Nystatin‐enhanced glutathione production by Saccharomyces cerevisiae depends on γ‐glutamylcysteine synthase activity and K+

Glutathione (GSH), an important tripeptide compound, is widely used as a therapeutic and in the food and cosmetic industries. To improve its production yield, we added the antibiotic nystatin to a batch fermentation of Saccharomyces cerevisiae, at different concentrations and at various times. Based on the results that nystatin can effectively stimulate GSH accumulation but at the same time inhibits cell growth, a three‐point addition strategy (0.05 mg/L at 8 h, 0.25 mg/L at 16 h, and 0.5 mg/L at 20 h) was developed to maximize GSH production. As a result, a maximum yield of 237.8 mg/L was obtained, which was by 50.6% higher than without the addition of nystatin. When combining this strategy with cysteine addition, the GSH yield increased to 278.9 mg/L. Subsequently, the γ‐glutamylcysteine synthetase (γ‐GCS) activity and K⁺ concentration were analyzed to investigate the possible mechanism involved in the increased production. It was found that the nystatin‐induced increase in the GSH yield was associated with a higher γ‐GCS activity and K⁺ concentration.     

酵母生产谷胱甘肽的培养条件研究

应用Plackett-Burman实验设计、响应面分析方法研究了酵母生产谷胱甘肽的培养条件,结果表明:最佳培养条件为初始pH 5.0,培养温度28℃,接种量10%,摇床转速200 r/min,种子液种龄22~23 h。葡萄糖1.95%,糖蜜1.95%,蛋白胨3%,Cys.HCl 0.10%,MgSO4.7H2O 0.5%,甲硫氨酸0.05%,在此优化的条件下培养,谷胱甘肽的产量达235.7 mg/L,比优化前提高45.4%。     

Differentiation of probiotic and environmental Saccharomyces cerevisiae strains in animal feed

Aims: To establish an identification system for probiotic Saccharomyces cerevisiae strains based on artificial neural network (ANN)–assisted Fourier‐transform infrared (FTIR) spectroscopy to improve quality control of animal feed. Methods and Results: The ANN‐based system for differentiating environmental from probiotic S. cerevisiae strains comprises five authorized feed additive strains plus environmental strains isolated from different habitats. A total of 108 isolates were used as reference strains to create the ANN. DHPLC analysis and δ‐PCR were used as reference methods to type probiotic yeast isolates. The performance of the FTIR‐ANN was tested in an internal validation using unknown spectra of each reference strain. This validation step yielded a classification rate of 99·1 %. For an external validation, a test data set comprising 965 spectra of 63 probiotic and environmental S. cerevisiae isolates unknown to the ANN was used, resulting in a classification rate of 98·2 %. Conclusions: Our results demonstrate that probiotic S. cerevisiae strains in feed can be differentiated successfully from environmental isolates using both genotypic approaches and ANN‐based FTIR spectroscopy. Significance and Impact of the Study: FTIR‐based artificial neural network analysis provides a rapid and inexpensive technique for yeast identification both at the species and at the strain level in routine diagnostic laboratories, using a single sample preparation.     

Crabtree/Warburg-like aerobic xylose fermentation by engineered Saccharomyces cerevisiae

Bottlenecks in the efficient conversion of xylose into cost-effective biofuels have limited the widespread use of plant lignocellulose as a renewable feedstock. The yeast Saccharomyces cerevisiae ferments glucose into ethanol with such high metabolic flux that it ferments high concentrations of glucose aerobically, a trait called the Crabtree/Warburg Effect. In contrast to glucose, most engineered S. cerevisiae strains do not ferment xylose at economically viable rates and yields, and they require respiration to achieve sufficient xylose metabolic flux and energy return for growth aerobically. Here, we evolved respiration-deficient S. cerevisiae strains that can grow on and ferment xylose to ethanol aerobically, a trait analogous to the Crabtree/Warburg Effect for glucose. Through genome sequence comparisons and directed engineering, we determined that duplications of genes encoding engineered xylose metabolism enzymes, as well as TKL1, a gene encoding a transketolase in the pentose phosphate pathway, were the causative genetic changes for the evolved phenotype. Reengineered duplications of these enzymes, in combination with deletion mutations in HOG1, ISU1, GRE3, and IRA2, increased the rates of aerobic and anaerobic xylose fermentation. Importantly, we found that these genetic modifications function in another genetic background and increase the rate and yield of xylose-to-ethanol conversion in industrially relevant switchgrass hydrolysate, indicating that these specific genetic modifications may enable the sustainable production of industrial biofuels from yeast. We propose a model for how key regulatory mutations prime yeast for aerobic xylose fermentation by lowering the threshold for overflow metabolism, allowing mutations to increase xylose flux and to redirect it into fermentation products.     

Mammalian proapoptotic factor ChaC1 and its homologues function as γ‐glutamyl cyclotransferases acting specifically on glutathione

ChaC1 is a mammalian proapoptic protein of unknown function induced during endoplasmic reticulum stress. We show using in vivo studies and novel in vitro assays that the ChaC family of proteins function as γ‐glutamyl cyclotransferases acting specifically to degrade glutathione but not other γ‐glutamyl peptides. The overexpression of these proteins (but not the catalytically dead E>Q mutants) led to glutathione depletion and enhanced apoptosis in yeast. The ChaC family is conversed across all phyla and represents a new pathway for glutathione degradation in living cells, and the first cytosolic pathway for glutathione degradation in mammalian cells.     

The significance of peroxisome function in chronological aging of Saccharomyces cerevisiae

We studied the chronological lifespan of glucose‐grown Saccharomyces cerevisiae in relation to the function of intact peroxisomes. We analyzed four different peroxisome‐deficient (pex) phenotypes. These included Δpex3 cells that lack peroxisomal membranes and in which all peroxisomal proteins are mislocalized together with Δpex6 in which all matrix proteins are mislocalized to the cytosol, whereas membrane proteins are still correctly sorted to peroxisomal ghosts. In addition, we analyzed two mutants in which the peroxisomal location of the β‐oxidation machinery is in part disturbed. We analyzed Δpex7 cells that contain virtually normal peroxisomes, except that all matrix proteins that contain a peroxisomal targeting signal type 2 (PTS2, also including thiolase), are mislocalized to the cytosol. In Δpex5 cells, peroxisomes only contain matrix proteins with a PTS2 in conjunction with all proteins containing a peroxisomal targeting signal type 1 (PTS1, including all β‐oxidation enzymes except thiolase) are mislocalized to the cytosol. We show that intact peroxisomes are an important factor in yeast chronological aging because all pex mutants showed a reduced chronological lifespan. The strongest reduction was observed in Δpex5 cells. Our data indicate that this is related to the complete inactivation of the peroxisomal β‐oxidation pathway in these cells due to the mislocalization of thiolase. Our studies suggest that during chronological aging, peroxisomal β‐oxidation contributes to energy generation by the oxidation of fatty acids that are released by degradation of storage materials and recycled cellular components during carbon starvation conditions.     

Improvement of glutathione production by metabolic engineering the sulfate assimilation pathway of Saccharomyces cerevisiae

Glutathione (GSH) is a valuable tri-peptide that is widely used in the pharmaceutical, food, and cosmetic industries. Glutathione is produced industrially by fermentation using Saccharomyces cerevisiae. In this study, we demonstrated that engineering in sulfate assimilation metabolism can significantly improve GSH production. The intracellular GSH content of MET14 and MET16 over-expressing strains increased up to 1.2 and 1.4-fold higher than that of the parental strain, respectively, whereas those of APA1 and MET3 over-expressing strains decreased. Especially, in the MET16 over-expressing strain, the volumetric GSH concentration was up to 1.7-fold higher than that of the parental strain as a result of the synergetic effect of the increases in the cell concentration and the intracellular GSH content. Additionally, combinatorial mutant strains that had been engineered to contain both the sulfur and the GSH synthetic metabolism synergistically increased the GSH production. External addition of cysteine to S. cerevisiae is well known as a way to increase the intracellular GSH content; however, it results a decrease in cell growth. This study showed that the engineering of sulfur metabolism in S. cerevisiae proves more valuable than addition of cysteine as a way to boost GSH production due to the increases in both the intracellular GSH content and the cell growth.     

Knockout of pgdS and ggt genes improves γ‐PGA yield in B. subtilis

One of the emerging biopolymers that are currently under active investigation is bacterial poly(γ‐glutamic acid) (γ‐PGA). However, before its full industrial exploitation, a substantial increase in microbial productivity is required. γ‐PGA obtained from the Bacillus subtilis laboratory strain 168 offers the advantage of a producer characterized by a well defined genetic framework and simple manipulation techniques. In this strain, the knockout of genes for the major γ‐PGA degrading enzymes, pgdS and ggt, leads to a considerable improvement in polymer yield, which attains levels analogous to the top wild γ‐PGA producer strains. This study highlights the convenience of using the laboratory strain of B. subtilis over wild isolates in designing strain improvement strategies aimed at increasing γ‐PGA productivity. Biotechnol. Bioeng. 2013; 110: 2006–2012. © 2013 Wiley Periodicals, Inc.     

Effect of thyroid hormone administration on the depletion of circulating glutathione in the isolated perfused rat liver and its relationship to basolateral γ-glutamyltransferase activity

The influence of thyroid hormone administration on liver glutathione (GSH) extraction in the isolated perfused liver was studied in fed rats for a period of 1–7 days following a single dose of 0.1 mg 3,5,3′-triiodothyronine (T₃)/kg. T₃ treatment led to an early and transient calorigenic response, as well as an enhancement in liver GSH removal, reaching a maximal effect at 2 days after hormone administration, which was normalized in the 3- to 7-day period studied. Addition of the γ-glutamyltransferase (γ-GT) inhibitor DL-serineborate (4 mM) to the perfusate abolished the increase in the hepatic removal of GSH elicited by T₃, and enhanced the sinusoidal concentration of GSH, studied at 2 days after hormone administration. These data support the role of hepatic basolateral γ-GT ectoactivity in the depletion of portally added and liver-derived GSH as an adaptive response to recover GSH levels after reduction by T₃-induced oxidative stress.     

The stress response neuropeptide CRF increases amyloid‐β production by regulating γ‐secretase activity

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid‐β (Aβ) production. In cells, CRF treatment increases Aβ production and triggers CRF receptor 1 (CRFR1) and γ‐secretase internalization. Co‐immunoprecipitation studies establish that γ‐secretase associates with CRFR1; this is mediated by β‐arrestin binding motifs. Additionally, CRFR1 and γ‐secretase co‐localize in lipid raft fractions, with increased γ‐secretase accumulation upon CRF treatment. CRF treatment also increases γ‐secretase activity in vitro, revealing a second, receptor‐independent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate γ‐secretase activity. Unexpectedly, CRFR1 antagonists also increased Aβ. These data collectively link CRF to increased Aβ through γ‐secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Aβ and in some cases preferentially increase Aβ42 via complex effects on γ‐secretase.     

Prion protein regulates glutathione metabolism and neural glutamate and cysteine uptake via excitatory amino acid transporter 3

Prion protein (PrP) plays crucial roles in regulating antioxidant systems to improve cell defenses against cellular stress. Here, we show that the interactions of PrP with the excitatory amino acid transporter 3 (EAAT3), γ‐glutamyl transpeptidase (γ‐GT), and multi‐drug resistance protein 1 (MRP1) in astrocytes and the interaction between PrP and EAAT3 in neurons regulate the astroglial and neuronal metabolism of the antioxidant glutathione. Ablation of PrP in astrocytes and cerebellar neurons leads to dysregulation of EAAT3‐mediated uptake of glutamate and cysteine, which are precursors for the synthesis of glutathione. In PrP‐deficient astrocytes, levels of intracellular glutathione are increased, and under oxidative stress, levels of extracellular glutathione are increased, due to (i) increased glutathione release via MRP1 and (ii) reduced activity of the glutathione‐degrading enzyme γ‐GT. In PrP‐deficient cerebellar neurons, cell death is enhanced under oxidative stress and glutamate excitotoxicity, when compared to wild‐type cerebellar neurons. These results indicate a functional interplay of PrP with EAAT3, MRP1 and γ‐GT in astrocytes and of PrP and EAAT3 in neurons, suggesting that these interactions play an important role in the metabolic cross‐talk between astrocytes and neurons and in protection of neurons by astrocytes from oxidative and glutamate‐induced cytotoxicity.