Fungal L-Methionine Biosynthesis Pathway Enzymes and Their Applications in Various Scientific and Commercial Fields

L-methionine (L-Met) is one of the nine proteinogenic amino acids essential for humans since, in human cells, there are no complete pathways for its biosynthesis from simple precursors. L-Met plays a crucial role in cellular function as it is required for proper protein synthesis, acting as an initiator. Additionally, this amino acid participates in various metabolic processes and serves as a precursor for the synthesis of S-adenosylmethionine (AdoMet), which is involved in the methylation of DNA molecules and phospholipids, as well as in maintaining genome stability. Due to its importance, fungal L-methionine biosynthesis pathway enzymes are being intensively studied. This review presents the current state of the art in terms of their cellular function, usefulness as molecular markers, antifungal targets, or industrial approaches.

Copper-based grape pest management has impacted wine aroma

Despite the high energetic cost of the reduction of sulfate to H2S, required for the synthesis of sulfur-containing amino acids, some wine Saccharomyces cerevisiae strains have been reported to produce excessive amounts of H2S during alcoholic fermentation, which is detrimental to wine quality. Surprisingly, in the presence of sulfite, used as a preservative, wine strains produce more H2S than wild (oak) or wine velum (flor) isolates during fermentation. Since copper resistance caused by the amplification of the sulfur rich protein Cup1p is a specific adaptation trait of wine strains, we analyzed the link between copper resistance mechanism, sulfur metabolism and H2S production. We show that a higher content of copper in the must increases the production of H2S, and that SO2 increases the resistance to copper. Using a set of 51 strains we observed a positive and then negative relation between the number of copies of CUP1 and H2S production during fermentation. This complex pattern could be mimicked using a multicopy plasmid carrying CUP1, confirming the relation between copper resistance and H2S production. The massive use of copper for vine sanitary management has led to the selection of resistant strains at the cost of a metabolic tradeoff: the overproduction of H2S, resulting in a decrease in wine quality.

QTL mapping reveals novel genes and mechanisms underlying variations in H2S production during alcoholic fermentation in Saccharomyces cerevisiae

Saccharomyces cerevisiae requirement for reduced sulfur to synthesize methionine and cysteine during alcoholic fermentation, is mainly fulfilled through the sulfur assimilation pathway. Saccharomyces cerevisiae reduces sulfate into sulfur dioxide (SO2) and sulfide (H2S), whose overproduction is a major issue in winemaking, due to its negative impact on wine aroma. The amount of H2S produced is highly strain-specific and also depends on SO2 concentration, often added to grape must. Applying a bulk segregant analysis to a 96-strain-progeny derived from two strains with different abilities to produce H2S, and comparing allelic frequencies along the genome of pools of segregants producing contrasting H2S quantities, we identified two causative regions involved in H2S production in the presence of SO2. A functional genetic analysis allowed the identification of variants in four genes able to impact H2S formation, viz; ZWF1, ZRT2, SNR2, and YLR125W, and involved in functions and pathways not associated with sulfur metabolism until now. These data point out that, in wine fermentation conditions, redox status, and zinc homeostasis are linked to H2S formation while providing new insights into the regulation of H2S production, and a new vision of the interplay between the sulfur assimilation pathway and cell metabolism.

Hydrogen sulfide production during early yeast fermentation correlates with volatile sulfur compound biogenesis but not thiol release

Yeasts undergo intensive metabolic changes during the early stages of fermentation. Previous reports suggest the early production of hydrogen sulfide (H2S) is associated with the release of a range of volatile sulfur compounds (VSCs), as well as the production of varietal thiol compounds 3-sulfanylhexan-1-ol (3SH) and 3-sulfanylhexyl acetate (3SHA) from six-carbon precursors, including (E)-hex-2-enal. In this study, we investigated the early H2S potential, VSCs/thiol output, and precursor metabolism of 11 commonly used laboratory and commercial Saccharomyces cerevisiae strains in chemically defined synthetic grape medium (SGM) within 12 h after inoculation. Considerable variability in early H2S potential was observed among the strains surveyed. Chemical profiling suggested that early H2S production correlates with the production of dimethyl disulfide, 2-mercaptoethanol, and diethyl sulfide, but not with 3SH or 3SHA. All strains were capable of metabolizing (E)-hex-2-enal, while the F15 strain showed significantly higher residue at 12 h. Early production of 3SH, but not 3SHA, can be detected in the presence of exogenous (E)-hex-2-enal and H2S. Therefore, the natural variability of early yeast H2S production contributes to the early output of selected VSCs, but the threshold of which is likely not high enough to contribute substantially to free varietal thiols in SGM.

Self-Produced Hydrogen Sulfide Improves Ethanol Fermentation by Saccharomyces cerevisiae and Other Yeast Species

Hydrogen sulfide (H2S) is a gas produced endogenously in organisms from the three domains of life. In mammals, it is involved in diverse physiological processes, including the regulation of blood pressure and its effects on memory. In contrast, in unicellular organisms, the physiological role of H2S has not been studied in detail. In yeast, for example, in the winemaking industry, H2S is an undesirable byproduct because of its rotten egg smell; however, its biological relevance during fermentation is not well understood. The effect of H2S in cells is linked to a posttranslational modification in cysteine residues known as S-persulfidation. In this paper, we evaluated S-persulfidation in the Saccharomyces cerevisiae proteome. We screened S-persulfidated proteins from cells growing in fermentable carbon sources, and we identified several glycolytic enzymes as S-persulfidation targets. Pyruvate kinase, catalyzing the last irreversible step of glycolysis, increased its activity in the presence of a H2S donor. Yeast cells treated with H2S increased ethanol production; moreover, mutant cells that endogenously accumulated H2S produced more ethanol and ATP during the exponential growth phase. This mechanism of the regulation of metabolism seems to be evolutionarily conserved in other yeast species, because H2S induces ethanol production in the pre-Whole-Genome Duplication species Kluyveromyces marxianus and Meyerozyma guilliermondii. Our results suggest a new role of H2S in the regulation of the metabolism during fermentation.

The mechanism of action of N-acetylcysteine (NAC): The emerging role of H2S and sulfane sulfur species

Initially adopted as a mucolytic about 60 years ago, the cysteine prodrug N-acetylcysteine (NAC) is the standard of care to treat paracetamol intoxication, and is included on the World Health Organization's list of essential medicines. Additionally, NAC increasingly became the epitome of an "antioxidant". Arguably, it is the most widely used "antioxidant" in experimental cell and animal biology, as well as clinical studies. Most investigators use and test NAC with the idea that it prevents or attenuates oxidative stress. Conventionally, it is assumed that NAC acts as (i) a reductant of disulfide bonds, (ii) a scavenger of reactive oxygen species and/or (iii) a precursor for glutathione biosynthesis. While these mechanisms may apply under specific circumstances, they cannot be generalized to explain the effects of NAC in a majority of settings and situations. In most cases the mechanism of action has remained unclear and untested. In this review, we discuss the validity of conventional assumptions and the scope of a newly discovered mechanism of action, namely the conversion of NAC into hydrogen sulfide and sulfane sulfur species. The antioxidative and cytoprotective activities of per- and polysulfides may explain many of the effects that have previously been ascribed to NAC or NAC-derived glutathione.

Identification and characterization of a sulfite reductase gene and new insights regarding the sulfur-containing amino acid metabolism in the basidiomycetous yeast Cryptococcus neoformans

The amino acid biosynthetic pathway of invasive pathogenic fungi has been studied as a potential antifungal drug target. Studies of the disruption of genes involved in amino acid biosynthesis have demonstrated the importance of this pathway in the virulence of Cryptococcus neoformans. Here, we identified the MET5 (CNL05500) and MET10 (CNG03990) genes in this pathway, both encoding sulfite reductase, which catalyzes the reduction of sulfite to sulfide. The MET14 (CNE03880) gene was also identified, which is responsible for the conversion of sulfate to sulfite. The use of cysteine as a sulfur source led to the production of methionine via hydrogen sulfide synthesis mediated by CYS4 (CNA06170), CYS3 (CNN01730), and MST1 (CND03690). MST1 exhibited high homology with the TUM1 gene of Saccharomyces cerevisiae, which has functional similarity with the 3-mercaptopyruvate sulfurtransferase (3-MST) gene in humans. Although the hypothesis that hydrogen sulfide is produced from cysteine via CYS4, CYS3, and MST1 warrants further study, the new insight into the metabolic pathway of sulfur-containing amino acids in C. neoformans provided here indicates the usefulness of this system in the development of screening tools for antifungal drug agents.

Iterative synthetic strategies and gene deletant experiments enable the first identification of polysulfides in Saccharomyces cerevisiae

Polysulfides, potential signalling molecules, were synthesised and then found and explored for the first time in yeast.

Novel biosynthetic pathway for sulfur amino acids in Cryptococcus neoformans

We elucidated a unique feature of sulfur metabolism in Cryptococcus neoformans. C. neoformans produces cysteine solely by the O-acetylserine pathway that consists of serine-O-acetyl transferase and cysteine synthase. We designated the gene encoding the former enzyme CYS2 (locus tag CNE02740) and the latter enzyme CYS1 (locus tag CNL05880). The cys1Δmutant strain was found to be avirulent in a murine infection model. Methionine practically does not support growth of the cys1Δ strain, and cysteine does not serve as a methionine source, indicating that the transsulfuration pathway does not contribute to sulfur amino acid synthesis in C. neoformans. Among the genes encoding enzymes catalyzing the reactions from homoserine to methionine, the gene corresponding to the Saccharomyces cerevisiae MET17 encoding O-acetylhomoserine sulfhydrylase (Met17p) had remained to be identified in C. neoformans. By genetic analysis of Met^- mutants obtained by Agrobacterium tumefaciens-mediated mutagenesis, we concluded that Cnc01220, most similar to Str2p (36% identity), cystathionine-γ-synthase, in the Saccharomyces genome, is the C. neoformans version of O-acetylhomoserine sulfhydrylase. We designated CNC01220 as MET17. The C. neoformans met3Δ mutant defective in the first step of the sulfate assimilation pathway, sulfate adenylyltransferase, barely uses methionine as a sulfur source, whereas it uses cysteine efficiently. The poor utilization of methionine by the met3Δ mutant is most probably due to the absence of the transsulfuration pathway, causing an incapability of C. neoformans to produce cysteine and hydrogen sulfide from methionine. When cysteine is used as a sulfur source, methionine is likely produced de novo by using hydrogen sulfide derived from cysteine via an unidentified pathway. Altogether, the unique features of sulfur amino acid metabolism in C. neoformans will make this fungus a valuable experimental system to develop anti-fungal agents and to investigate physiology of hydrogen sulfide.

Yeast genes involved in regulating cysteine uptake affect production of hydrogen sulfide from cysteine during fermentation

An early burst of hydrogen sulfide (H2S) produced by Saccharomyces cerevisiae during fermentation could increase varietal thiols and therefore enhance desirable tropical aromas in varieties such as Sauvignon Blanc. Here we attempted to identify genes affecting H2S formation from cysteine by screening yeast deletion libraries via a colony colour assay on media resembling grape juice. Both Δlst4 and Δlst7 formed lighter coloured colonies and produced significantly less H2S than the wild type on high concentrations of cysteine, likely because they are unable to take up cysteine efficiently. We then examined the nine known cysteine permeases and found that deletion of AGP1, GNP1 and MUP1 led to reduced production of H2S from cysteine. We further showed that deleting genes involved in the SPS-sensing pathway such as STP1 and DAL81 also reduced H2S from cysteine. Together, this study indirectly confirms that Agp1p, Gnp1p and Mup1p are the major cysteine permeases and that they are regulated by the SPS-sensing and target of rapamycin pathways under the grape juice-like, cysteine-supplemented, fermentation conditions. The findings highlight that cysteine transportation could be a limiting factor for yeast to generate H2S from cysteine, and therefore selecting wine yeasts without defects in cysteine uptake could maximise thiol production potential.