The many facets of sulfur incorporation in natural product biosynthesis

Fulvic acid impact on constructed wetland-microbial electrolysis cell system performance: Metagenomic insights

This study explores the roles of fulvic acid (FA) in both a conventionally constructed wetland (CCW) and a newly constructed wetland-microbial electrolysis cell (ECW). The results showed that FA increased the average removal efficiency of chemical oxygen demand, total phosphorus, total nitrogen, and ammonia nitrogen in ECW by 8.6, 46.2, 33.0, and 27.9 %, respectively, compared to CCW, and reduced the global warming potential by > 60 %. FA promoted the proliferation of electroactive bacteria (e.g., Chlorobaculum and Candidatus Tenderia) and FA-degrading bacteria (e.g., Anaerolineaceae and Gammaproteobacteria) and reduced methanogens (e.g., Methanothrix) via type-changing. The study's findings suggest that FA influences pollutant removal and microbiome dynamics by altering dissolved oxygen levels and redox potential. In summary, FA and ECW enhanced the efficiency of constructed wetlands by facilitating electron transfer and consumption, and supporting microbial growth and metabolism.

Structural insights into the convergent evolution of sulfoxide synthase EgtB-IV, an ergothioneine-biosynthetic homolog of ovothiol synthase OvoA

Non-heme iron-dependent sulfoxide/selenoxide synthases (NHISS) constitute a unique metalloenzyme class capable of installing a C-S/Se bond onto histidine to generate thio/selenoimidazole antioxidants, such as ergothioneine and ovothiol. These natural products are increasingly recognized for their health benefits. Among associated ergothioneine-biosynthetic enzymes, type IV EgtBs stand out, as they exhibit low sequence similarity with other EgtB subfamilies due to their recent divergence from the ovothiol-biosynthetic enzyme OvoA. Herein, we present crystal structures of two representative EgtB-IV enzymes, offering insights into the basis for this evolutionary convergence and enhancing our understanding of NHISS active site organization more broadly. The ability to interpret how key residues modulate substrate specificity and regioselectivity has implications for downstream identification of divergent reactivity within the NHISS family. To this end, we identify a previously unclassified clade of OvoA-like enzymes with a seemingly hybrid set of characteristics, suggesting they may represent an evolutionary intermediate between OvoA and EgtB-IV.

Thioimidate Solutions to Thioamide Problems during Thionopeptide Deprotection

Thioamides have structural and chemical similarity to peptide bonds, offering valuable insights when probing peptide backbone interactions, but are prone to side reactions during solid-phase peptide synthesis (SPPS). Thioimidates have been demonstrated to be effective protecting groups for thioamides during peptide elongation. We further demonstrate how thioimidates can assist thioamides through the most yield-crippling step of thionopeptide deprotection, allowing for the first isolation of an important benchmark α-helical peptide that had previously eluded synthesis and isolation.

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.

Biochemical and Structural Characterization of OvoATh2: A Mononuclear Nonheme Iron Enzyme from Hydrogenimonas thermophila for Ovothiol Biosynthesis

Ovothiol A and ergothioneine are thiol-histidine derivatives with sulfur substitutions at the δ-carbon or ε-carbon of the l-histidine imidazole ring, respectively. Both ovothiol A and ergothioneine have protective effects on many aging-related diseases, and the sulfur substitution plays a key role in determining their chemical and biological properties, while factors governing sulfur incorporation regioselectivities in ovothiol and ergothioneine biosynthesis in the corresponding enzymes (OvoA, Egt1, or EgtB) are not yet known. In this study, we have successfully obtained the first OvoA crystal structure, which provides critical information to explain their C-S bond formation regioselectivity. Furthermore, OvoATh2 exhibits several additional activities: (1) ergothioneine sulfoxide synthase activity akin to Egt1 in ergothioneine biosynthesis; (2) cysteine dioxygenase activity using l-cysteine and l-histidine analogues as substrates; (3) cysteine dioxygenase activity upon mutation of an active site tyrosine residue (Y406). The structural insights and diverse chemistries demonstrated by OvoATh2 pave the way for future comprehensive structure-function correlation studies.

A new sulphur-containing metabolite from a mangrove endophytic fungus Aspergillus sp. GXNU-MA

A new sulphur-containing metabolite, asperiguxidione A (1), was isolated from a mangrove endophytic fungus Aspergillus sp. GXNU-MA, and four known alkaloids 2-5, were isolated together from this strain. Their structures were determined by the com-bination of 1D and 2D NMR spectroscopy, HR-ESI-MS, and ECD analysis. Compounds 1 and 2 exhibited mediate activity against Staphylococcus aureus and Enterobacter aerogenes with equal MIC values of 12.5 μg/mL. Compound 3 reduced NO production in LPS-stimulated cells with an IC50 value of 13.329 ± 0.53 μg/mL in the anti-inflammatory assay.