Crabtree-negative characteristics of recombinant xylose-utilizing Saccharomyces cerevisiae

Advances in biosynthesis of 7-Dehydrocholesterol through de novo cell factory strategies

7-Dehydrocholesterol (7-DHC) is an important sterol for maintaining human health and is present in the skin. After sun exposure, 7-DHC in the skin is converted to vitamin D3 to strengthen the immune system. In recent years, synthetic biology has gained importance due to the effective and efficient production of various important compounds using microorganisms. Despite the understanding of the mechanisms and pathways of 7-DHC biosynthesis, achieving higher production yields remains a significant challenge. This review aims to provide a comprehensive overview of the current state of 7-DHC biosynthesis. Various synthetic strategies including optimization of rate-limiting enzymes, metabolic fluxes, redox balance, and subcellular localization are discussed. Moreover, the role of omics technology in designing important proteins and gene editing techniques for strain modification to efficiently synthesize 7-DHC will also be discussed.

Engineering of Saccharomyces cerevisiae for co-fermentation of glucose and xylose: Current state and perspectives

The use of non-food lignocellulosic biomass to produce ethanol fits into the strategy of a global circular economy with low dependence on fossil energy resources. Xylose is the second most abundant sugar in lignocellulosic hydrolysate, and its utilization in fermentation is a key issue in making the full use of raw plant materials for ethanol production and reduce production costs. Saccharomyces cerevisiae is the best ethanol producer but the organism is not a native xylose user. In recent years, great efforts have been made in the construction of xylose utilizing S. cerevisiae strains by metabolic and evolutionary engineering approaches. In addition, managing global transcriptional regulation works provides an effective means to increase the xylose utilization capacity of recombinant strains. Here we review the common strategies and research advances in the research field in order to facilitate the researches in xylose metabolism and xylose-based fermentation.

The impact of transcription factors Znf1, Sip4, Adr1, Tup1, and Hap4 on xylose alcoholic fermentation in the engineered yeast Saccharomyces cerevisiae

Lignocellulosic biomass is an attractive sustainable platform for fuel ethanol production. Xylose is a second after glucose most abounded sugar in lignocellulosic hydrolysates. Effective conversion of xylose to ethanol is one of key prerequisite for the development of an efficient conversion of biomass to ethanol. Engineered Saccharomyces cerevisiae strains are able to xylose fermentation. However, the yield and productivities of xylose fermentation remains lower in comparison with glucose fermentation. In this work, we studied impact of transcription factors Znf1, Sip4, Adr1, Tup1, and Hap4 on xylose catabolism. We have isolated znf1Δ, adr1Δ, tup1Δ and hap4Δ mutants, and strains overexpressing SIP4, ADR1 and HAP4 genes on the background of xylose-fermenting strain of S. cerevisiae aiming to explore involvement of these transcription factors in regulation of xylose growth and fermentation. It was shown that hap4Δ reveal 1.8-fold increase of ethanol production from xylose as compared to that of parental strain. The hap4Δ mutant accumulates 10.38 g l^-1 of ethanol with an overall ethanol yield reaching 0.41 g g^-1 of consumed xylose. While the other constructed strains revealed a decrease in ethanol production from this pentose.

D-glucose overflow metabolism in an evolutionary engineered high-performance D-xylose consuming Saccharomyces cerevisiae strain

Co-consumption of D-xylose and D-glucose by Saccharomyces cerevisiae is essential for cost-efficient cellulosic bioethanol production. There is a need for improved sugar conversion rates to minimize fermentation times. Previously, we have employed evolutionary engineering to enhance D-xylose transport and metabolism in the presence of D-glucose in a xylose-fermenting S. cerevisiae strain devoid of hexokinases. Re-introduction of Hxk2 in the high performance xylose-consuming strains restored D-glucose utilization during D-xylose/D-glucose co-metabolism, but at rates lower than the non-evolved strain. In the absence of D-xylose, D-glucose consumption was similar to the parental strain. The evolved strains accumulated trehalose-6-phosphate during sugar co-metabolism, and showed an increased expression of trehalose pathway genes. Upon the deletion of TSL1, trehalose-6-phosphate levels were decreased and D-glucose consumption and growth on mixed sugars was improved. The data suggest that D-glucose/D-xylose co-consumption in high-performance D-xylose consuming strains causes the glycolytic flux to saturate. Excess D-glucose is phosphorylated enters the trehalose pathway resulting in glucose recycling and energy dissipation, accumulation of trehalose-6-phosphate which inhibits the hexokinase activity, and release of trehalose into the medium.

Transcription Analysis of Recombinant Trichoderma reesei HJ-48 to Compare the Molecular Basis for Fermentation of Glucose and Xylose

Profiling the transcriptome changes involved in xylose metabolism by the fungus Trichoderma reesei allows for the identification of potential targets for ethanol production processing. In the present study, the transcriptome of T. reesei HJ-48 grown on xylose versus glucose was analyzed using nextgeneration sequencing technology. During xylose fermentation, numerous genes related to central metabolic pathways, including xylose reductase (XR) and xylitol dehydrogenase (XDH), were expressed at higher levels in T. reesei HJ-48. Notably, growth on xylose did not fully repress the genes encoding enzymes of the tricarboxylic acid and respiratory pathways. In addition, increased expression of several sugar transporters was observed during xylose fermentation. This study provides a valuable dataset for further investigation of xylose fermentation and provides a deeper insight into the various genes involved in this process.

A Thi2p Regulatory Network Controls the Post-glucose Effect of Xylose Utilization in Saccharomyces cerevisiae

The complete and efficient utilization of both glucose and xylose is necessary for the economically viable production of biofuels and chemicals using lignocellulosic feedstocks. Although recently obtained recombinant Saccharomyces cerevisiae strains metabolize xylose well when xylose is the sole carbon source in the medium (henceforth referred to as "X stage"), their xylose consumption rate is significantly reduced during the xylose-only consumption phase of glucose-xylose co-fermentation ("GX stage"). This post-glucose effect seriously decreases overall fermentation efficiency. We showed in previous work that THI2 deletion can alleviate this post-glucose effect, but the underlying mechanisms were ill-defined. In the present study, we profiled the transcriptome of a thi2Δ strain growing at the GX stage. Thi2p in GX stage cells regulates genes involved in the cell cycle, stress tolerance, and cell viability. Importantly, the regulation of Thi2p differs from a previous regulatory network that functions when glucose is the sole carbon source, which suggests that the function of Thi2p depends on the carbon source. Modeling research seeking to optimize metabolic engineering via TFs should account for this important carbon source difference. Building on our initial study, we confirmed that several identified factors did indeed increase fermentation efficiency. Specifically, overexpressing STT4, RGI2, and TFC3 increases specific xylose utilization rate of the strain by 36.9, 29.7, 42.8%, respectively, in the GX stage of anaerobic fermentation. Our study thus illustrates a promising strategy for the rational engineering of yeast for lignocellulosic ethanol production.

L-Lactic acid production from glucose and xylose with engineered strains of Saccharomyces cerevisiae: aeration and carbon source influence yields and productivities

BACKGROUND

Saccharomyces cerevisiae, engineered for L-lactic acid production from glucose and xylose, is a promising production host for lignocellulose-to-lactic acid processes. However, the two principal engineering strategies-pyruvate-to-lactic acid conversion with and without disruption of the competing pyruvate-to-ethanol pathway-have not yet resulted in strains that combine high lactic acid yields (Y_LA) and productivities (Q_LA) on both sugar substrates. Limitations seemingly arise from a dependency on the carbon source and the aeration conditions, but the underlying effects are poorly understood. We have recently presented two xylose-to-lactic acid converting strains, IBB14LA1 and IBB14LA1_5, which have the L-lactic acid dehydrogenase from Plasmodium falciparum (pfLDH) integrated at the pdc1 (pyruvate decarboxylase) locus. IBB14LA1_5 additionally has its pdc5 gene knocked out. In this study, the influence of carbon source and oxygen on Y_LA and Q_LA in IBB14LA1 and IBB14LA1_5 was investigated.

RESULTS

In anaerobic fermentation IBB14LA1 showed a higher Y_LA on xylose (0.27 g g _Xyl^-1 ) than on glucose (0.18 g g _Glc^-1 ). The ethanol yields (Y_EtOH, 0.15 g g _Xyl^-1 and 0.32 g g _Glc^-1 ) followed an opposite trend. In IBB14LA1_5, the effect of the carbon source on Y_LA was less pronounced (~ 0.80 g g _Xyl^-1 , and 0.67 g g _Glc^-1 ). Supply of oxygen accelerated glucose conversions significantly in IBB14LA1 (Q_LA from 0.38 to 0.81 g L^-1 h^-1) and IBB14LA1_5 (Q_LA from 0.05 to 1.77 g L^-1 h^-1) at constant Y_LA (IBB14LA1 ~ 0.18 g g _Glc^-1 ; IBB14LA1_5 ~ 0.68 g g _Glc^-1 ). In aerobic xylose conversions, however, lactic acid production ceased completely in IBB14LA1 and decreased drastically in IBB14LA1_5 (Y_LA aerobic ≤ 0.25 g g _Xyl^-1 and anaerobic ~ 0.80 g g _Xyl^-1 ) at similar Q_LA (~ 0.04 g L^-1 h^-1). Switching from aerobic to microaerophilic conditions (pO₂ ~ 2%) prevented lactic acid metabolization, observed for fully aerobic conditions, and increased Q_LA and Y_LA up to 0.11 g L^-1 h^-1 and 0.38 g g _Xyl^-1 , respectively. The pfLDH and PDC activities in IBB14LA1 were measured and shown to change drastically dependent on carbon source and oxygen.

CONCLUSION

Evidence from conversion time courses together with results of activity measurements for pfLDH and PDC show that in IBB14LA1 the distribution of fluxes at the pyruvate branching point is carbon source and oxygen dependent. Comparison of the performance of strain IBB14LA1 and IBB14LA1_5 in conversions under different aeration conditions (aerobic, anaerobic, and microaerophilic) further suggest that xylose, unlike glucose, does not repress the respiratory response in both strains. This study proposes new genetic engineering targets for rendering genetically engineering S. cerevisiae better suited for lactic acid biorefineries.

Real-time monitoring of the sugar sensing in Saccharomyces cerevisiae indicates endogenous mechanisms for xylose signaling

Background

The sugar sensing and carbon catabolite repression in Baker’s yeast Saccharomyces cerevisiae is governed by three major signaling pathways that connect carbon source recognition with transcriptional regulation. Here we present a screening method based on a non-invasive in vivo reporter system for real-time, single-cell screening of the sugar signaling state in S. cerevisiae in response to changing carbon conditions, with a main focus on the response to glucose and xylose.

Results

The artificial reporter system was constructed by coupling a green fluorescent protein gene (yEGFP3) downstream of endogenous yeast promoters from the Snf3p/Rgt2p, SNF1/Mig1p and cAMP/PKA signaling pathways: HXT1p/2p/4p; SUC2p, CAT8p; TPS1p/2p and TEF4p respectively. A panel of eight biosensors strains was generated by single copy chromosomal integration of the different constructs in a W303-derived strain. The signaling biosensors were validated for their functionality with flow cytometry by comparing the fluorescence intensity (FI) response in the presence of high or nearly depleted glucose to the known induction/repression conditions of the eight different promoters. The FI signal correlated with the known patterns of the selected promoters while maintaining a non-invasive property on the cellular phenotype, as was demonstrated in terms of growth, metabolites and enzyme activity.

Conclusions

Once verified, the sensors were used to evaluate the signaling response to varying conditions of extracellular glucose, glycerol and xylose by screening in 96-well microtiter plates. We show that these yeast strains, which do not harbor any recombinant pathways for xylose utilization, are lacking a signaling response for extracellular xylose. However, for the HXT2p/4p sensors, a shift in the flow cytometry population dynamics indicated that internalized xylose does affect the signaling. These results suggest that the previously observed effects of this pentose on the S. cerevisiae physiology and gene regulation can be attributed to xylose and not only to a lack of glucose.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0580-x) contains supplementary material, which is available to authorized users.

Increased ethanol production by deletion of HAP4 in recombinant xylose-assimilating Saccharomyces cerevisiae

The Saccharomyces cerevisiae HAP4 gene encodes a transcription activator that plays a key role in controlling the expression of genes involved in mitochondrial respiration and reductive pathways. This work examines the effect of knockout of the HAP4 gene on aerobic ethanol production in a xylose-utilizing S. cerevisiae strain. A hap4-deleted recombinant yeast strain (B42-DHAP4) showed increased maximum concentration, production rate, and yield of ethanol compared with the reference strain MA-B42, irrespective of cultivation medium (glucose, xylose, or glucose/xylose mixtures). Notably, B42-DHAP4 was capable of producing ethanol from xylose as the sole carbon source under aerobic conditions, whereas no ethanol was produced by MA-B42. Moreover, the rate of ethanol production and ethanol yield (0.44 g/g) from the detoxified hydrolysate of wood chips was markedly improved in B42-DHAP4 compared to MA-B42. Thus, the results of this study support the view that deleting HAP4 in xylose-utilizing S. cerevisiae strains represents a useful strategy in ethanol production processes.

Simultaneously improving xylose fermentation and tolerance to lignocellulosic inhibitors through evolutionary engineering of recombinant Saccharomyces cerevisiae harbouring xylose isomerase

BACKGROUND

Yeasts tolerant to toxic inhibitors from steam-pretreated lignocellulose with xylose co-fermentation capability represent an appealing approach for 2nd generation ethanol production. Whereas rational engineering, mutagenesis and evolutionary engineering are established techniques for either improved xylose utilisation or enhancing yeast tolerance, this report focuses on the simultaneous enhancement of these attributes through mutagenesis and evolutionary engineering of Saccharomyces cerevisiae harbouring xylose isomerase in anoxic chemostat culture using non-detoxified pretreatment liquor from triticale straw.

RESULTS

Following ethyl methanesulfonate (EMS) mutagenesis, Saccharomyces cerevisiae strain D5A⁺ (ATCC 200062 strain platform), harbouring the xylose isomerase (XI) gene for pentose co-fermentation was grown in anoxic chemostat culture for 100 generations at a dilution rate of 0.10 h⁻¹ in a medium consisting of 60% (v/v) non-detoxified hydrolysate liquor from steam-pretreated triticale straw, supplemented with 20 g/L xylose as carbon source. In semi-aerobic batch cultures in the same medium, the isolated strain D5A(+H) exhibited a slightly lower maximum specific growth rate (μ(max) = 0.12 ± 0.01 h⁻¹) than strain TMB3400, with no ethanol production observed by the latter strain. Strain D5A(+H) also exhibited a shorter lag phase (4 h vs. 30 h) and complete removal of HMF, furfural and acetic acid from the fermentation broth within 24 h, reaching an ethanol concentration of 1.54 g/L at a yield (Y(p/s)) of 0.06 g/g xylose and a specific productivity of 2.08 g/gh. Evolutionary engineering profoundly affected the yeast metabolism, given that parental strain D5A+ exhibited an oxidative metabolism on xylose prior to strain development.

CONCLUSIONS

Physiological adaptations confirm improvements in the resistance to and conversion of inhibitors from pretreatment liquor with simultaneous enhancement of xylose to ethanol fermentation. These data support the sequential application of random mutagenesis followed by continuous culture under simultaneous selective pressure from inhibitors and xylose as primary carbon source.

Evaluation of novel xylose-fermenting yeast strains from Brazilian forests for hemicellulosic ethanol production from sugarcane bagasse

Bioconversion of hemicellulosic hydrolysates into ethanol with the desired yields plays a pivotal role for the overall success of biorefineries. This paper aims to evaluate the ethanol production potential of four native strains of Scheffersomyces shehatae (syn. Candida shehatae) viz. S. shehatae BR6-2AI, CG8-8BY, PT1-1BASP and BR6-2AY, isolated from Brazilian forests. These strains were grown in commercial d-xylose-supplemented synthetic medium and sugarcane bagasse hemicellulose hydrolysate. S. shehatae BR6-2AY showed maximum ethanol production [0.48 ± 0.019 g g⁻¹, 95 ± 3.78 % fermentation efficiency (FE)] followed by S. shehatae CG8-8BY (0.47 ± 0.016 g g⁻¹, 93 ± 3.12 % FE), S. shehatae BR6-2AI (0.45 ± 0.01 g g⁻¹, 89 ± 1.71 % FE) and S. shehatae PT1-1BASP (0.44 ± 0.02 g g⁻¹, 86 ± 3.37 % FE) when grown in synthetic medium. During the fermentation of hemicellulose hydrolysates, S. shehatae CG8-8BY and S. shehatae BR6-2AY showed ethanol production (0.30 ± 0.05 g g⁻¹, 58 ± 0.02 % FE) and (0.21 ± 0.01 g g⁻¹, 40 ± 1.93 % FE), respectively.

Enhanced xylose fermentation capacity related to an altered glucose sensing and repression network in a recombinant Saccharomyces cerevisiae

The co-fermentation of glucose and xylose is one of the issues in decreasing the price of biofuel or chemicals produced from lignocellulosic materials. A glucose and xylose co-utilizing Saccharomyces cerevisiae was obtained through rational genetic manipulation. Non-rational evolution in xylose was performed, and the xylose utilization efficiency of the engineered strain was significantly enhanced. The results of transcriptome study suggested that Snf1/Mig1-mediated regulation, a part of glucose sensing and repression network, was altered in the evolved strain and might be related to the enhancement of xylose utilization.

Reconstruction and analysis of a genome-scale metabolic model for Scheffersomyces stipitis

Background

Fermentation of xylose, the major component in hemicellulose, is essential for economic conversion of lignocellulosic biomass to fuels and chemicals. The yeast Scheffersomyces stipitis (formerly known as Pichia stipitis) has the highest known native capacity for xylose fermentation and possesses several genes for lignocellulose bioconversion in its genome. Understanding the metabolism of this yeast at a global scale, by reconstructing the genome scale metabolic model, is essential for manipulating its metabolic capabilities and for successful transfer of its capabilities to other industrial microbes.

Results

We present a genome-scale metabolic model for Scheffersomyces stipitis, a native xylose utilizing yeast. The model was reconstructed based on genome sequence annotation, detailed experimental investigation and known yeast physiology. Macromolecular composition of Scheffersomyces stipitis biomass was estimated experimentally and its ability to grow on different carbon, nitrogen, sulphur and phosphorus sources was determined by phenotype microarrays. The compartmentalized model, developed based on an iterative procedure, accounted for 814 genes, 1371 reactions, and 971 metabolites. In silico computed growth rates were compared with high-throughput phenotyping data and the model could predict the qualitative outcomes in 74% of substrates investigated. Model simulations were used to identify the biosynthetic requirements for anaerobic growth of Scheffersomyces stipitis on glucose and the results were validated with published literature. The bottlenecks in Scheffersomyces stipitis metabolic network for xylose uptake and nucleotide cofactor recycling were identified by in silico flux variability analysis. The scope of the model in enhancing the mechanistic understanding of microbial metabolism is demonstrated by identifying a mechanism for mitochondrial respiration and oxidative phosphorylation.

Conclusion

The genome-scale metabolic model developed for Scheffersomyces stipitis successfully predicted substrate utilization and anaerobic growth requirements. Useful insights were drawn on xylose metabolism, cofactor recycling and mechanism of mitochondrial respiration from model simulations. These insights can be applied for efficient xylose utilization and cofactor recycling in other industrial microorganisms. The developed model forms a basis for rational analysis and design of Scheffersomyces stipitis metabolic network for the production of fuels and chemicals from lignocellulosic biomass.

The glucose/xylose facilitator Gxf1 from Candida intermedia expressed in a xylose-fermenting industrial strain of Saccharomyces cerevisiae increases xylose uptake in SSCF of wheat straw

Ethanolic fermentation of lignocellulose raw materials requires industrial xylose-fermenting strains capable of complete and efficient D-xylose consumption. A central question in xylose fermentation by Saccharomyces cerevisiae engineered for xylose fermentation is to improve the xylose uptake. In the current study, the glucose/xylose facilitator Gxf1 from Candida intermedia, was expressed in three different xylose-fermenting S. cerevisiae strains of industrial origin. The in vivo effect on aerobic xylose growth and the initial xylose uptake rate were assessed. The expression of Gxf1 resulted in enhanced aerobic xylose growth only for the TMB3400 based strain. It displayed more than a 2-fold higher affinity for D-xylose than the parental strain and approximately 2-fold higher initial specific growth rate at 4 g/L D-xylose. Enhanced xylose consumption was furthermore observed when the GXF1-strain was assessed in simultaneous saccharification and co-fermentation (SSCF) of pretreated wheat straw. However, the ethanol yield remained unchanged due to increased by-product formation. Metabolic flux analysis suggested that the expression of the Gxf1 transporter had shifted the control of xylose catabolism from transport to the NAD(+) dependent oxidation of xylitol to xylulose.

The deletion of YLR042c improves ethanolic xylose fermentation by recombinant Saccharomyces cerevisiae

In a recent study combining transcriptome analyses of a number of recombinant laboratory and industrial S. cerevisiae strains with improved xylose utilization and their respective control strains, the ORF YLR042c was identified as a downregulated gene and it was shown that the gene deletion improved aerobic growth on xylose in the tested strain background. In the present study, the influence of deleting YLR042c on xylose fermentation was investigated in two different xylose-fermenting strains: TMB3001, which expresses genes from the initial xylose catabolizing pathway, including heterologous xylose reductase (XR) and xylitol dehydrogenase (XDH) and endogenous xylulokinase (XK); and TMB3057, which, in addition to the initial xylose catabolizing pathway, overexpresses the endogenous genes encoding the non-oxidative pentose phosphate pathway enzymes. The deletion of YLR042c led to improved aerobic growth on xylose in both strain backgrounds. However, the effect was more significant in the strain with the poorer growth rate on xylose (TMB3001). Under anaerobic conditions, the deletion of YLR042c increased the specific xylose consumption rate and the ethanol and xylitol yields. In strain TMB3057, xylose consumption was also improved at low concentrations and during co-fermentation of xylose and glucose. The effect of the gene deletion and overexpression was also tested for different carbon sources. Altogether, these results suggest that YLR042c influences xylose and the assimilation of carbon sources other than glucose, and that the effect could be at the level of sugar transport or sugar signalling.

Fermentation of mixed glucose-xylose substrates by engineered strains of Saccharomyces cerevisiae: role of the coenzyme specificity of xylose reductase, and effect of glucose on xylose utilization

Background

In spite of the substantial metabolic engineering effort previously devoted to the development of Saccharomyces cerevisiae strains capable of fermenting both the hexose and pentose sugars present in lignocellulose hydrolysates, the productivity of reported strains for conversion of the naturally most abundant pentose, xylose, is still a major issue of process efficiency. Protein engineering for targeted alteration of the nicotinamide cofactor specificity of enzymes catalyzing the first steps in the metabolic pathway for xylose was a successful approach of reducing xylitol by-product formation and improving ethanol yield from xylose. The previously reported yeast strain BP10001, which expresses heterologous xylose reductase from Candida tenuis in mutated (NADH-preferring) form, stands for a series of other yeast strains designed with similar rational. Using 20 g/L xylose as sole source of carbon, BP10001 displayed a low specific uptake rate q_xylose (g xylose/g dry cell weight/h) of 0.08. The study presented herein was performed with the aim of analysing (external) factors that limit q_xylose of BP10001 under xylose-only and mixed glucose-xylose substrate conditions. We also carried out a comprehensive investigation on the currently unclear role of coenzyme utilization, NADPH compared to NADH, for xylose reduction during co-fermentation of glucose and xylose.

Results

BP10001 and BP000, expressing C. tenuis xylose reductase in NADPH-preferring wild-type form, were used. Glucose and xylose (each at 10 g/L) were converted sequentially, the corresponding q_substrate values being similar for each strain (glucose: 3.0; xylose: 0.05). The distribution of fermentation products from glucose was identical for both strains whereas when using xylose, BP10001 showed enhanced ethanol yield (BP10001 0.30 g/g; BP000 0.23 g/g) and decreased yields of xylitol (BP10001 0.26 g/g; BP000 0.36 g/g) and glycerol (BP10001 0.023 g/g; BP000 0.072 g/g) as compared to BP000. Increase in xylose concentration from 10 to 50 g/L resulted in acceleration of substrate uptake by BP10001 (0.05 - 0.14 g/g CDW/h) and reduction of the xylitol yield (0.28 g/g - 0.15 g/g). In mixed substrate batches, xylose was taken up at low glucose concentrations (< 4 g/L) and up to fivefold enhanced xylose uptake rate was found towards glucose depletion. A fed-batch process designed to maintain a "stimulating" level of glucose throughout the course of xylose conversion provided a q_xylose that had an initial value of 0.30 ± 0.04 g/g CDW/h and decreased gradually with time. It gave product yields of 0.38 g ethanol/g total sugar and 0.19 g xylitol/g xylose. The effect of glucose on xylose utilization appears to result from the enhanced flux of carbon through glycolysis and the pentose phosphate pathway under low-glucose reaction conditions.

Conclusions

Relative improvements in the distribution of fermentation products from xylose that can be directly related to a change in the coenzyme preference of xylose reductase from NADPH in BP000 to NADH in BP10001 increase in response to an increase in the initial concentration of the pentose substrate from 10 to 50 g/L. An inverse relationship between xylose uptake rate and xylitol yield for BP10001 implies that xylitol by-product formation is controlled not only by coenzyme regeneration during two-step oxidoreductive conversion of xylose into xylulose. Although xylose is not detectably utilized at glucose concentrations greater than 4 g/L, the presence of a low residual glucose concentration (< 2 g/L) promotes the uptake of xylose and its conversion into ethanol with only moderate xylitol by-product formation. A fed-batch reaction that maintains glucose in the useful concentration range and provides a constant q_glucose may be useful for optimizing q_xylose in processes designed for co-fermentation of glucose and xylose.

Increased expression of the oxidative pentose phosphate pathway and gluconeogenesis in anaerobically growing xylose-utilizing Saccharomyces cerevisiae

Background

Fermentation of xylose to ethanol has been achieved in S. cerevisiae by genetic engineering. Xylose utilization is however slow compared to glucose, and during anaerobic conditions addition of glucose has been necessary for cellular growth. In the current study, the xylose-utilizing strain TMB 3415 was employed to investigate differences between anaerobic utilization of glucose and xylose. This strain carried a xylose reductase (XYL1 K270R) engineered for increased NADH utilization and was capable of sustained anaerobic growth on xylose as sole carbon source. Metabolic and transcriptional characterization could thus for the first time be performed without addition of a co-substrate or oxygen.

Results

Analysis of metabolic fluxes showed that although the specific ethanol productivity was an order of magnitude lower on xylose than on glucose, product yields were similar for the two substrates. In addition, transcription analysis identified clear regulatory differences between glucose and xylose. Respiro-fermentative metabolism on glucose during aerobic conditions caused repression of cellular respiration, while metabolism on xylose under the same conditions was fully respiratory. During anaerobic conditions, xylose repressed respiratory pathways, although notably more weakly than glucose. It was also observed that anaerobic xylose growth caused up-regulation of the oxidative pentose phosphate pathway and gluconeogenesis, which may be driven by an increased demand for NADPH during anaerobic xylose catabolism.

Conclusion

Co-factor imbalance in the initial twp steps of xylose utilization may reduce ethanol productivity by increasing the need for NADP+ reduction and consequently increase reverse flux in glycolysis.