probability analysis, and a comparative assessment of experimental and theoretical electronic circular dichroism (ECD) spectra. A plausible biosynthetic pathway for these compounds was proposed. Compounds 1-3 exhibited inhibitory activity against the human neurotropic coxsackievirus B3 (CVB3); 1 was the most potent, surpassing the positive control ribavirin with a higher therapeutic index.

Characterization of Terpenoids from the Ambrosia Beetle Symbiont and Laurel Wilt Pathogen Harringtonia lauricola

Little is known concerning terpenoids produced by members of the fungal order Ophiostomales, with the member Harringtonia lauricola having the unique lifestyle of being a beetle symbiont but potentially devastating tree pathogen. Nine known terpenoids, including six labdane diterpenoids (1-6) and three hopane triterpenes (7-9), were isolated from H. lauricola ethyl acetate (EtOAc) extracts for the first time. All compounds were tested for various in vitro bioactivities. Six compounds, 2, 4, 5, 6, 7, and 9, are described functionally. Compounds 2, 4, 5, and 9 expressed potent antiproliferative activity against the MCF-7, HepG2 and A549 cancer cell lines, with half-maximal inhibitory concentrations (IC50s) ~12.54-26.06 μM. Antimicrobial activity bioassays revealed that compounds 4, 5, and 9 exhibited substantial effects against Gram-negative bacteria (Escherichia coli and Ralstonia solanacearum) with minimum inhibitory concentration (MIC) values between 3.13 and 12.50 μg/mL. Little activity was seen towards Gram-positive bacteria for any of the compounds, whereas compounds 2, 4, 7, and 9 expressed antifungal activities (Fusarium oxysporum) with MIC values ranging from 6.25 to 25.00 μg/mL. Compounds 4, 5, and 9 also displayed free radical scavenging abilities towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) and superoxide (O2-), with IC50 values of compounds 2, 4, and 6 ~3.45-14.04 μg/mL and 22.87-53.31 μg/mL towards DPPH and O2-, respectively. These data provide an insight into the biopharmaceutical potential of terpenoids from this group of fungal insect symbionts and plant pathogens.

Extending the Structural Diversity of Labdane Diterpenoids from Marine-Derived Fungus Talaromyces sp. HDN151403 Using Heterologous Expression

Heterologous biosynthesis has become an effective means to activate fungal silent biosynthetic gene clusters (BGCs) and efficiently utilize fungal genetic resources. Herein, thirteen labdane diterpene derivatives, including five undescribed ones named talarobicins A-E (3-7), were discovered via heterologous expression of a silent BGC (labd) in Aspergillus nidulans. Their structures with absolute configurations were elucidated using extensive MS and NMR spectroscopic methods, as well as electronic circular dichroism (ECD) calculations. These labdanes belong to four skeleton types, and talarobicin B (4) is the first 3,18-dinor-2,3:4,18-diseco-labdane diterpene with the cleavage of the C2-C3 bond in ring A and the decarboxylation at C-3 and C-18. Talarobicin B (4) represents the key intermediate in the biosynthesis of penioxalicin and compound 13. The combinatorial heterologous expression and feeding experiments revealed that the cytochrome P450 enzymes LabdC, LabdE, and LabdF were responsible for catalyzing various chemical reactions, such as oxidation, decarboxylation, and methylation. All of the compounds are noncytotoxic, and compounds 2 and 8 displayed inhibitory effects against methicillin-resistant coagulase-negative staphylococci (MRCNS) and Bacillus cereus.

Decoding Catalysis by Terpene Synthases

The review by Christianson, published in 2017 on the twentieth anniversary of the emergence of the field, summarizes the foundational discoveries and key advances in terpene synthase/cyclase (TS) biocatalysis (Christianson, D. W. Chem Rev2017, 117 (17), 11570-11648. DOI: 10.1021/acs.chemrev.7b00287). Here, we review the TS literature published since then, bringing the field up to date and looking forward to what could be the near future of TS rational design. Many revealing discoveries have been made in recent years, building on the knowledge and fundamental principles uncovered during those initial two decades of study. We use these to explore TS reaction chemistry and see how a combined experimental and computational approach helps to decipher the complexities of TS catalysis. Revealed are a suite of catalytic motifs which control product outcome in TSs, some obvious, some more subtle. We examine each in detail, using the most recent papers and insights to illustrate how exactly this fascinating class of enzymes takes a single acyclic substrate and turns it into the many thousands of complex terpenoids found in Nature. We then explore some of the recent strategies for TS engineering, including machine learning and other data-driven approaches. From this, rational and predictive engineering of TSs, "designer terpene synthases", will begin to emerge as a realistic goal.

Actinomycetes as Producers of Biologically Active Terpenoids: Current Trends and Patents

Terpenes and their derivatives (terpenoids and meroterpenoids, in particular) constitute the largest class of natural compounds, which have valuable biological activities and are promising therapeutic agents. The present review assesses the biosynthetic capabilities of actinomycetes to produce various terpene derivatives; reports the main methodological approaches to searching for new terpenes and their derivatives; identifies the most active terpene producers among actinomycetes; and describes the chemical diversity and biological properties of the obtained compounds. Among terpene derivatives isolated from actinomycetes, compounds with pronounced antifungal, antiviral, antitumor, anti-inflammatory, and other effects were determined. Actinomycete-produced terpenoids and meroterpenoids with high antimicrobial activity are of interest as a source of novel antibiotics effective against drug-resistant pathogenic bacteria. Most of the discovered terpene derivatives are produced by the genus Streptomyces; however, recent publications have reported terpene biosynthesis by members of the genera Actinomadura, Allokutzneria, Amycolatopsis, Kitasatosporia, Micromonospora, Nocardiopsis, Salinispora, Verrucosispora, etc. It should be noted that the use of genetically modified actinomycetes is an effective tool for studying and regulating terpenes, as well as increasing productivity of terpene biosynthesis in comparison with native producers. The review includes research articles on terpene biosynthesis by Actinomycetes between 2000 and 2022, and a patent analysis in this area shows current trends and actual research directions in this field.

Structure-guided product determination of the bacterial type II diterpene synthase Tpn2

A grand challenge in terpene synthase (TS) enzymology is the ability to predict function from protein sequence. Given the limited number of characterized bacterial TSs and significant sequence diversities between them and their eukaryotic counterparts, this is currently impossible. To contribute towards understanding the sequence-structure-function relationships of type II bacterial TSs, we determined the structure of the terpentedienyl diphosphate synthase Tpn2 from Kitasatospora sp. CB02891 by X-ray crystallography and made structure-guided mutants to probe its mechanism. Substitution of a glycine into a basic residue changed the product preference from the clerodane skeleton to a syn-labdane skeleton, resulting in the first syn-labdane identified from a bacterial TS. Understanding how a single residue can dictate the cyclization pattern in Tpn2, along with detailed bioinformatics analysis of bacterial type II TSs, sets the stage for the investigation of the functional scope of bacterial type II TSs and the discovery of novel bacterial terpenoids.

The Biosynthetic Gene Cluster for Sestermobaraenes-Discovery of a Geranylfarnesyl Diphosphate Synthase and a Multiproduct Sesterterpene Synthase from Streptomyces mobaraensis

A biosynthetic gene cluster from Streptomyces mobaraensis encoding the first cases of a bacterial geranylfarnesyl diphosphate synthase and a type I sesterterpene synthase was identified. The structures of seven sesterterpenes produced by these enzymes were elucidated, including their absolute configurations. The enzyme mechanism of the sesterterpene synthase was investigated by extensive isotope labeling experiments.

Strepimidazoles A-G from the Plant Endophytic Streptomyces sp. PKU-EA00015 with Inhibitory Activities against a Plant Pathogenic Fungus

Seven new 4-acyl-2-aminoimidazoles, designated strepimidazoles A-G (1-7), were discovered from the endophytic Streptomyces sp. PKU-EA00015 isolated from Salvia miltiorrhiza Bunge, whose dry root "Danshen" is one of the most widely used traditional Chinese medicines. The resonance signals of the 2-aminoimidazole moiety in 1-7 were absent in the NMR spectra due to tautomerization, and the structures of 1-7 were identified after preparation of their acetylation products 1a-7a, respectively. Compounds 1-7 represent a new family of 2-aminoimidazole-containing natural products, enriching the structural diversity of natural products from endophytic origin. Compounds 1-7 showed different degrees of inhibitory activities against the plant pathogenic fungus Verticillium dahliae V991, revealing structure-activity relationships on the acyl moieties. The plant pathogenic fungus V. dahliae has been confirmed to cause serious chlorosis of cultivated S. miltiorrhiza Bunge in China. This study opens the door for further investigation of mutualistic relationships between S. miltiorrhiza Bunge and their endophytic actinomycetes and for possible antifungal agent development for biological control of V. dahliae in the future.

Dimeric Pyranonaphthoquinone Glycosides with Anti-HIV and Cytotoxic Activities from a Soil-Derived Streptomyces

Eight new sulfur-bridged pyranonaphthoquinone (PNQ) dimers, naquihexcins C-J (1-8), a new PNQ monomer, naquihexcin K (10), and three known analogues (9, 11, and 12) were isolated from Streptomyces sp. KIB3133. The new structures were elucidated by interpretation of spectroscopic data. Dimer 4 was synthesized via a cascade S_N2 reactions between two monomers and sodium sulfide, an approach motivated by the proposed biosynthetic pathway of dimeric pyranonaphthoquinones. Naquihexcin E (3) exhibited moderate HIV-1 inhibitory activity. Naquihexcins C (1), E (3), and I (7) showed inhibitory effects against two tumor cell lines (HL-60 and MCF-7) with IC₅₀ values ranging from 1.4 to 16.1 μM.

A unique spiro-β-triazinedione-γ-hydantoin type alkaloid with antiviral activity against tobacco mosaic virus from Streptomyces gamaensis

Chemical investigation of a soil actinomycete strain Streptomyces gamaensis NEAU-Gz11 led to the isolation of three alkaloids 1–3 with antiviral activity.