Kinetic modelling reveals current limitations in the production of ethanol from xylose by recombinant Saccharomyces cerevisiae

Structural and biochemical insights of xylose MFS and SWEET transporters in microbial cell factories: challenges to lignocellulosic hydrolysates fermentation

The production of bioethanol from lignocellulosic biomass requires the efficient conversion of glucose and xylose to ethanol, a process that depends on the ability of microorganisms to internalize these sugars. Although glucose transporters exist in several species, xylose transporters are less common. Several types of transporters have been identified in diverse microorganisms, including members of the Major Facilitator Superfamily (MFS) and Sugars Will Eventually be Exported Transporter (SWEET) families. Considering that Saccharomyces cerevisiae lacks an effective xylose transport system, engineered yeast strains capable of efficiently consuming this sugar are critical for obtaining high ethanol yields. This article reviews the structure-function relationship of sugar transporters from the MFS and SWEET families. It provides information on several tools and approaches used to identify and characterize them to optimize xylose consumption and, consequently, second-generation ethanol production.

Molecular Modification Enhances Xylose Uptake by the Sugar Transporter KM_SUT5 of Kluyveromyces marxianus

This research cloned and expressed the sugar transporter gene KM_SUT5 from Kluyveromyces marxianus GX-UN120, which displayed remarkable sugar transportation capabilities, including pentose sugars. To investigate the impact of point mutations on xylose transport capacity, we selected four sites, predicted the suitable amino acid sites by molecular docking, and altered their codons to construct the corresponding mutants, Q74D, Y195K, S460H, and Q464F, respectively. Furthermore, we conducted site-directed truncation on six sites of KM_SUT5p. The molecular modification resulted in significant changes in mutant growth and the D-xylose transport rate. Specifically, the S460H mutant exhibited a higher growth rate and demonstrated excellent performance across 20 g L-1 xylose, achieving the highest xylose accumulation under xylose conditions (49.94 μmol h-1 gDCW-1, DCW mean dry cell weight). Notably, mutant delA554-, in which the transporter protein SUT5 is truncated at position delA554-, significantly increased growth rates in both D-xylose and D-glucose substrates. These findings offer valuable insights into potential modifications of other sugar transporters and contribute to a deeper understanding of the C-terminal function of sugar transporters.

Metabolic Engineering of E. coli for Xylose Production from Glucose as the Sole Carbon Source

Xylose is the raw material for the synthesis of many important platform compounds. At present, xylose is commercially produced by chemical extraction. However, there are still some bottlenecks in the extraction of xylose, including complicated operation processes and the chemical substances introduced, leading to the high cost of xylose and of synthesizing the downstream compounds of xylose. The current market price of xylose is 8× that of glucose, so using low-cost glucose as the substrate to produce the downstream compounds of xylose can theoretically reduce the cost by 70%. Here, we designed a pathway for the biosynthesis of xylose from glucose in Escherichia coli. This biosynthetic pathway was achieved by overexpressing five genes, namely, zwf, pgl, gnd, rpe, and xylA, while replacing the native xylulose kinase gene xylB with araL from B. subtilis, which displays phosphatase activity toward d-xylulose 5-phosphate. The yield of xylose was increased to 3.3 g/L by optimizing the metabolic pathway. Furthermore, xylitol was successfully synthesized by introducing the xyl1 gene, which suggested that the biosynthetic pathway of xylose from glucose is universally applicable for the synthesis of xylose downstream compounds. This is the first study to synthesize xylose and its downstream compounds by using glucose as a substrate, which not only reduces the cost of raw materials, but also alleviates carbon catabolite repression (CCR), providing a new idea for the synthesis of downstream compounds of xylose.

Identification and analysis of sugar transporters capable of co-transporting glucose and xylose simultaneously

Simultaneous co-fermentation of glucose and xylose is a key desired trait of engineered Saccharomyces cerevisiae for efficient and rapid production of biofuels and chemicals. However, glucose strongly inhibits xylose transport by endogenous hexose transporters of S. cerevisiae. We identified structurally distant sugar transporters (Lipomyces starkeyi LST1_205437 and Arabidopsis thaliana AtSWEET7) capable of co-transporting glucose and xylose from previously unexplored oleaginous yeasts and plants. Kinetic analysis showed that LST1_205437 had lenient glucose inhibition on xylose transport and AtSWEET7 transported glucose and xylose simultaneously with no inhibition. Modelling studies of LST1_205437 revealed that Ala335 residue at sugar binding site can accommodates both glucose and xylose. Docking studies with AtSWEET7 revealed that Trp59, Trp183, Asn145, and Asn179 residues stabilized the interactions with sugars, allowing both xylose and glucose to be co-transported. In addition, we altered sugar preference of LST1_205437 by single amino acid mutation at Asn365. Our findings provide a new mechanistic insight on glucose and xylose transport mechanism of sugar transporters and the identified sugar transporters can be employed to develop engineered yeast strains for producing cellulosic biofuels and chemicals.

Optimization of 1,2,4-butanetriol production from xylose in Saccharomyces cerevisiae by metabolic engineering of NADH/NADPH balance

1,2,4-Butanetriol (BT) is used as a precursor for the synthesis of various pharmaceuticals and the energetic plasticizer 1,2,4-butanetriol trinitrate. In Saccharomyces cerevisiae, BT is biosynthesized from xylose via heterologous four enzymatic reactions catalyzed by xylose dehydrogenase, xylonate dehydratase, 2-ketoacid decarboxylase, and alcohol dehydrogenase. We here aimed to improve the BT yield in S. cerevisiae by genetic engineering. First, the amount of the key intermediate 2-keto-3-deoxy-xylonate as described previously was successfully reduced in 41% by multiple integrations of Lactococcus lactis 2-ketoacid decarboxylase gene kdcA into the yeast genome. Since the heterologous BT synthetic pathway is independent of yeast native metabolism, this manipulation has led to NADH/NADPH imbalance and deficiency during BT production. Overexpression of the NADH kinase POS5Δ17 lacking the mitochondrial targeting sequence to relieve NADH/NADPH imbalance resulted in the BT titer of 2.2 g/L (31% molar yield). Feeding low concentrations of glucose and xylose to support the supply of NADH resulted in BT titer of 6.6 g/L with (57% molar yield). Collectively, improving the NADH/NADPH ratio and supply from glucose are essential for the construction of a xylose pathway, such as the BT synthetic pathway, independent of native yeast metabolism.

Novel xylose transporter Cs4130 expands the sugar uptake repertoire in recombinant Saccharomyces cerevisiae strains at high xylose concentrations

BACKGROUND

The need to restructure the world's energy matrix based on fossil fuels and mitigate greenhouse gas emissions stimulated the development of new biobased technologies for renewable energy. One promising and cleaner alternative is the use of second-generation (2G) fuels, produced from lignocellulosic biomass sugars. A major challenge on 2G technologies establishment is the inefficient assimilation of the five-carbon sugar xylose by engineered Saccharomyces cerevisiae strains, increasing fermentation time. The uptake of xylose across the plasma membrane is a critical limiting step and the budding yeast S. cerevisiae is not designed with a broad transport system and regulatory mechanisms to assimilate xylose in a wide range of concentrations present in 2G processes.

RESULTS

Assessing diverse microbiomes such as the digestive tract of plague insects and several decayed lignocellulosic biomasses, we isolated several yeast species capable of using xylose. Comparative fermentations selected the yeast Candida sojae as a potential source of high-affinity transporters. Comparative genomic analysis elects four potential xylose transporters whose properties were evaluated in the transporter null EBY.VW4000 strain carrying the xylose-utilizing pathway integrated into the genome. While the traditional xylose transporter Gxf1 allows an improved growth at lower concentrations (10 g/L), strains containing Cs3894 and Cs4130 show opposite responses with superior xylose uptake at higher concentrations (up to 50 g/L). Docking and normal mode analysis of Cs4130 and Gxf1 variants pointed out important residues related to xylose transport, identifying key differences regarding substrate translocation comparing both transporters.

CONCLUSIONS

Considering that xylose concentrations in second-generation hydrolysates can reach high values in several designed processes, Cs4130 is a promising novel candidate for xylose uptake. Here, we demonstrate a novel eukaryotic molecular transporter protein that improves growth at high xylose concentrations and can be used as a promising target towards engineering efficient pentose utilization in yeast.

Biochemical routes for uptake and conversion of xylose by microorganisms

Xylose is a major component of lignocellulose and the second most abundant sugar present in nature. Efficient utilization of xylose is required for the development of economically viable processes to produce biofuels and chemicals from biomass. However, there are still some bottlenecks in the bioconversion of xylose, including the fact that some microorganisms cannot assimilate xylose naturally and that the uptake and metabolism of xylose are inhibited by glucose, which is usually present with xylose in lignocellulose hydrolysate. To overcome these issues, numerous efforts have been made to discover, characterize, and engineer the transporters and enzymes involved in xylose utilization to relieve glucose inhibition and to develop recombinant microorganisms to produce fuels and chemicals from xylose. Here we describe a recent advancement focusing on xylose-utilizing pathways, biosynthesis of chemicals from xylose, and engineering strategies used to improve the conversion efficiency of xylose.

Construction of advanced producers of first- and second-generation ethanol in Saccharomyces cerevisiae and selected species of non-conventional yeasts (Scheffersomyces stipitis, Ogataea polymorpha)

This review summarizes progress in the construction of efficient yeast ethanol producers from glucose/sucrose and lignocellulose. Saccharomyces cerevisiae is the major industrial producer of first-generation ethanol. The different approaches to increase ethanol yield and productivity from glucose in S. cerevisiae are described. Construction of the producers of second-generation ethanol is described for S. cerevisiae, one of the best natural xylose fermenters, Scheffersomyces stipitis and the most thermotolerant yeast known Ogataea polymorpha. Each of these organisms has some advantages and drawbacks. S. cerevisiae is the primary industrial ethanol producer and is the most ethanol tolerant natural yeast known and, however, cannot metabolize xylose. S. stipitis can effectively ferment both glucose and xylose and, however, has low ethanol tolerance and requires oxygen for growth. O. polymorpha grows and ferments at high temperatures and, however, produces very low amounts of ethanol from xylose. Review describes how the mentioned drawbacks could be overcome.

Reassessment of requirements for anaerobic xylose fermentation by engineered, non-evolved Saccharomyces cerevisiae strains

Expression of a heterologous xylose isomerase, deletion of the GRE3 aldose-reductase gene and overexpression of genes encoding xylulokinase (XKS1) and non-oxidative pentose-phosphate-pathway enzymes (RKI1, RPE1, TAL1, TKL1) enables aerobic growth of Saccharomyces cerevisiae on d-xylose. However, literature reports differ on whether anaerobic growth on d-xylose requires additional mutations. Here, CRISPR-Cas9-assisted reconstruction and physiological analysis confirmed an early report that this basic set of genetic modifications suffices to enable anaerobic growth on d-xylose in the CEN.PK genetic background. Strains that additionally carried overexpression cassettes for the transaldolase and transketolase paralogs NQM1 and TKL2 only exhibited anaerobic growth on d-xylose after a 7–10 day lag phase. This extended lag phase was eliminated by increasing inoculum concentrations from 0.02 to 0.2 g biomass L⁻¹. Alternatively, a long lag phase could be prevented by sparging low-inoculum-density bioreactor cultures with a CO₂/N₂-mixture, thus mimicking initial CO₂ concentrations in high-inoculum-density, nitrogen-sparged cultures, or by using l-aspartate instead of ammonium as nitrogen source. This study resolves apparent contradictions in the literature on the genetic interventions required for anaerobic growth of CEN.PK-derived strains on d-xylose. Additionally, it indicates the potential relevance of CO₂ availability and anaplerotic carboxylation reactions for anaerobic growth of engineered S. cerevisiae strains on d-xylose.

Kinetic modeling of Stickland reactions-coupled methanogenesis for a methanogenic culture

Studying amino acid catabolism-coupled methanogenesis is the important standpoints to decipher the metabolic behavior of a methanogenic culture. l-Glycine and l-alanine are acted as sole carbon and nitrogen sources for acidogenic bacteria. One amino acid is oxidized and another one is reduced for acetate production via pyruvate by oxidative deamination process in the Stickland reactions. Herein, we have developed a kinetic model for the Stickland reactions-coupled methanogenesis (SRCM) and simulated objectively to maximize the rate of methane production. We collected the metabolic information from enzyme kinetic parameters for amino acid catabolism of Clostridium acetobutylicum ATCC 824 and methanogenesis of Methanosarcina acetivorans C2A. The SRCM model of this study consisted of 18 reactions and 61 metabolites with enzyme kinetic parameters derived experimental data. The internal or external metabolic flux rate of this system found to control the acidogenesis and methanogenesis in a methanogenic culture. Using the SRCM model, flux distributions were calculated for each reaction and metabolite in order to maximize the methane production rate from the glycine–alanine pair. Results of this study, we demonstrated the metabolic behavior, metabolite pairing while mutually interact, and advantages of syntrophic metabolism of amino acid-directed methane production in a methanogenic starter culture.

Rewired cellular signaling coordinates sugar and hypoxic responses for anaerobic xylose fermentation in yeast

Microbes can be metabolically engineered to produce biofuels and biochemicals, but rerouting metabolic flux toward products is a major hurdle without a systems-level understanding of how cellular flux is controlled. To understand flux rerouting, we investigated a panel of Saccharomyces cerevisiae strains with progressive improvements in anaerobic fermentation of xylose, a sugar abundant in sustainable plant biomass used for biofuel production. We combined comparative transcriptomics, proteomics, and phosphoproteomics with network analysis to understand the physiology of improved anaerobic xylose fermentation. Our results show that upstream regulatory changes produce a suite of physiological effects that collectively impact the phenotype. Evolved strains show an unusual co-activation of Protein Kinase A (PKA) and Snf1, thus combining responses seen during feast on glucose and famine on non-preferred sugars. Surprisingly, these regulatory changes were required to mount the hypoxic response when cells were grown on xylose, revealing a previously unknown connection between sugar source and anaerobic response. Network analysis identified several downstream transcription factors that play a significant, but on their own minor, role in anaerobic xylose fermentation, consistent with the combinatorial effects of small-impact changes. We also discovered that different routes of PKA activation produce distinct phenotypes: deletion of the RAS/PKA inhibitor IRA2 promotes xylose growth and metabolism, whereas deletion of PKA inhibitor BCY1 decouples growth from metabolism to enable robust fermentation without division. Comparing phosphoproteomic changes across ira2Δ and bcy1Δ strains implicated regulatory changes linked to xylose-dependent growth versus metabolism. Together, our results present a picture of the metabolic logic behind anaerobic xylose flux and suggest that widespread cellular remodeling, rather than individual metabolic changes, is an important goal for metabolic engineering.

Computational Approaches on Stoichiometric and Kinetic Modeling for Efficient Strain Design

Engineering biological systems that are capable of overproducing products of interest is the ultimate goal of any biotechnology application. To this end, stoichiometric (or steady state) and kinetic models are increasingly becoming available for a variety of organisms including prokaryotes, eukaryotes, and microbial communities. This ever-accelerating pace of such model reconstructions has also spurred the development of optimization-based strain design techniques. This chapter highlights a number of such frameworks developed in recent years in order to generate testable hypotheses (in terms of genetic interventions), thus addressing the challenges in metabolic engineering. In particular, three major methods are covered in detail including two methods for designing strains (i.e., one stoichiometric model-based and the other by integrating kinetic information into a stoichiometric model) and one method for analyzing microbial communities.

New Protocol Based on UHPLC-MS/MS for Quantitation of Metabolites in Xylose-Fermenting Yeasts

Xylose fermentation is a bottleneck in second-generation ethanol production. As such, a comprehensive understanding of xylose metabolism in naturally xylose-fermenting yeasts is essential for prospection and construction of recombinant yeast strains. The objective of the current study was to establish a reliable metabolomics protocol for quantification of key metabolites of xylose catabolism pathways in yeast, and to apply this protocol to Spathaspora arborariae. Ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS) was used to quantify metabolites, and afterwards, sample preparation was optimized to examine yeast intracellular metabolites. S. arborariae was cultivated using xylose as a carbon source under aerobic and oxygen-limited conditions. Ion pair chromatography (IPC) and hydrophilic interaction liquid chromatography-tandem mass spectrometry (HILIC-MS/MS) were shown to efficiently quantify 14 and 5 metabolites, respectively, in a more rapid chromatographic protocol than previously described. Thirteen and eleven metabolites were quantified in S. arborariae under aerobic and oxygen-limited conditions, respectively. This targeted metabolomics protocol is shown here to quantify a total of 19 metabolites, including sugars, phosphates, coenzymes, monosaccharides, and alcohols, from xylose catabolism pathways (glycolysis, pentose phosphate pathway, and tricarboxylic acid cycle) in yeast. Furthermore, to our knowledge, this is the first time that intracellular metabolites have been quantified in S. arborariae after xylose consumption. The results indicated that fine control of oxygen levels during fermentation is necessary to optimize ethanol production by S. arborariae. The protocol presented here may be applied to other yeast species and could support yeast genetic engineering to improve second generation ethanol production. Graphical Abstract ᅟ.

A design-build-test cycle using modeling and experiments reveals interdependencies between upper glycolysis and xylose uptake in recombinant S. cerevisiae and improves predictive capabilities of large-scale kinetic models

BACKGROUND

Recent advancements in omics measurement technologies have led to an ever-increasing amount of available experimental data that necessitate systems-oriented methodologies for efficient and systematic integration of data into consistent large-scale kinetic models. These models can help us to uncover new insights into cellular physiology and also to assist in the rational design of bioreactor or fermentation processes. Optimization and Risk Analysis of Complex Living Entities (ORACLE) framework for the construction of large-scale kinetic models can be used as guidance for formulating alternative metabolic engineering strategies.

RESULTS

We used ORACLE in a metabolic engineering problem: improvement of the xylose uptake rate during mixed glucose-xylose consumption in a recombinant Saccharomyces cerevisiae strain. Using the data from bioreactor fermentations, we characterized network flux and concentration profiles representing possible physiological states of the analyzed strain. We then identified enzymes that could lead to improved flux through xylose transporters (XTR). For some of the identified enzymes, including hexokinase (HXK), we could not deduce if their control over XTR was positive or negative. We thus performed a follow-up experiment, and we found out that HXK2 deletion improves xylose uptake rate. The data from the performed experiments were then used to prune the kinetic models, and the predictions of the pruned population of kinetic models were in agreement with the experimental data collected on the HXK2-deficient S. cerevisiae strain.

CONCLUSIONS

We present a design-build-test cycle composed of modeling efforts and experiments with a glucose-xylose co-utilizing recombinant S. cerevisiae and its HXK2-deficient mutant that allowed us to uncover interdependencies between upper glycolysis and xylose uptake pathway. Through this cycle, we also obtained kinetic models with improved prediction capabilities. The present study demonstrates the potential of integrated "modeling and experiments" systems biology approaches that can be applied for diverse applications ranging from biotechnology to drug discovery.

Xylose Fermentation by Saccharomyces cerevisiae: Challenges and Prospects

Many years have passed since the first genetically modified Saccharomyces cerevisiae strains capable of fermenting xylose were obtained with the promise of an environmentally sustainable solution for the conversion of the abundant lignocellulosic biomass to ethanol. Several challenges emerged from these first experiences, most of them related to solving redox imbalances, discovering new pathways for xylose utilization, modulation of the expression of genes of the non-oxidative pentose phosphate pathway, and reduction of xylitol formation. Strategies on evolutionary engineering were used to improve fermentation kinetics, but the resulting strains were still far from industrial application. Lignocellulosic hydrolysates proved to have different inhibitors derived from lignin and sugar degradation, along with significant amounts of acetic acid, intrinsically related with biomass deconstruction. This, associated with pH, temperature, high ethanol, and other stress fluctuations presented on large scale fermentations led the search for yeasts with more robust backgrounds, like industrial strains, as engineering targets. Some promising yeasts were obtained both from studies of stress tolerance genes and adaptation on hydrolysates. Since fermentation times on mixed-substrate hydrolysates were still not cost-effective, the more selective search for new or engineered sugar transporters for xylose are still the focus of many recent studies. These challenges, as well as under-appreciated process strategies, will be discussed in this review.

Kinetic modeling of cell metabolism for microbial production

Kinetic models of cellular metabolism are important tools for the rational design of metabolic engineering strategies and to explain properties of complex biological systems. The recent developments in high-throughput experimental data are leading to new computational approaches for building kinetic models of metabolism. Herein, we briefly survey the available databases, standards and software tools that can be applied for kinetic models of metabolism. In addition, we give an overview about recently developed ordinary differential equations (ODE)-based kinetic models of metabolism and some of the main applications of such models are illustrated in guiding metabolic engineering design. Finally, we review the kinetic modeling approaches of large-scale networks that are emerging, discussing their main advantages, challenges and limitations.

Identification of novel metabolic interactions controlling carbon flux from xylose to ethanol in natural and recombinant yeasts

BACKGROUND

Unlike xylose-converting natural yeasts, recombinant strains of Saccharomyces cerevisiae expressing the same xylose assimilation pathway produce under anaerobic conditions xylitol rather than ethanol from xylose at low specific xylose conversion rates. Despite intense research efforts over the last two decades, differences in these phenotypes cannot be explained by current metabolic and kinetic models. To improve our understanding how metabolic flux of xylose carbon to ethanol is controlled, we developed a novel kinetic model based on enzyme mechanisms and applied quantitative metabolite profiling together with enzyme activity analysis to study xylose-to-ethanol metabolisms of Candida tenuis CBS4435 (q xylose = 0.10 g/gdc/h, 25 °C; Y ethanol = 0.44 g/g; Y xylitol = 0.09 g/g) and the recombinant S. cerevisiae strain BP000 (q xylose = 0.07 g/gdc/h, 30 °C; Y ethanol = 0.24 g/g; Y xylitol = 0.43 g/g), both expressing the same xylose reductase (XR), comprehensively.

RESULTS

Results from strain-to-strain metabolic control analysis indicated that activity levels of XR and the maximal flux capacity of the upper glycolysis (UG; both ≥ tenfold higher in CBS4435) contributed predominantly to phenotype differentiation while reactions from the oxidative pentose phosphate pathway played minor roles. Intracellular metabolite profiles supported results obtained from kinetic modeling and indicated a positive correlation between pool sizes of UG metabolites and carbon flux through the UG. For CBS4435, fast carbon flux through the UG could be associated with an allosteric control of 6-phosphofructokinase (PFK) activity by fructose 6-phosphate. The ability of CBS4435 to keep UG metabolites at high levels could be explained by low glycerol 3-phosphate phosphatase (GPP, 17-fold lower in CBS4435) and high XR activities.

CONCLUSIONS

By applying a systems biology approach in which we combined results obtained from metabolic control analysis based on kinetic modeling with data obtained from quantitative metabolite profiling and enzyme activity analyses, we could provide new insights into metabolic and kinetic interactions contributing to the control of carbon flux from xylose to ethanol. Supported by evidences presented two new targets, PFK and GPP, could be identified that aside from XR play pivotal roles in phenotype differentiation. Design of efficient and fast microbial ethanol producers in the future can certainly benefit from results presented in this study.

Rapid ethanol production at elevated temperatures by engineered thermotolerant Kluyveromyces marxianus via the NADP(H)-preferring xylose reductase-xylitol dehydrogenase pathway

Conversion of xylose to ethanol by yeasts is a challenge because of the redox imbalances under oxygen-limited conditions. The thermotolerant yeast Kluyveromyces marxianus grows well with xylose as a carbon source at elevated temperatures, but its xylose fermentation ability is weak. In this study, a combination of the NADPH-preferring xylose reductase (XR) from Neurospora crassa and the NADP(+)-preferring xylitol dehydrogenase (XDH) mutant from Scheffersomyces stipitis (Pichia stipitis) was constructed. The xylose fermentation ability and redox balance of the recombinant strains were improved significantly by over-expression of several downstream genes. The intracellular concentrations of coenzymes and the reduced coenzyme/oxidized coenzyme ratio increased significantly in these metabolic strains. The byproducts, such as glycerol and acetic acid, were significantly reduced by the disruption of glycerol-3-phosphate dehydrogenase (GPD1). The resulting engineered K. marxianus YZJ088 strain produced 44.95 g/L ethanol from 118.39 g/L xylose with a productivity of 2.49 g/L/h at 42 °C. Additionally, YZJ088 realized glucose and xylose co-fermentation and produced 51.43 g/L ethanol from a mixture of 103.97 g/L xylose and 40.96 g/L glucose with a productivity of 2.14 g/L/h at 42 °C. These promising results validate the YZJ088 strain as an excellent producer of ethanol from xylose through the synthetic xylose assimilation pathway.

Applications of computational modeling in metabolic engineering of yeast

Generally, a microorganism's phenotype can be described by its pattern of metabolic fluxes. Although fluxes cannot be measured directly, inference of fluxes is well established. In biotechnology the aim is often to increase the capacity of specific fluxes. For this, metabolic engineering methods have been developed and applied extensively. Many of these rely on balancing of intracellular metabolites, redox, and energy fluxes, using genome-scale models (GEMs) that in combination with appropriate objective functions and constraints can be used to predict potential gene targets for obtaining a preferred flux distribution. These methods point to strategies for altering gene expression; however, fluxes are often controlled by post-transcriptional events. Moreover, GEMs are usually not taking into account metabolic regulation, thermodynamics and enzyme kinetics. To facilitate metabolic engineering, tools from synthetic biology have emerged, enabling integration and assembly of naturally nonexistent, but well-characterized components into a living organism. To describe these systems kinetic models are often used and to integrate these systems with the standard metabolic engineering approach, it is necessary to expand the modeling of metabolism to consider kinetics of individual processes. This review will give an overview about models available for metabolic engineering of yeast and discusses their applications.

Metabolic engineering and classical selection of the methylotrophic thermotolerant yeast Hansenula polymorpha for improvement of high-temperature xylose alcoholic fermentation

Background

The methylotrophic yeast, Hansenula polymorpha is an industrially important microorganism, and belongs to the best studied yeast species with well-developed tools for molecular research. The complete genome sequence of the strain NCYC495 of H. polymorpha is publicly available. Some of the well-studied strains of H. polymorpha are known to ferment glucose, cellobiose and xylose to ethanol at elevated temperature (45 – 50°C) with ethanol yield from xylose significantly lower than that from glucose and cellobiose. Increased yield of ethanol from xylose was demonstrated following directed metabolic changes but, still the final ethanol concentration achieved is well below what is considered feasible for economic recovery by distillation.

Results

In this work, we describe the construction of strains of H. polymorpha with increased ethanol production from xylose using an ethanol-non-utilizing strain (2EthOH⁻) as the host. The transformants derived from 2EthOH⁻ overexpressing modified xylose reductase (XYL1m) and native xylitol dehydrogenase (XYL2) were isolated. These transformants produced 1.5-fold more ethanol from xylose than the original host strain. The additional overexpression of XYL3 gene coding for xylulokinase, resulted in further 2.3-fold improvement in ethanol production with no measurable xylitol formed during xylose fermentation. The best ethanol producing strain obtained by metabolic engineering approaches was subjected to selection for resistance to the known inhibitor of glycolysis, the anticancer drug 3-bromopyruvate. The best mutant selected had an ethanol yield of 0.3 g/g xylose and produced up to 9.8 g of ethanol/l during xylose alcoholic fermentation at 45°C without correction for ethanol evaporation.

Conclusions

Our results indicate that xylose conversion to ethanol at elevated temperature can be significantly improved in H. polymorpha by combining methods of metabolic engineering and classical selection.

KiMoSys: a web-based repository of experimental data for KInetic MOdels of biological SYStems

BACKGROUND

The kinetic modeling of biological systems is mainly composed of three steps that proceed iteratively: model building, simulation and analysis. In the first step, it is usually required to set initial metabolite concentrations, and to assign kinetic rate laws, along with estimating parameter values using kinetic data through optimization when these are not known. Although the rapid development of high-throughput methods has generated much omics data, experimentalists present only a summary of obtained results for publication, the experimental data files are not usually submitted to any public repository, or simply not available at all. In order to automatize as much as possible the steps of building kinetic models, there is a growing requirement in the systems biology community for easily exchanging data in combination with models, which represents the main motivation of KiMoSys development.

DESCRIPTION

KiMoSys is a user-friendly platform that includes a public data repository of published experimental data, containing concentration data of metabolites and enzymes and flux data. It was designed to ensure data management, storage and sharing for a wider systems biology community. This community repository offers a web-based interface and upload facility to turn available data into publicly accessible, centralized and structured-format data files. Moreover, it compiles and integrates available kinetic models associated with the data.KiMoSys also integrates some tools to facilitate the kinetic model construction process of large-scale metabolic networks, especially when the systems biologists perform computational research.

CONCLUSIONS

KiMoSys is a web-based system that integrates a public data and associated model(s) repository with computational tools, providing the systems biology community with a novel application facilitating data storage and sharing, thus supporting construction of ODE-based kinetic models and collaborative research projects.The web application implemented using Ruby on Rails framework is freely available for web access at http://kimosys.org, along with its full documentation.

Kinetic models in industrial biotechnology - Improving cell factory performance

An increasing number of industrial bioprocesses capitalize on living cells by using them as cell factories that convert sugars into chemicals. These processes range from the production of bulk chemicals in yeasts and bacteria to the synthesis of therapeutic proteins in mammalian cell lines. One of the tools in the continuous search for improved performance of such production systems is the development and application of mathematical models. To be of value for industrial biotechnology, mathematical models should be able to assist in the rational design of cell factory properties or in the production processes in which they are utilized. Kinetic models are particularly suitable towards this end because they are capable of representing the complex biochemistry of cells in a more complete way compared to most other types of models. They can, at least in principle, be used to in detail understand, predict, and evaluate the effects of adding, removing, or modifying molecular components of a cell factory and for supporting the design of the bioreactor or fermentation process. However, several challenges still remain before kinetic modeling will reach the degree of maturity required for routine application in industry. Here we review the current status of kinetic cell factory modeling. Emphasis is on modeling methodology concepts, including model network structure, kinetic rate expressions, parameter estimation, optimization methods, identifiability analysis, model reduction, and model validation, but several applications of kinetic models for the improvement of cell factories are also discussed.

Engineering of yeast hexose transporters to transport D-xylose without inhibition by D-glucose

All known D-xylose transporters are competitively inhibited by D-glucose, which is one of the major reasons hampering simultaneous fermentation of D-glucose and D-xylose, two primary sugars present in lignocellulosic biomass. We have set up a yeast growth-based screening system for mutant D-xylose transporters that are insensitive to the presence of D-glucose. All of the identified variants had a mutation at either a conserved asparagine residue in transmembrane helix 8 or a threonine residue in transmembrane helix 5. According to a homology model of the yeast hexose transporter Gal2 deduced from the crystal structure of the D-xylose transporter XylE from Escherichia coli, both residues are found in the same region of the protein and are positioned slightly to the extracellular side of the central sugar-binding pocket. Therefore, it is likely that alterations sterically prevent D-glucose but not D-xylose from entering the pocket. In contrast, changing amino acids that are supposed to directly interact with the C6 hydroxymethyl group of D-glucose negatively affected transport of both D-glucose and D-xylose. Determination of kinetic properties of the mutant transporters revealed that Gal2-N376F had the highest affinity for D-xylose, along with a moderate transport velocity, and had completely lost the ability to transport hexoses. These transporter versions should prove valuable for glucose-xylose cofermentation in lignocellulosic hydrolysates by Saccharomyces cerevisiae and other biotechnologically relevant organisms. Moreover, our data contribute to the mechanistic understanding of sugar transport because the decisive role of the conserved asparagine residue for determining sugar specificity has not been recognized before.

Effects of production and market factors on ethanol profitability for an integrated first and second generation ethanol plant using the whole sugarcane as feedstock

BACKGROUND

Sugarcane is an attractive feedstock for ethanol production, especially if the lignocellulosic fraction can also be treated in second generation (2G) ethanol plants. However, the profitability of 2G ethanol is affected by the processing conditions, operating costs and market prices. This study focuses on the minimum ethanol selling price (MESP) and maximum profitability of ethanol production in an integrated first and second generation (1G + 2G) sugarcane-to-ethanol plant. The feedstock used was sugarcane juice, bagasse and leaves. The lignocellulosic fraction was hydrolysed with enzymes. Yields were assumed to be 95% of the theoretical for each of the critical steps in the process (steam pretreatment, enzymatic hydrolysis (EH), fermentation, solid/liquid separation, anaerobic digestion) in order to obtain the best conditions possible for ethanol production, to assess the lowest production costs. Techno-economic analysis was performed for various combinations of process options (for example use of pentoses, addition of leaves), EH conditions (water-insoluble solids (WIS) and residence time), operating cost (enzymes) and market factors (wholesale prices of electricity and ethanol, cost of the feedstock).

RESULTS

The greatest reduction in 2G MESP was achieved when using the pentoses for the production of ethanol rather than biogas. This was followed, in decreasing order, by higher enzymatic hydrolysis efficiency (EHE), by increasing the WIS to 30% and by a short residence time (48 hours) in the EH. The addition of leaves was found to have a slightly negative impact on 1G + 2G MESP, but the effect on 2G MESP was negligible. Sugarcane price significantly affected 1G + 2G MESP, while the price of leaves had a much lower impact. Net present value (NPV) analysis of the most interesting case showed that integrated 1G + 2G ethanol production including leaves could be more profitable than 1G ethanol, despite the fact that the MESP was higher than in 1G ethanol production.

CONCLUSIONS

A combined 1G + 2G ethanol plant could potentially outperform a 1G plant in terms of NPV, depending on market wholesale prices of ethanol and electricity. Therefore, although it is more expensive than 1G ethanol production, 2G ethanol production can make the integrated 1G + 2G process more profitable.

On the role of GAPDH isoenzymes during pentose fermentation in engineered Saccharomyces cerevisiae

In the metabolic network of the cell, many intermediary products are shared between different pathways. d-Glyceraldehyde-3-phosphate, a glycolytic intermediate, is a substrate of GAPDH but is also utilized by transaldolase and transketolase in the scrambling reactions of the nonoxidative pentose phosphate pathway. Recent efforts to engineer baker's yeast strains capable of utilizing pentose sugars present in plant biomass rely on increasing the carbon flux through this pathway. However, the competition between transaldolase and GAPDH for d-glyceraldehyde-3-phosphate produced in the first transketolase reaction compromises the carbon balance of the pathway, thereby limiting the product yield. Guided by the hypothesis that reduction in GAPDH activity would increase the availability of d-glyceraldehyde-3-phosphate for transaldolase and thereby improve ethanol production during fermentation of pentoses, we performed a comprehensive characterization of the three GAPDH isoenzymes in baker's yeast, Tdh1, Tdh2, and Tdh3 and analyzed the effect of their deletion on xylose utilization by engineered strains. Our data suggest that overexpression of transaldolase is a more promising strategy than reduction in GAPDH activity to increase the flux through the nonoxidative pentose phosphate pathway.

Engineering acidic Streptomyces rubiginosus D-xylose isomerase by rational enzyme design

To maximize bioethanol production from lignocellulosic biomass, all sugars must be utilized. Yeast fermentation can be improved by introducing the d-xylose isomerase enzyme to convert the pentose sugar d-xylose, which cannot be fermented by Saccharomyces cerevisiae, into the fermentable ketose d-xylulose. The low activity of d-xylose isomerase, especially at the low pH required for optimal fermentation, limits its use. A rational enzyme engineering approach was undertaken, and seven amino acid positions were replaced to improve the activity of Streptomyces rubiginosus d-xylose isomerase towards its physiological substrate at pH values below 6. The active-site design was guided by mechanistic insights and the knowledge of amino acid protonation states at low pH obtained from previous joint X-ray/neutron crystallographic experiments. Tagging the enzyme with 6 or 12 histidine residues at the N-terminus resulted in a significant increase in the active-site affinity towards substrate at pH 5.8. Substituting an asparagine at position 215, which hydrogen bonded to the metal-bound Glu181 and Asp245, with an aspartate gave a variant with almost an order of magnitude lower KM than measured for the native enzyme, with a 4-fold increase in activity. Other studied variants showed similar (Asp57Asn, Glu186Gln/Asn215Asp), lower (Asp57His, Asn247Asp, Lys289His, Lys289Glu) or no (Gln256Asp, Asp287Asn, ΔAsp287) activity in acidic conditions relative to the native enzyme.

Construction of fast xylose-fermenting yeast based on industrial ethanol-producing diploid Saccharomyces cerevisiae by rational design and adaptive evolution

Background

It remains a challenge for recombinant S. cerevisiae to convert xylose in lignocellulosic biomass hydrolysates to ethanol. Although industrial diploid strains are more robust compared to laboratory haploid strains, however, industrial diploid S. cerevisiae strains have been less pursued in previous studies. This work aims to construct fast xylose-fermenting yeast using an industrial ethanol-producing diploid S. cerevisiae strain as a host.

Results

Fast xylose-fermenting yeast was constructed by genome integration of xylose-utilizing genes and adaptive evolution, including 1) Piromyces XYLA was introduced to enable the host strain to convert xylose to xylulose; 2) endogenous genes (XKS1, RKI1, RPE1, TKL1, and TAL1) were overexpressed to accelerate conversion of xylulose to ethanol; 3) Candida intermedia GXF1, which encodes a xylose transporter, was introduced at the GRE3 locus to improve xylose uptake; 4) aerobic evolution in rich xylose media was carried out to increase growth and xylose consumption rates. The best evolved strain CIBTS0735 consumed 80 g/l glucose and 40 g/l xylose in rich media within 24 hours at an initial OD₆₀₀ of 1.0 (0.63 g DCW/l) and produced 53 g/l ethanol.

Conclusions

Based on the above fermentation performance, we conclude that CIBTS0735 shows great potential for ethanol production from lignocellulosic biomass.

Enhanced expression of genes involved in initial xylose metabolism and the oxidative pentose phosphate pathway in the improved xylose-utilizing Saccharomyces cerevisiae through evolutionary engineering

Fermentation of xylose in lignocellulosic hydrolysates by Saccharomyces cerevisiae has been achieved through heterologous expression of the xylose reductase (XR)-xylitol dehydrogenase (XDH) pathway. However, the fermentation efficiency is far from the requirement for industrial application due to high yield of the byproduct xylitol, low ethanol yield, and low xylose consumption rate. Through evolutionary engineering, an improved xylose-utilizing strain SyBE005 was obtained with 78.3 % lower xylitol production and a 2.6-fold higher specific ethanol production rate than those of the parent strain SyBE004, which expressed an engineered NADP(+)-preferring XDH. The transcriptional differences between SyBE005 and SyBE004 were investigated by quantitative RT-PCR. Genes including XYL1, XYL2, and XKS1 in the initial xylose metabolic pathway showed the highest up-regulation in SyBE005. The increased expression of XYL1 and XYL2 correlated with enhanced enzymatic activities of XR and XDH. In addition, the expression level of ZWF1 in the oxidative pentose phosphate pathway increased significantly in SyBE005, indicating an elevated demand for NADPH from XR. Genes involved in the TCA cycle (LAT1, CIT1, CIT2, KGD1, KGD, SDH2) and gluconeogenesis (ICL1, PYC1) were also up-regulated in SyBE005. Genomic analysis revealed that point mutations in transcriptional regulators CYC8 and PHD1 might be responsible for the altered expression. In addition, a mutation (Y89S) in ZWF1 was identified which might improve NADPH production in SyBE005. Our results suggest that increasing the expression of XYL1, XYL2, XKS1, and enhancing NADPH supply are promising strategies to improve xylose fermentation in recombinant S. cerevisiae.

Improved xylose fermentation of Kluyveromyces marxianus at elevated temperature through construction of a xylose isomerase pathway

To improve the xylose fermentation ability of Kluyveromyces marxianus, a xylose assimilation pathway through xylose isomerase was constructed. The genes encoding xylose reductase (KmXyl1) and xylitol dehydrogenase (KmXyl2) were disrupted in K. marxianus YHJ010 and the resultant strain was named YRL002. A codon-optimized xylose isomerase gene from Orpinomyces was transformed into K. marxianus YRL002 and expressed under GAPDH promoter. The transformant was adapted in the SD medium containing 1 % casamino acid with 2 % xylose as sole carbon source. After 32 times of trans-inoculation, a strain named YRL005, which can grow at a specific growth rate of 0.137/h with xylose as carbon source, was obtained. K. marxianus YRL005 could ferment 30.15 g/l of xylose and produce 11.52 g/l ethanol with a yield of 0.38 g/g, production rate of 0.069 g/l/h at 42 °C, and also could ferment 16.60 g/l xylose to produce 5.21 g/l ethanol with a yield of 0.31 g/g, and production rate of 0.054 g/l h at 45 °C. Co-fermentation with 2 % glucose could not improve the amount and yield of ethanol fermented from xylose obviously, but it could improve the production rate. Furthermore, K. marxianus YRL005 can ferment with the corn cob hydrolysate, which contained 20.04 g/l xylose to produce 8.25 g/l ethanol. It is a good platform to construct thermo-tolerant xylose fermentation yeast.

Deletion of FPS1, encoding aquaglyceroporin Fps1p, improves xylose fermentation by engineered Saccharomyces cerevisiae

Accumulation of xylitol in xylose fermentation with engineered Saccharomyces cerevisiae presents a major problem that hampers economically feasible production of biofuels from cellulosic plant biomass. In particular, substantial production of xylitol due to unbalanced redox cofactor usage by xylose reductase (XR) and xylitol dehydrogenase (XDH) leads to low yields of ethanol. While previous research focused on manipulating intracellular enzymatic reactions to improve xylose metabolism, this study demonstrated a new strategy to reduce xylitol formation and increase carbon flux toward target products by controlling the process of xylitol secretion. Using xylitol-producing S. cerevisiae strains expressing XR only, we determined the role of aquaglyceroporin Fps1p in xylitol export by characterizing extracellular and intracellular xylitol. In addition, when FPS1 was deleted in a poorly xylose-fermenting strain with unbalanced XR and XDH activities, the xylitol yield was decreased by 71% and the ethanol yield was substantially increased by nearly four times. Experiments with our optimized xylose-fermenting strain also showed that FPS1 deletion reduced xylitol production by 21% to 30% and increased ethanol yields by 3% to 10% under various fermentation conditions. Deletion of FPS1 decreased the xylose consumption rate under anaerobic conditions, but the effect was not significant in fermentation at high cell density. Deletion of FPS1 resulted in higher intracellular xylitol concentrations but did not significantly change the intracellular NAD(+)/NADH ratio in xylose-fermenting strains. The results demonstrate that Fps1p is involved in xylitol export in S. cerevisiae and present a new gene deletion target, FPS1, and a mechanism different from those previously reported to engineer yeast for improved xylose fermentation.

Feasibility of xylose fermentation by engineered Saccharomyces cerevisiae overexpressing endogenous aldose reductase (GRE3), xylitol dehydrogenase (XYL2), and xylulokinase (XYL3) from Scheffersomyces stipitis

Saccharomyces cerevisiae has been engineered for producing ethanol from xylose, the second most abundant sugar in cellulosic biomass hydrolyzates. Heterologous expressions of xylose reductase (XYL1) and xylitol dehydrogenase (XYL2), or of xylose isomerase (xylA), either case of which being accompanied by overexpression of xylulokinase (XKS1 or XYL3), are known as the prevalent strategies for metabolic engineering of S. cerevisiae to ferment xylose. In this study, we propose an alternative strategy that employs overexpression of GRE3 coding for endogenous aldose reductase instead of XYL1 to construct efficient xylose-fermenting S. cerevisiae. Replacement of XYL1 with GRE3 has been regarded as an undesirable approach because NADPH-specific aldose reductase (GRE3) would aggravate redox balance with xylitol dehydrogenase (XYL2) using NAD(+) exclusively. Here, we demonstrate that engineered S. cerevisiae overexpressing GRE3, XYL2, and XYL3 can ferment xylose as well as a mixture of glucose and xylose with higher ethanol yields (0.29-0.41 g g(-1) sugars) and productivities (0.13-0.85 g L(-1) h(-1)) than those (0.23-0.39 g g(-1) sugars, 0.10-0.74 g L(-1) h(-1)) of an isogenic strain overexpressing XYL1, XYL2, and XYL3 under oxygen-limited conditions. We found that xylose fermentation efficiency of a strain overexpressing GRE3 was dramatically increased by high expression levels of XYL2. Our results suggest that optimized expression levels of GRE3, XYL2, and XYL3 could overcome redox imbalance during xylose fermentation by engineered S. cerevisiae under oxygen-limited conditions.

Improving ethanol and xylitol fermentation at elevated temperature through substitution of xylose reductase in Kluyveromyces marxianus

Thermo-tolerant yeast Kluyveromyces marxianus is able to utilize a wide range of substrates, including xylose; however, the xylose fermentation ability is weak because of the redox imbalance under oxygen-limited conditions. Alleviating the intracellular redox imbalance through engineering the coenzyme specificity of NADPH-preferring xylose reductase (XR) and improving the expression of XR should promote xylose consumption and fermentation. In this study, the native xylose reductase gene (Kmxyl1) of the K. marxianus strain was substituted with XR or its mutant genes from Pichia stipitis (Scheffersomyces stipitis). The ability of the resultant recombinant strains to assimilate xylose to produce xylitol and ethanol at elevated temperature was greatly improved. The strain YZB014 expressing mutant PsXR N272D, which has a higher activity with both NADPH and NADH as the coenzyme, achieved the best results, and produced 3.55 g l(-1) ethanol and 11.32 g l(-1) xylitol-an increase of 12.24- and 2.70-fold in product at 42 °C, respectively. A 3.94-fold increase of xylose consumption was observed compared with the K. marxianus YHJ010 harboring KmXyl1. However, the strain YZB015 expressing a mutant PsXR K21A/N272D, with which co-enzyme preference was completely reversed from NADPH to NADH, failed to ferment due to the low expression. So in order to improve xylose consumption and fermentation in K. marxianus, both higher activity and co-enzyme specificity change are necessary.

Strategies for the production of high concentrations of bioethanol from seaweeds: production of high concentrations of bioethanol from seaweeds

Bioethanol has attracted attention as an alternative to petroleum-derived fuel. Seaweeds have been proposed as some of the most promising raw materials for bioethanol production because they have several advantages over lignocellulosic biomass. However, because seaweeds contain low contents of glucans, i.e., polysaccharides composed of glucose, the conversion of only the glucans from seaweed is not sufficient to produce high concentrations of ethanol. Therefore, it is also necessary to produce ethanol from other specific carbohydrate components of seaweeds, including sulfated polysaccharides, mannitol, alginate, agar and carrageenan. This review summarizes the current state of research on the production of ethanol from seaweed carbohydrates for which the conversion of carbohydrates to sugars is a key step and makes comparisons with the production of ethanol from lignocellulosic biomass. This review provides valuable information necessary for the production of high concentrations of ethanol from seaweeds.

Co-fermentation of xylose and cellobiose by an engineered Saccharomyces cerevisiae

We have integrated and coordinately expressed in Saccharomyces cerevisiae a xylose isomerase and cellobiose phosphorylase from Ruminococcus flavefaciens that enables fermentation of glucose, xylose, and cellobiose under completely anaerobic conditions. The native xylose isomerase was active in cell-free extracts from yeast transformants containing a single integrated copy of the gene. We improved the activity of the enzyme and its affinity for xylose by modifications to the 5′-end of the gene, site-directed mutagenesis, and codon optimization. The improved enzyme, designated RfCO*, demonstrated a 4.8-fold increase in activity compared to the native xylose isomerase, with a Km for xylose of 66.7 mM and a specific activity of 1.41 μmol/min/mg. In comparison, the native xylose isomerase was found to have a Km for xylose of 117.1 mM and a specific activity of 0.29 μmol/min/mg. The coordinate over-expression of RfCO* along with cellobiose phosphorylase, cellobiose transporters, the endogenous genes GAL2 and XKS1, and disruption of the native PHO13 and GRE3 genes allowed the fermentation of glucose, xylose, and cellobiose under completely anaerobic conditions. Interestingly, this strain was unable to utilize xylose or cellobiose as a sole carbon source for growth under anaerobic conditions, thus minimizing yield loss to biomass formation and maximizing ethanol yield during their fermentation.