Characterization of the glutathione S-transferases that belong to the GSTFuA class in Ceriporiopsis subvermispora: Implications in intracellular detoxification and metabolism of wood-derived compounds

Fungal removal of cyanotoxins in constructed wetlands: The forgotten degraders

Harmful cyanobacterial blooms have increased globally, releasing hazardous cyanotoxins that threaten the safety of water resources. Constructed wetlands (CWs) are a nature-based and low-cost solution to purify and remove cyanotoxins from water. However, bio-mechanistic understanding of the biotransformation processes expected to drive cyanotoxin removal in such systems is poor, and primarily focused on bacteria. Thus, the present study aimed at exploring the fungal contribution to microcystin-LR and cylindrospermopsin biodegradation in CWs. Based on CW mesocosms, two experimental approaches were taken: a) amplicon sequencing studies were conducted to investigate the involvement of the fungal community; and b) CW fungal isolates were tested for their microcystin-LR and cylindrospermopsin degradation capabilities. The data uncovered effects of seasonality (spring or summer), cyanotoxin exposure, vegetation (unplanted, Juncus effusus or Phragmites australis) and substratum (sand or gravel) on the fungal community structure. Additionally, the arbuscular mycorrhizal fungus Rhizophagus and the endophyte Myrmecridium showed positive correlations with cyanotoxin removal. Fungal isolates revealed microcystin-LR-removal potentials of approximately 25 % in in vitro biodegradation experiments, while the extracellular chemical fingerprint of the cultures suggested a potential intracellular metabolization. The results from this study may help us understand the fungal contribution to cyanotoxin removal, as well as their ecology in CWs.

Enzymatic Degradation of Deoxynivalenol with the Engineered Detoxification Enzyme Fhb7

In the era of global climate change, the increasingly severe Fusarium head blight (FHB) and deoxynivalenol (DON) contamination have caused economic losses and brought food and feed safety concerns. Recently, an FHB resistance gene Fhb7 coding a glutathione-S transferase (GST) to degrade DON by opening the critical toxic epoxide moiety was identified and opened a new window for wheat breeding and DON detoxification. However, the poor stability of Fhb7 and the elusiveness of the catalytic mechanism hinder its practical application. Herein, we report the first structure of Fhb7 at 2.41 Å and reveal a unique catalytic mechanism of epoxide opening transformation in GST family proteins. Furthermore, variants V29P and M10 showed that 5.5-fold and 266.7-fold longer half-life time than wild-type, respectively, were identified. These variants offer broad substrate scope, and the engineered biosafe Bacillus subtilis overexpressing the variants shows excellent DON degradation performance, exhibiting potential at bacterium engineering to achieve DON detoxification in the feed and biomedicine industry. This work provides a profound mechanistic insight into the enzymatic activities of Fhb7 and paves the way for further utilizing Fhb7-related enzymes in crop breeding and DON detoxification by synthetic biology.

Diversity of Omega Glutathione Transferases in mushroom-forming fungi revealed by phylogenetic, transcriptomic, biochemical and structural approaches

The Omega class of glutathione transferases (GSTs) forms a distinct class within the cytosolic GST superfamily because most of them possess a catalytic cysteine residue. The human GST Omega 1 isoform was first characterized twenty years ago, but it took years of work to clarify the roles of the human isoforms. Concerning the kingdom of fungi, little is known about the cellular functions of Omega glutathione transferases (GSTOs), although they are widely represented in some of these organisms. In this study, we re-assess the phylogeny and the classification of GSTOs based on 240 genomes of mushroom-forming fungi (Agaricomycetes). We observe that the number of GSTOs is not only extended in the order of Polyporales but also in other orders such as Boletales. Our analysis leads to a new classification in which the fungal GSTOs are divided into two Types A and B. The catalytic residue of Type-A is either cysteine or serine, while that of Type-B is cysteine. The present study focuses on Trametes versicolor GSTO isoforms that possess a catalytic cysteine residue. Transcriptomic data show that Type-A GSTOs are constitutive enzymes while Type-B are inducible ones. The crystallographic analysis reveals substantial structural differences between the two types while they have similar biochemical profiles in the tested conditions. Additionally, these enzymes have the ability to bind antioxidant molecules such as wood polyphenols in two possible binding sites as observed from X-ray structures. The multiplication of GSTOs could allow fungal organisms to adapt more easily to new environments.

Identification of key residues for activities of atypical glutathione S-transferase of Ceriporiopsis subvermispora, a selective degrader of lignin in woody biomass, by crystallography and functional mutagenesis

Ceriporiopsis subvermispora (C. subvermispora) is a selective degrader of lignin in the woody biomass. Glutathione S-transferases (GSTs) are multifunctional enzymes that play important roles in cellular detoxification and metabolism. The crystal structures of a GST of C. subvermispora, CsGST83044, in GSH-free and -bound forms were solved at 1.95 and 2.19 Å resolution, respectively. The structure of the GSH-bound form revealed that CsGST83044 can be categorized as an atypical-type of GST. In the GSH-bound form of CsGST83044, Asn22, Asn24, and Tyr46 are located closest to the sulfur atom and form hydrogen bonds with the thiol group. The functional mutagenesis indicated that they are critical for the enzymatic activities of CsGST83044. The critical residues of an atypical-type GST belonging to the GSTFuA class were revealed for the first time. A previous study indicated that CsGST83044 and another GST, CsGST63524, differ in substrate preference; CsGST83044 prefers smaller substrates than CsGST63524 for its esterase activity. The GSH-bound pocket of CsGST83044 turns out to be small, which may explain the preference for smaller substrates. Protein engineering of GSTs of C. subvermispora in the light of the obtained insight may pave a path in the future for utilization of the woody biomass.

Structure of a serine-type glutathione S-transferase of Ceriporiopsis subvermispora and identification of the enzymatically important non-canonical residues by functional mutagenesis

Ceriporiopsis subvermispora (C. subvermispora), one of the white-rot fungi, is known as a selective lignin degrader of the woody biomass. Glutathione S-transferases (GSTs) are multifunctional enzymes that are capable of catalyzing the reactions involved in detoxification and metabolic pathways. In this study, a GST of C. subvermispora, named CsGST63524, was overexpressed in E. coli, and then purified by affinity, anion exchange, and size exclusion column chromatography. The crystal structures of the CsGST63524 in ligand-free and complex with GSH were refined at 2.45 and 2.50 Å resolutions, respectively. The sulfur atom of glutathione forms a hydrogen bond with Ser21 of CsGST63524, indicating it is a serine-type GST. Mutagenesis of Ser21 unexpectedly indicated that this serine residue is not essential for the enzymatic activity of CsGST63524. Comparative sequence and structural analyses, together with functional mutagenesis, newly identified the enzymatically important non-canonical amino acid residues, Asn23 and Tyr45, other than the serine residue.