Effects of Lys and His supplementations on the regulation of nitrogen metabolism in lager yeast

Metabolomics Reveals the Regulatory Mechanisms of Antioxidant Dipeptides Enhancing the Tolerance of Lager Yeast against Ethanol Stress

The antioxidant dipeptides (Ala-His, AH; Thr-Tyr, TY; and Phe-Cys, FC) significantly enhanced the lager yeast tolerance of ethanol stress. The enhancement mechanisms were further elucidated through physiological responses and metabolomics analysis. The results indicated that antioxidant dipeptides significantly increased the lager yeast biomass and budding rate. The primary mechanisms by which antioxidant dipeptides improved lager yeast tolerance involved decreasing intracellular reactive oxygen species (ROS) levels and increasing energy metabolism. Specifically, the addition of FC resulted in a 27.44% reduction in intracellular ROS content and a 26.14% increase in the ATP level compared to the control. Metabolomics analysis further explored the potential mechanisms underlying the protective effects of FC, identifying 63 upregulated and 103 downregulated metabolites. The analysis revealed that FC altered intracellular metabolites related to glutathione metabolism, purine metabolism, starch and sucrose metabolism, and ABC transporters, thereby enhancing yeast stress tolerance. The results suggest that FC is an effective enhancer for improving lager yeast tolerance to ethanol stress.

Volatile Aroma Compound Production Is Affected by Growth Rate in S. cerevisiae

The initial growth rate of a yeast strain is a key parameter in the production of fermented beverages. Fast growth is linked with higher fermentative capacity and results in less slow and stuck fermentations unable to reach the expected final gravity. As concentrations of metabolites are in a constant state of flux, quantitative data on how growth rate affects the production of aromatic compounds becomes an important factor for brewers. Chemostats allow to set and keep a specific dilution rate throughout the fermentation and are ideal system to study the effect of growth on aroma production. In this study, we ran chemostats alongside batch and fed-batch cultures, compared volatile profiles detected at different growth rates, and identified those affected by the different feeding profiles. Specifically, we quantified six abundant aroma compounds produced in anaerobic glucose-limited continuous cultivations of S. cerevisiae at different dilution rates. We found that volatile production was affected by the growth rate in four out of six compounds assayed, with higher alcohols and esters following opposite trends. Batch and fed-batch fermentations were devised to study the extent by which the final concentration of volatile compounds is influenced by glucose availability. Compared with the batch system, fed-batch fermentations, where the yeast growth was artificially limited by a slow constant release of nutrients in the media, resulted in a significant increase in concentration of higher alcohols, mirroring the results obtained in continuous fermentations. This study paves the way to further process development optimization for the production of fermented beverages. IMPORTANCE The production of fermentation beverages will need to quickly adapt to changes in both the climate and customer demands, requiring the development of new strains and processes. Breakthroughs in the field are hindered by the limited knowledge on the interplay between physiology and aroma compound production in yeast. No quantitative data on how growth rate affects aroma profile is available in the literature to guide optimization of the complex flavors in fermented beverages. In this study, we exploited the chemostat system, alongside with batch and fed-batch cultures, to compare volatile profiles at different growth rates. We identified the aromatic compounds affected by the different feeding profiles and nutrient limitations. Moreover, we uncovered the correlation between yeast growth, esters, and higher alcohols production. This study showcases the potential of the application of feeding profiles for the manipulation of aroma in the craft beverage industry.

Screening and transcriptomic analysis of the ethanol-tolerant mutant Saccharomyces cerevisiae YN81 for high-gravity brewing

Ethanol stress is one of the major limiting factors for high-gravity brewing. Breeding of yeast strain with high ethanol tolerance, and revealing the ethanol tolerance mechanism of Saccharomyces cerevisiae is of great significance to the production of high-gravity beer. In this study, the mutant YN81 was obtained by ultraviolet-diethyl sulfate (UV-DES) cooperative mutagenesis from parental strain CS31 used in high-gravity craft beer brewing. The ethanol tolerance experiment results showed that cell growth and viability of YN81 were significantly greater than that of CS31 under ethanol stress. The ethanol tolerance mechanisms of YN81 were studied through observation of cell morphology, intracellular trehalose content, and transcriptomic analysis. Results from scanning electron microscope (SEM) showed alcohol toxicity caused significant changes in the cell morphology of CS31, while the cell morphology of YN81 changed slightly, indicating the cell morphology of CS31 got worse (the formation of hole and cell wrinkle). In addition, compared with ethanol-free stress, the trehalose content of YN81 and CS31 increased dramatically under ethanol stress, but there was no significant difference between YN81 and CS31, whether with or without ethanol stress. GO functional annotation analysis showed that under alcohol stress, the number of membrane-associated genes in YN81 was higher than that without alcohol stress, as well as CS31, while membrane-associated genes in YN81 were expressed more than CS31 under alcohol stress. KEGG functional enrichment analysis showed unsaturated fatty acid synthesis pathways and amino acid metabolic pathways were involved in ethanol tolerance of YN81. The mutant YN81 and its ethanol tolerance mechanism provide an optimal strain and theoretical basis for high-gravity craft beer brewing.

Microbial succession and exploration of higher alcohols-producing core bacteria in northern Huangjiu fermentation

Higher alcohols (HAs) are abundant compounds that provide important flavors in Huangjiu, but they also cause hangover. Previous studies have shown the production of HAs to be related to yeast, but the correlations between HAs and other microorganisms are rarely reported. In this study, we detected changes in levels of HAs and microbial dynamics during the Huangjiu fermentation process. Relationships were characterized using Pearson’s correlation coefficient. The functional core HA-producing bacteria were selected by bidirectional orthogonal partial least squares (O2PLS). The result showed that 2-methyl-1-propanol, phenethyl alcohol and 3-methyl-1-butanol were the principle HAs present at high levels. Lactococcus and Saccharomyces were predominant at the genus level of bacteria and fungi, respectively. A total of 684 correlations between HAs and microorganisms were established. Five genera were screened as functional core HA-producing bacteria. Our findings might provide some new inspiration for controlling the content of HAs, enhancing international prestige and market expansion of Huangjiu.

Nitrogenous Compound Utilization and Production of Volatile Organic Compounds among Commercial Wine Yeasts Highlight Strain-Specific Metabolic Diversity

Genetic background and environmental conditions affect the production of sensory impact compounds by Saccharomyces cerevisiae. The relative importance of the strain-specific metabolic capabilities for the production of volatile organic compounds (VOCs) remains unclear. We investigated which amino acids contribute to VOC production and whether amino acid-VOC relations are conserved among yeast strains. Amino acid consumption and production of VOCs during grape juice fermentation was investigated using four commercial wine yeast strains: Elixir, Opale, R2, and Uvaferm. Principal component analysis of the VOC data demonstrated that Uvaferm correlated with ethyl acetate and ethyl hexanoate production, R2 negatively correlated with the acetate esters, and Opale positively correlated with fusel alcohols. Biomass formation was similar for all strains, pointing to metabolic differences in the utilization of nutrients to form VOCs. Partial least-squares linear regression showed that total aroma production is a function of nitrogen utilization (R2 = 0.87). We found that glycine, tyrosine, leucine, and lysine utilization were positively correlated with fusel alcohols and acetate esters. Mechanistic modeling of the yeast metabolic network via parsimonious flux balance analysis and flux enrichment analysis revealed enzymes with crucial roles, such as transaminases and decarboxylases. Our work provides insights in VOC production in wine yeasts. IMPORTANCE Saccharomyces cerevisiae is widely used in grape juice fermentation to produce wines. Along with the genetic background, the nitrogen in the environment in which S. cerevisiae grows impacts its regulation of metabolism. Also, commercial S. cerevisiae strains exhibit immense diversity in their formation of aromas, and a desirable aroma bouquet is an essential characteristic for wines. Since nitrogen affects aroma formation in wines, it is essential to know the extent of this connection and how it leads to strain-dependent aroma profiles in wines. We evaluated the differences in the production of key aroma compounds among four commercial wine strains. Moreover, we analyzed the role of nitrogen utilization on the formation of various aroma compounds. This work illustrates the unique aroma-producing differences among industrial yeast strains and suggests more intricate, nitrogen-associated routes influencing those aroma-producing differences.

Screening lager yeast with higher ethyl-acetate production by adaptive laboratory evolution in high concentration of acetic acid

Ethyl-acetate is important for the flavor and aroma of the alcoholic beverages, therefore, there have been extensive efforts toward increasing its production by engineering yeast strains. In this study, we reported a new approach to breed non-genetic modified producing yeast strain with higher ethyl-acetate production for beer brewing. First, we demonstrated the positive effect of higher acetic acid concentration on inducing the expression of acetyl-CoA synthetase (ACS). Then, we applied adaptive laboratory evolution method to evolve strain with higher expression level of ACS. As a result, we obtained several evolved strains with increased ACS expression level as well as ethyl-acetate production. In 3 L scale fermentation, the optimal strain EA60 synthesized more ethyl-acetate than M14 at the same time point. At the end of fermentation, the ethyl-acetate production in EA60 was 21.4% higher than M14, while the other flavor components except for acetic acid were changed in a moderate degree, indicating this strain had a bright prospect in industrial application. Moreover, this study also indicated that ACS1 played a more important role in increasing the acetic acid tolerance of yeast, while ACS2 contributed to the synthesis of cytosol acetyl-CoA, thereby facilitating the production of ethyl-acetate during fermentation.

Wheat gluten hydrolysates promotes fermentation performance of brewer's yeast in very high gravity worts

The effects of wheat gluten hydrolysates (WGH) and their ethanol elution fractions obtained on XAD-16 resin on physiological activity and fermentation performance of brewer's yeast during very-high-gravity (VHG) worts fermentation were investigated. The results showed that the addition of WGH and their elution fractions in VHG worts significantly enhanced yeast biomass and viability, and further increased the fermentability, ethanol yield and productivity of yeast. Supplementation with 40% ethanol fraction exhibited the highest biomass (6.9 g/L dry cell), cell viability, fermentability (82.05%), ethanol titer (12.19%, v/v) and ethanol productivity during VHG worts fermentation. In addition, 40% ethanol fraction supplementation also caused the most consumption of amino acid and the highest accumulation of intracellular glycerol and trehalose, 15.39% of increase in cell-membrane integrity, 39.61% of enhancement in mitochondrial membrane potential (MMP), and 18.94% of reduction in intracellular reactive oxygen species (ROS) level in yeast under VHG conditions. Therefore, WGH supplementation was an efficient method to improve fermentation performance of brewer's yeast during VHG worts.

Occurrence of melatonin and indolic compounds derived from l-tryptophan yeast metabolism in fermented wort and commercial beers

Melatonin and serotonin are bioactive compounds present in foods and beverages and related to neuroprotection and anti-angiogenesis, among other activities. They have been described in wines and the role of yeast in their formation is clear. Thus, this study evaluates the content of these bioactives and other related indolic compounds in beer. For this purpose, commercial beers were analyzed by a validated UHPLC-HRMS method and sample treatment optimized due to the low concentrations expected. Moreover, a wort was fermented with different commercial beer yeast (Abbaye, Diamond, SafAle, SafLager) in order to monitor the formation of these bioactives during the elaboration process. Results show that indolic compounds such as N-acetylserotonin and 3-indoleacetic acid are produced during the alcoholic fermentation of wort. Moreover, the occurrence of four indolic compounds (5-hydroxytryptophan, N-acetylserotonin, 3-indoleacetic acid, l-tryptophan ethyl ester) in commercial beers is reported for the first time.

Evaluation of craft beers through the direct determination of amino acids by capillary electrophoresis and principal component analysis

The composition of beer wort in terms of amino acid (AA) content affects the final product quality, once it is related to the vitality of yeast during the initial exponential growth phase and throughout fermentation. The objective of this work was the use of a capillary zone electrophoresis method with UV-vis detection in association with Principal Component (Data) Analysis for craft beer classification. Cysteine, histidine, phenylalanine, lysine, tryptophan and arginine were the monitored AAs in wort and finished beer, which were extracted through cation exchange resin. Good differentiation among samples according to their production was obtained, showing a profile of AAs (<LQ-99 µg mL^-1) for each wort, which can be used as indicator of the quality of a beer. One of the samples had the mashing step monitored, showing an increasing profile in the concentrations of AAs (6.5-55 µg mL^-1), which can be explained by the protein cleavage.

Effects of N-Acetylcysteine on the reproductive performance, oxidative stress and RNA sequencing of Nubian goats

N-acetylcysteine (NAC) has been found to enhance the protective ability of cells to counter balance oxidative stress and inflammation. To investigate the effects of dietary NAC supplementation on the reproductive performance of goats, the reproductive performance and endometrial transcriptome of goats fed with diets with NAC (NAC group) and without NAC supplementation (control group) were compared. Results showed that the goats fed with 0.03% and 0.05% NAC had similar litter size, birth weight, nitric oxide (NO), sex hormones and amino acids levels compared with the goats of the control group. However, feeding with 0.07% NAC supplementation from day 0 to day 30 of gestation remarkably increased the litter size of goats. The goats of the 0.07% NAC group presented increased levels of NO relative to the control group, but their sex hormones and amino acids showed no differences. Comparative transcriptome analysis identified 207 differentially expressed genes (DEGs) in the endometrium between the control and the 0.07% NAC groups. These DEGs included 146 upregulated genes and 61 downregulated genes in the 0.07% NAC group. They were primarily involved in the cellular response to toxic substances, oxidoreductase activity, immune receptor activity, signalling receptor binding, cytokine-cytokine receptor interactions, PI3K-Akt signalling pathway and PPAR signalling pathway. In conclusion, results showed that dietary 0.07% NAC supplementation exerted a beneficial effect on the survival of goat embryos at the early pregnancy stage. Such positive outcome might be due to the increased NO production and affected expression of genes involved in the anti-inflammation pathways of the endometrium.

[Synthesis and regulation of flavor compounds derived from brewing yeast: fusel alcohols]

Among the main beer components, fusel alcohols are important because of their influence on the flavor of the final product, and therefore on its quality. During the production process, these compounds are generated by yeasts through the metabolism of amino acids. The yeasts, fermentation conditions and wort composition affect fusel alcohols profiles and their concentrations. In this review, we provide detailed information about the enzymes involved in fusel alcohols formation and their regulation. Moreover, we describe how the type of yeast used, the fermentation temperature and the composition of carbohydrates and nitrogen source in wort, among other fermentation parameters, affect the biosynthesis of these alcohols. Knowing how fusel alcohol levels vary during beer production provides a relevant tool for brewers to achieve the desired characteristics in the final product and at the same time highlights the aspects still unknown to science.

Wheat gluten hydrolysates separated by macroporous resins enhance the stress tolerance in brewer's yeast

Wheat gluten hydrolysates (WGH) were used to examine their adsorption-desorption kinetics and thermodynamics characteristics on six macroporous resins, and their effects on the stress tolerance in brewer's yeast. Results showed that the pseudo second-order kinetics, Langmuir and Freundlich model could illuminate the adsorption mechanism, and the adsorption process was physical, spontaneous and exothermic. The 40% ethanol fraction separated by XAD-16 resin improved significantly the ethanol tolerance and the viability of brewer's yeast, while the 0% ethanol fraction separated by XAD-16 resin increased obviously the osmotic stress tolerance and the viability of brewer's yeast. Results from scanning electron microscopy showed that both these WGH fractions could increase budding rate and maintain normal morphology of yeast cells under various environmental stress. Thus, WGH separated by macroporous resin could be used in high gravity brewing to enhance the ethanol and osmotic stress tolerance in brewer's yeast.

Improvement of Multiple-Stress Tolerance and Ethanol Production in Yeast during Very-High-Gravity Fermentation by Supplementation of Wheat-Gluten Hydrolysates and Their Ultrafiltration Fractions

The effects of wheat-gluten hydrolysates (WGH) and their ultrafiltration fractions on multiple-stress tolerance and ethanol production in yeast during very-high-gravity (VHG) fermentation were examined. The results showed that WGH and WHG-ultrafiltration-fraction supplementations could significantly enhance the growth and viability of yeast and further improve the tolerance of yeast to osmotic stress and ethanol stress. The addition of MW < 1 kDa fractions led to 51.08 and 21.70% enhancements in cell-membrane integrity, 30.74 and 10.43% decreases in intracellular ROS accumulation, and 34.18 and 26.16% increases in mitochondrial membrane potential (ΔΨm) in yeast under osmotic stress and ethanol stress, respectively. Moreover, WGH and WHG-ultrafiltration-fraction supplementations also improved the growth and ethanol production of yeast during VHG fermentation, and supplementation with the <1 kDa fraction resulted in a maximum biomass of 16.47 g/L dry cell and an ethanol content of 18.50% (v/v) after VHG fermentation.

Peptide (Lys-Leu) and amino acids (Lys and Leu) supplementations improve physiological activity and fermentation performance of brewer's yeast during very high-gravity (VHG) wort fermentation

Lys and Leu were generally considered as the key amino acids for brewer's yeast during beer brewing. In the present study, peptide Lys-Leu and a free amino acid (FAA) mixture of Lys and Leu (Lys + Leu) were supplemented in 24 °P wort to examine their effects on physiological activity and fermentation performance of brewer's yeast during very high-gravity (VHG) wort fermentation. Results showed that although both peptide Lys-Leu and their FAA mixture supplementations could increase the growth and viability, intracellular trehalose and glycerol content, wort fermentability, and ethanol content for brewer's yeast during VHG wort fermentation, and peptide was better than their FAA mixture at promoting growth and fermentation for brewer's yeast when the same dose was kept. Moreover, peptide Lys-Leu supplementation significantly increased the assimilation of Asp, but decreased the assimilation of Gly, Ala, Val, (Cys)2, Ile, Leu, Tyr, Phe, Lys, Arg, and Pro. However, the FAA mixture supplementation only promoted the assimilation of Lys and Leu, while reduced the absorption of total amino acids to a greater extent. Thus, the peptide Lys-Leu was more effective than their FAA mixture on the improvement of physiological activity, fermentation performance, and nitrogen metabolism of brewer's yeast during VHG wort fermentation.

[Synthesis and regulation of flavor compounds derived from brewing yeast: Esters]

During brewing process yeast produce more than 500 chemical compounds that can negatively and positively impact beer at the organoleptic level. In recent years, and particularly thanks to the advancement of molecular biology and genomics, there has been considerable progress in our understanding about the molecular and cellular basis of the synthesis and regulation of many of these flavor compounds. This article focuses on esters, responsible for the floral and fruity beer flavor. Its formation depends on various enzymes and factors such as the concentration of wort nutrients, the amount of dissolved oxygen and carbon dioxide, fermentation temperature and mainly the genetics of the yeast used. We provide information about how the esters originate and how is the impact of different fermentative parameters on the final concentrations of these compounds and the quality of the end product.

Impact of Wort Amino Acids on Beer Flavour: A Review

The process by which beer is brewed has not changed significantly since its discovery thousands of years ago. Grain is malted, dried, crushed and mixed with hot water to produce wort. Yeast is added to the sweet, viscous wort, after which fermentation occurs. The biochemical events that occur during fermentation reflect the genotype of the yeast strain used, and its phenotypic expression is influenced by the composition of the wort and the conditions established in the fermenting vessel. Although wort is complex and not completely characterized, its content in amino acids indubitably affects the production of some minor metabolic products of fermentation which contribute to the flavour of beer. These metabolic products include higher alcohols, esters, carbonyls and sulfur-containing compounds. The formation of these products is comprehensively reviewed in this paper. Furthermore, the role of amino acids in the beer flavour, in particular their relationships with flavour active compounds, is discussed in light of recent data.

Effects of wheat gluten hydrolysates fractionated by different methods on the growth and fermentation performances of brewer's yeast under high gravity fermentation

Wheat gluten hydrolysates (WGH) were fractionated through ultrafiltration membrane with molecular weight (Mw) cut‐off of 3 kDa and ethanol precipitation, respectively. WGH and their fractions were used to examine their effects on the growth and fermentation performances of brewer's yeast under high gravity fermentation. Results showed that WGH and their fractions exhibited significant differences in biomass accumulation, viability, ethanol yield, free amino nitrogen and sugar consumptions under high gravity fermentation. Compared to WGH, the fractions with Mw < 3 kDa and the supernatant of WGH treated with ethanol precipitation showed better fermentation performance for brewer's yeast. The relatively lower molecular weight and the higher levels of Leu, Lys, His and Arg in these two fractions might be responsible for their bioactivity for brewer's yeast. Thus, both ultrafiltration and ethanol precipitation could be used as efficient methods for enriching peptides with significant growth‐ and fermentation‐promoting activity for brewer's yeast under high gravity fermentation.

Physiology, ecology and industrial applications of aroma formation in yeast

Yeast cells are often employed in industrial fermentation processes for their ability to efficiently convert relatively high concentrations of sugars into ethanol and carbon dioxide. Additionally, fermenting yeast cells produce a wide range of other compounds, including various higher alcohols, carbonyl compounds, phenolic compounds, fatty acid derivatives and sulfur compounds. Interestingly, many of these secondary metabolites are volatile and have pungent aromas that are often vital for product quality. In this review, we summarize the different biochemical pathways underlying aroma production in yeast as well as the relevance of these compounds for industrial applications and the factors that influence their production during fermentation. Additionally, we discuss the different physiological and ecological roles of aroma-active metabolites, including recent findings that point at their role as signaling molecules and attractants for insect vectors.

Identification of Nitrogen Consumption Genetic Variants in Yeast Through QTL Mapping and Bulk Segregant RNA-Seq Analyses

Saccharomyces cerevisiae is responsible for wine must fermentation. In this process, nitrogen represents a limiting nutrient and its scarcity results in important economic losses for the wine industry. Yeast isolates use different strategies to grow in poor nitrogen environments and their genomic plasticity enables adaptation to multiple habitats through improvements in nitrogen consumption. Here, we used a highly recombinant S. cerevisiae multi-parent population (SGRP-4X) derived from the intercross of four parental strains of different origins to identify new genetic variants responsible for nitrogen consumption differences during wine fermentation. Analysis of 165 fully sequenced F12 segregants allowed us to map 26 QTL in narrow intervals for 14 amino acid sources and ammonium, the majority of which represent genomic regions previously unmapped for these traits. To complement this strategy, we performed Bulk segregant RNA-seq (BSR-seq) analysis in segregants exhibiting extremely high and low ammonium consumption levels. This identified several QTL overlapping differentially expressed genes and refined the gene candidate search. Based on these approaches, we were able to validate ARO1, PDC1, CPS1, ASI2, LYP1, and ALP1 allelic variants underlying nitrogen consumption differences between strains, providing evidence of many genes with small phenotypic effects. Altogether, these variants significantly shape yeast nitrogen consumption with important implications for evolution, ecological, and quantitative genomics.

Evaluating the effects of biocompatible cholinium ionic liquids on microbial lipid production by Trichosporon fermentans

BACKGROUND

Microbial lipid is a potential raw material for large-scale biodiesel production and lignocellulosic hydrolysate has been considered as promising low-cost substrate for lipid fermentation. Lignocellulosic biomass needs to be pretreated before enzymatic hydrolysis, and biocompatible cholinium ionic liquids (ILs) have been demonstrated to be highly efficient for pretreatment. However, the impact of these ILs residues in hydrolysates on downstream biotransformation remains unknown. Therefore, the influence of three typical cholinium ILs on the lipid production by Trichosporon fermentans was first investigated.

RESULTS

The cell growth of T. fermentans was stimulated in the presence of cholinium lysine ([Ch][Lys]) and cholinium serine ([Ch][Ser]), while the lipid accumulation was inhibited by [Ch][Lys]) and [Ch][Ser]. Both cell growth and lipid accumulation of T. fermentans were inhibited in the presence of cholinium acetate ([Ch][OAc]). Despite the reduction in lipid content, the lipid production by T. fermentans was improved in the presence of low concentrations of [Ch][Lys] (≤30 mM) and [Ch][Ser] (≤20 mM) due to the remarkable increase of biomass. It was found that cholinium cation had minor influence on lipid production. However, the anions of [Ch][Lys] and [Ch][Ser] could be assimilated as nitrogen source by T. fermentans and the reduced C/N ratio accounts for the inhibition of lipid accumulation, which could be alleviated by improving C/N ratio of medium. In addition, the anion of [Ch][OAc] could be metabolized by T. fermentans, leading to a rapid alkaline-pH shift and strong inhibition of lipid production. And this inhibitory effect on lipid production could be significantly reduced by controlling culture pH.

CONCLUSIONS

The anions of [Ch][Lys], [Ch][Ser] and [Ch][OAc] play an important role in affecting the cell growth and lipid accumulation of T. fermentans, and the inhibition of these three ILs on lipid production can be alleviated by careful fermentation condition control. Hence, T. fermentans is a promising strain for microbial lipid production from cholinium ILs-pretreated lignocellulosic hydrolysates.

Transcriptomic and biochemical evidence for the role of lysine biosynthesis against linoleic acid hydroperoxide-induced stress in Saccharomyces cerevisiae

Amino acid biosynthesis forms part of an integrated stress response against oxidants in Saccharomyces cerevisiae and higher eukaryotes. Here we show an essential protective role of the l-lysine biosynthesis pathway in response to the oxidative stress condition induced by the lipid oxidant-linoleic acid hydroperoxide (LoaOOH), by means of transcriptomic profiling and phenotypic analysis, and using the deletion mutant dal80∆ and lysine auxotroph lys1∆. A comprehensive up-regulation of lysine biosynthetic genes (LYS1, LYS2, LYS4, LYS9, LYS12, LYS20 and LYS21) was revealed in dal80Δ following the oxidant challenge. The lysine auxotroph (lys1∆) exhibited a significant decrease in growth compared with that of BY4743 upon exposure to LoaOOH, albeit with the sufficient provision of lysine in the medium. Furthermore, the growth of wild type BY4743 exposed to LoaOOH was also greatly reduced in lysine-deficient conditions, despite a full complement of lysine biosynthetic genes. Amino acid analysis of LoaOOH-treated yeast showed that the level of cellular lysine remained unchanged throughout oxidant challenge, suggesting that the induced lysine biosynthesis leads to a steady-state metabolism as compared to the untreated yeast cells. Together, these findings demonstrate that lysine availability and its biosynthesis pathway play an important role in protecting the cell from lipid peroxide-induced oxidative stress, which is directly related to understanding environmental stress and industrial yeast management in brewing, wine making and baking.

Yeast: the soul of beer's aroma--a review of flavour-active esters and higher alcohols produced by the brewing yeast

Among the most important factors influencing beer quality is the presence of well-adjusted amounts of higher alcohols and esters. Thus, a heavy body of literature focuses on these substances and on the parameters influencing their production by the brewing yeast. Additionally, the complex metabolic pathways involved in their synthesis require special attention. More than a century of data, mainly in genetic and proteomic fields, has built up enough information to describe in detail each step in the pathway for the synthesis of higher alcohols and their esters, but there is still place for more. Higher alcohols are formed either by anabolism or catabolism (Ehrlich pathway) of amino acids. Esters are formed by enzymatic condensation of organic acids and alcohols. The current paper reviews the up-to-date knowledge in the pathways involving the synthesis of higher alcohols and esters by brewing yeasts. Fermentation parameters affecting yeast response during biosynthesis of these aromatic substances are also fully reviewed.