Reconstitution of Enzymatic Carbon-Sulfur Bond Formation Reveals Detoxification-Like Strategy in Fungal Toxin Biosynthesis

Verticillins: fungal epipolythiodioxopiperazine alkaloids with chemotherapeutic potential

Covering: 1970 through June of 2023Verticillins are epipolythiodioxopiperazine (ETP) alkaloids, many of which possess potent, nanomolar-level cytotoxicity against a variety of cancer cell lines. Over the last decade, their in vivo activity and mode of action have been explored in detail. Notably, recent studies have indicated that these compounds may be selective inhibitors of histone methyltransferases (HMTases) that alter the epigenome and modify targets that play a crucial role in apoptosis, altering immune cell recognition, and generating reactive oxygen species. Verticillin A (1) was the first of 27 analogues reported from fungal cultures since 1970. Subsequent genome sequencing identified the biosynthetic gene cluster responsible for producing verticillins, allowing a putative pathway to be proposed. Further, molecular sequencing played a pivotal role in clarifying the taxonomic characterization of verticillin-producing fungi, suggesting that most producing strains belong to the genus Clonostachys (i.e., Bionectria), Bionectriaceae. Recent studies have explored the total synthesis of these molecules and the generation of analogues via both semisynthetic and precursor-directed biosynthetic approaches. In addition, nanoparticles have been used to deliver these molecules, which, like many natural products, possess challenging solubility profiles. This review summarizes over 50 years of chemical and biological research on this class of fungal metabolites and offers insights and suggestions on future opportunities to push these compounds into pre-clinical and clinical development.

Formation of Bridged Disulfide in Epidithiodioxopiperazines

Epidithiodioxopiperazine (ETP) alkaloids, featuring a 2,5-diketopiperazine core and transannular disulfide bridge, exhibit a broad spectrum of biological activities. However, the structural complexity has prevented efficient chemical synthesis and further clinical research. In the past few decades, many achievements have been made in the biosynthesis of ETPs. Here, we discuss the biosynthetic progress and summarize them as two comprehensible metabolic principles for better understanding the complex pathways of α, α'- and α, β'-disulfide bridged ETPs. Specifically, we systematically outline the catalytic machineries to install α, α'- and α, β'-disulfide by flavin-containing oxygenases. This concept would contribute to the medical and industrial applications of ETPs.

Bacterial Isothiocyanate Biosynthesis by Rhodanese-Catalyzed Sulfur Transfer onto Isonitriles

Natural products bearing isothiocyanate (ITC) groups are an important group of specialized metabolites that play various roles in health, nutrition, and ecology. Whereas ITC biosynthesis via glucosinolates in plants has been studied in detail, there is a gap in understanding the bacterial route to specialized metabolites with such reactive heterocumulene groups, as in the antifungal sinapigladioside from Burkholderia gladioli. Here we propose an alternative ITC pathway by enzymatic sulfur transfer onto isonitriles catalyzed by rhodanese-like enzymes (thiosulfate:cyanide sulfurtransferases). Mining the B. gladioli genome revealed six candidate genes (rhdA-F), which were individually expressed in E. coli. By means of a synthetic probe, the gene products were evaluated for their ability to produce the key ITC intermediate in the sinapigladioside pathway. In vitro biotransformation assays identified RhdE, a prototype single-domain rhodanese, as the most potent ITC synthase. Interestingly, while RhdE also efficiently transforms cyanide into thiocyanate, it shows high specificity for the natural pathway intermediate, indicating that the sinapigladioside pathway has recruited a ubiquitous detoxification enzyme for the formation of a bioactive specialized metabolite. These findings not only elucidate an elusive step in bacterial ITC biosynthesis but also reveal a new function of rhodanese-like enzymes in specialized metabolism.

Enhancing gliotoxins production in deep-sea derived fungus Dichotomocyes cejpii by engineering the biosynthetic pathway

Gliotoxin can be developed as potent biopesticide. In this study, the positive transcriptional factor gliZ, glutathione-S transferase encoding gene gliG and gliN were firstly deleted by CRISPR/Cas9 system, which abolished the production of gliotoxin-like compounds in Dichotomomyces cejpii. CRISPR/dCas9 system targeting promoter of gliG was used to activate the biosynthetic genes in gli cluster. The overexpression of gliZ, gliN and gliG can significantly improve the yield of gliotoxin-like compunds. The gliotoxin yields was improved by 16.38 ± 1.36 fold, 18.98 ± 1.28 fold through gliZ overexpression and gliM deletion in D. cejpii FS110. In addtion, gliN was heterologously expressed in E. coli, the purified GliN can catalyze gliotoxin into methyl-gliotoxin. Furthermore, the binding sequences of GliZ in the promoters of gliG was determined by Dnase footprinting. This study firstly illustrated the transcriptional regulatory mechanism of DcGliZ for the gliotoxin biosynthesis in D. cejpii, and improved the yields of gliotoxins significantly in D. cejpii via biosynthetic approaches.

Epipolythiodioxopiperazine-Based Natural Products: Building Blocks, Biosynthesis and Biological Activities

Epipolythiodioxopiperazines (ETPs) are fungal secondary metabolites that share a 2,5-diketopiperazine scaffold built from two amino acids and bridged by a sulfide moiety. Modifications of the core and the amino acid side chains, for example by methylations, acetylations, hydroxylations, prenylations, halogenations, cyclizations, and truncations create the structural diversity of ETPs and contribute to their biological activity. However, the key feature responsible for the bioactivities of ETPs is their sulfide moiety. Over the last years, combinations of genome mining, reverse genetics, metabolomics, biochemistry, and structural biology deciphered principles of ETP production. Sulfurization via glutathione and uncovering of the thiols followed by either oxidation or methylation crystallized as fundamental steps that impact expression of the biosynthesis cluster, toxicity and secretion of the metabolite as well as self-tolerance of the producer. This article showcases structure and activity of prototype ETPs such as gliotoxin and discusses the current knowledge on the biosynthesis routes of these exceptional natural products.

Enantioselective [1,2]-Stevens Rearrangement of Thiosulfonates to Construct Dithio-Substituted Quaternary Carbon Centers

An enantioselective [1,2] Stevens rearrangement was realized by using chiral guanidine and copper(i) complexes. Bis-sulfuration of α-diazocarbonyl compounds was developed through using thiosulfonates as the sulfenylating agent. It was undoubtedly an atom-economic process providing an efficient route to access novel chiral dithioketal derivatives, affording the corresponding products in good yields (up to 90% yield) and enantioselectivities (up to 96 : 4 er). A novel catalytic cycle was proposed to rationalize the reaction process and enantiocontrol.

An asymmetric [1,2] Stevens rearrangement was realized via chiral guanidine and copper(i) complexes. A series of novel chiral dithioketal derivatives were obtained with good yields (up to 90% yield) and enantioselectivities (up to 96 : 4 er).

Structural and Mechanistic Insights into C-S Bond Formation in Gliotoxin

Glutathione‐S‐transferases (GSTs) usually detoxify xenobiotics. The human pathogenic fungus Aspergillus fumigatus however uses the exceptional GST GliG to incorporate two sulfur atoms into its virulence factor gliotoxin. Because these sulfurs are essential for biological activity, glutathionylation is a key step of gliotoxin biosynthesis. Yet, the mechanism of carbon−sulfur linkage formation from a bis‐hydroxylated precursor is unresolved. Here, we report structures of GliG with glutathione (GSH) and its reaction product cyclo[‐l‐Phe‐l‐Ser]‐bis‐glutathione, which has been purified from a genetically modified A. fumigatus strain. The structures argue for stepwise processing of first the Phe and second the Ser moiety. Enzyme‐mediated dehydration of the substrate activates GSH and a helix dipole stabilizes the resulting anion via a water molecule for the nucleophilic attack. Activity assays with mutants validate the interactions of GliG with the ligands and enrich our knowledge about enzymatic C−S bond formation in gliotoxin and epipolythiodioxopiperazine (ETP) natural compounds in general.

Fungal Epithiodiketopiperazines Carrying α,β-Polysulfide Bridges from Penicillium steckii YE, and Their Chemical Interconversion

Some fungal epithiodiketopiperazine alkaloids display α,β-polysulfide bridges alongside diverse structural variations. However, the logic of their chemical diversity has rarely been explored. Here, we report the identification of three new (2, 3, 8) and five known (1, 4-7) epithiodiketopiperazines of this subtype from a marine-derived Penicillium sp. The structure elucidation was supported by multiple spectroscopic analyses. Importantly, we observed multiple nonenzymatic interconversions of these analogues in aqueous solutions and organic solvents. Furthermore, the same biosynthetic origin of these compounds was supported by one mined gene cluster. The dominant analogue (1) demonstrated selective cytotoxicity to androgen-sensitive prostate cancer cells and HIF-depleted colorectal cells and mild antiaging activities, linking the bioactivity to oxidative stress. These results provide crucial insight into the formation of fungal epithiodiketopiperazines through chemical interconversions.

The Aspergilli and Their Mycotoxins: Metabolic Interactions With Plants and the Soil Biota

Species of the highly diverse fungal genus Aspergillus are well-known agricultural pests, and, most importantly, producers of various mycotoxins threatening food safety worldwide. Mycotoxins are studied predominantly from the perspectives of human and livestock health. Meanwhile, their roles are far less known in nature. However, to understand the factors behind mycotoxin production, the roles of the toxins of Aspergilli must be understood from a complex ecological perspective, taking mold-plant, mold-microbe, and mold-animal interactions into account. The Aspergilli may switch between saprophytic and pathogenic lifestyles, and the production of secondary metabolites, such as mycotoxins, may vary according to these fungal ways of life. Recent studies highlighted the complex ecological network of soil microbiotas determining the niches that Aspergilli can fill in. Interactions with the soil microbiota and soil macro-organisms determine the role of secondary metabolite production to a great extent. While, upon infection of plants, metabolic communication including fungal secondary metabolites like aflatoxins, gliotoxin, patulin, cyclopiazonic acid, and ochratoxin, influences the fate of both the invader and the host. In this review, the role of mycotoxin producing Aspergillus species and their interactions in the ecosystem are discussed. We intend to highlight the complexity of the roles of the main toxic secondary metabolites as well as their fate in natural environments and agriculture, a field that still has important knowledge gaps.

Diketopiperazine Formation in Fungi Requires Dedicated Cyclization and Thiolation Domains

Cyclization of linear dipeptidyl precursors derived from nonribosomal peptide synthetases (NRPSs) into 2,5-diketopiperazines (DKPs) is a crucial step in the biosynthesis of a large number of bioactive natural products. However, the mechanism of DKP formation in fungi has remained unclear, despite extensive studies of their biosyntheses. Here we show that DKP formation en route to the fungal virulence factor gliotoxin requires a seemingly extraneous couplet of condensation (C) and thiolation (T) domains in the NRPS GliP. In vivo truncation of GliP to remove the CT couplet or just the T domain abrogated production of gliotoxin and all other gli pathway metabolites. Point mutation of conserved active sites in the C and T domains diminished cyclization activity of GliP in vitro and abolished gliotoxin biosynthesis in vivo. Verified NRPSs of other fungal DKPs terminate with similar CT domain couplets, suggesting a conserved strategy for DKP biosynthesis by fungal NRPSs.

Enzymes for Detoxification of Various Mycotoxins: Origins and Mechanisms of Catalytic Action

Mycotoxins are highly dangerous natural compounds produced by various fungi. Enzymatic transformation seems to be the most promising method for detoxification of mycotoxins. This review summarizes current information on enzymes of different classes to convert various mycotoxins. An in-depth analysis of 11 key enzyme mechanisms towards dozens of major mycotoxins was realized. Additionally, molecular docking of mycotoxins to enzymes' active centers was carried out to clarify some of these catalytic mechanisms. Analyzing protein homologues from various organisms (plants, animals, fungi, and bacteria), the prevalence and availability of natural sources of active biocatalysts with a high practical potential is discussed. The importance of multifunctional enzyme combinations for detoxification of mycotoxins is posed.