Production of Ethanol from d-Xylose by Using d-Xylose Isomerase and Yeasts

Metabolic engineering for pentose utilization in Saccharomyces cerevisiae

The introduction of pentose utilization pathways in baker's yeast Saccharomyces cerevisiae is summarized together with metabolic engineering strategies to improve ethanolic pentose fermentation. Bacterial and fungal xylose and arabinose pathways have been expressed in S. cerevisiae but do not generally convey significant ethanolic fermentation traits to this yeast. A large number of rational metabolic engineering strategies directed among others toward sugar transport, initial pentose conversion, the pentose phosphate pathway, and the cellular redox metabolism have been exploited. The directed metabolic engineering approach has often been combined with random approaches including adaptation, mutagenesis, and hybridization. The knowledge gained about pentose fermentation in S. cerevisiae is primarily limited to genetically and physiologically well-characterized laboratory strains. The translation of this knowledge to strains performing in an industrial context is discussed.

Efficient production of L-lactic acid from xylose by Pichia stipitis

Microbial conversion of renewable raw materials to useful products is an important objective in industrial biotechnology. Pichia stipitis, a yeast that naturally ferments xylose, was genetically engineered for l-(+)-lactate production. We constructed a P. stipitis strain that expressed the l-lactate dehydrogenase (LDH) from Lactobacillus helveticus under the control of the P. stipitis fermentative ADH1 promoter. Xylose, glucose, or a mixture of the two sugars was used as the carbon source for lactate production. The constructed P. stipitis strain produced a higher level of lactate and a higher yield on xylose than on glucose. Lactate accumulated as the main product in xylose-containing medium, with 58 g/liter lactate produced from 100 g/liter xylose. Relatively efficient lactate production also occurred on glucose medium, with 41 g/liter lactate produced from 94 g/liter glucose. In the presence of both sugars, xylose and glucose were consumed simultaneously and converted predominantly to lactate. Lactate was produced at the expense of ethanol, whose production decreased to approximately 15 to 30% of the wild-type level on xylose-containing medium and to 70 to 80% of the wild-type level on glucose-containing medium. Thus, LDH competed efficiently with the ethanol pathway for pyruvate, even though the pathway from pyruvate to ethanol was intact. Our results show, for the first time, that lactate production from xylose by a yeast species is feasible and efficient. This is encouraging for further development of yeast-based bioprocesses to produce lactate from lignocellulosic raw material.

Alcoholic fermentation of carbon sources in biomass hydrolysates by Saccharomyces cerevisiae: current status

Fuel ethanol production from plant biomass hydrolysates by Saccharomyces cerevisiae is of great economic and environmental significance. This paper reviews the current status with respect to alcoholic fermentation of the main plant biomass-derived monosaccharides by this yeast. Wild-type S. cerevisiae strains readily ferment glucose, mannose and fructose via the Embden-Meyerhof pathway of glycolysis, while galactose is fermented via the Leloir pathway. Construction of yeast strains that efficiently convert other potentially fermentable substrates in plant biomass hydrolysates into ethanol is a major challenge in metabolic engineering. The most abundant of these compounds is xylose. Recent metabolic and evolutionary engineering studies on S. cerevisiae strains that express a fungal xylose isomerase have enabled the rapid and efficient anaerobic fermentation of this pentose. L: -Arabinose fermentation, based on the expression of a prokaryotic pathway in S. cerevisiae, has also been established, but needs further optimization before it can be considered for industrial implementation. In addition to these already investigated strategies, possible approaches for metabolic engineering of galacturonic acid and rhamnose fermentation by S. cerevisiae are discussed. An emerging and major challenge is to achieve the rapid transition from proof-of-principle experiments under 'academic' conditions (synthetic media, single substrates or simple substrate mixtures, absence of toxic inhibitors) towards efficient conversion of complex industrial substrate mixtures that contain synergistically acting inhibitors.

Nectar xylose metabolism in a rodent pollinator (Aethomys namaquensis): defining the role of gastrointestinal microflora using 14C-labeled xylose

The Namaqua rock mouse Aethomys namaquensis, a rodent pollinator of certain geoflorous Protea species, consumes nectar containing xylose. Xylose is not known to be efficiently utilized by mammals. However, it is fermented by certain bacteria, yeasts, and fungi, particularly gastrointestinal bacteria. The end products of microbial fermentation are utilized by the host in oxidative metabolism. Here we investigate the degree to which intestinal bacteria of A. namaquensis contribute to xylose metabolism. Mice were caught during Protea humiflora flowering and nonflowering seasons and given an oral dose of 14C-labeled xylose. Exhaled CO2 and excreted urine and feces were continuously collected for 30 h thereafter, and label recovery was determined. Each mouse was then treated with antibiotics to reduce gut microflora, and the experiment was repeated. With their natural gut flora population intact, mice caught during the flowering season exhaled significantly more 14CO2 than did mice caught during the nonflowering season. Also, during both seasons, mice exhaled significantly more 14CO2 before antibiotic treatment than after. Antibiotic treatment caused a significant increase in the proportion of 14C-labeled xylose that was excreted in the urine. The mouse diet likely influences the composition of the gastrointestinal community. Aethomys namaquensis relies on its gut microflora to ferment xylose, thereby converting it into end products that are used by the mice for metabolism.

Endogenous xylose pathway in Saccharomyces cerevisiae

The baker's yeast Saccharomyces cerevisiae is generally classified as a non-xylose-utilizing organism. We found that S. cerevisiae can grow on D-xylose when only the endogenous genes GRE3 (YHR104w), coding for a nonspecific aldose reductase, and XYL2 (YLR070c, ScXYL2), coding for a xylitol dehydrogenase (XDH), are overexpressed under endogenous promoters. In nontransformed S. cerevisiae strains, XDH activity was significantly higher in the presence of xylose, but xylose reductase (XR) activity was not affected by the choice of carbon source. The expression of SOR1, encoding a sorbitol dehydrogenase, was elevated in the presence of xylose as were the genes encoding transketolase and transaldolase. An S. cerevisiae strain carrying the XR and XDH enzymes from the xylose-utilizing yeast Pichia stipitis grew more quickly and accumulated less xylitol than did the strain overexpressing the endogenous enzymes. Overexpression of the GRE3 and ScXYL2 genes in the S. cerevisiae CEN.PK2 strain resulted in a growth rate of 0.01 g of cell dry mass liter(-1) h(-1) and a xylitol yield of 55% when xylose was the main carbon source.

Hemicellulose bioconversion

Various agricultural residues, such as corn fiber, corn stover, wheat straw, rice straw, and sugarcane bagasse, contain about 20-40% hemicellulose, the second most abundant polysaccharide in nature. The conversion of hemicellulose to fuels and chemicals is problematic. In this paper, various pretreatment options as well as enzymatic saccharification of lignocellulosic biomass to fermentable sugars is reviewed. Our research dealing with the pretreatment and enzymatic saccharification of corn fiber and development of novel and improved enzymes such as endo-xylanase, beta-xylosidase, and alpha- l-arabinofuranosidase for hemicellulose bioconversion is described. The barriers, progress, and prospects of developing an environmentally benign bioprocess for large-scale conversion of hemicellulose to fuel ethanol, xylitol, 2,3-butanediol, and other value-added fermentation products are highlighted.

Production of ethanol and xylitol from corn cobs by yeasts

Saccharomyces cerevisiae and Candida tropicalis were used separately and as co-culture for simultaneous saccharification and fermentation (SSF) of 5-20% (w/v) dry corn cobs. A maximal ethanol concentration of 27, 23, 21 g/l (w/v) from 200 g/l (w/v) dry corn cobs was obtained by S. cerevisiae, C. tropicalis and the co-culture, respectively, after 96 h of fermentation. However, theoretical yields of 82%, 71% and 63% were observed from 50 g/l dry corn cobs for the above cultures, respectively. Maximal xylitol concentration of 21, 20 and 15 g/l from 200 g/l (w/v) dry corn cobs was obtained by C. tropicalis, co-culture, and S. cerevisiae, respectively. Maximum theoretical yields of 79.0%, 77.0% and 58% were observed from 50 g/l of corn cobs, respectively. The volumetric productivities for ethanol and xylitol increased with the increase in substrate concentration, whereas, yield decreased. Glycerol and acetic acid were formed as minor by-products. S. cerevisiae and C. tropicalis resulted in better product yields (0.42 and 0.36 g/g) for ethanol and (0.52 and 0.71 g/g) for xylitol, respectively, whereas, the co-culture showed moderate level of ethanol (0.32 g/g) and almost maximal levels of xylitol (0.69 g/g).

A view of the history of biochemical engineering

The authors present a view of biochemical engineering by describing their personal interests and experience over the years involving mostly conversion of lignocellulosics into fuels and chemicals and the associated engineering subjects.

Successful design and development of genetically engineered Saccharomyces yeasts for effective cofermentation of glucose and xylose from cellulosic biomass to fuel ethanol

Ethanol is an effective, environmentally friendly, nonfossil, transportation biofuel that produces far less pollution than gasoline. Furthermore, ethanol can be produced from plentiful, domestically available, renewable, cellulosic biomass. However, cellulosic biomass contains two major sugars, glucose and xylose, and a major obstacle in this process is that Saccharomyces yeasts, traditionally used and still the only microorganisms currently used for large scale industrial production of ethanol from glucose, are unable to ferment xylose to ethanol. This makes the use of these safest, most effective Saccharomyces yeasts for conversion of biomass to ethanol economically unfeasible. Since 1980, scientists worldwide have actively been trying to develop genetically engineered Saccharomyces yeasts to ferment xylose. In 1993, we achieved a historic breakthrough to succeed in the development of the first genetically engineered Saccharomyces yeasts that can effectively ferment both glucose and xylose to ethanol. This was accomplished by carefully redesigning the yeast metabolic pathway for fermenting xylose to ethanol, including cloning three xylose-metabolizing genes, modifying the genetic systems controlling gene expression, changing the dynamics of the carbon flow, etc. As a result, our recombinant yeasts not only can effectively ferment both glucose and xylose to ethanol when these sugars are present separately in the medium, but also can effectively coferment both glucose and xylose present in the same medium simultaneously to ethanol. This has made it possible because we have genetically engineered the Saccharomyces yeasts as such that they are able to overcome some of the natural barrier present in all microorganisms, such as the synthesis of the xylose metabolizing enzymes not to be affected by the presence of glucose and by the absence of xylose in the medium. This first generation of genetically engineered glucose-xylose-cofermenting Saccharomyces yeasts relies on the presence of a high-copy-number 2 mu-based plasmid that contains the three cloned genetically modified xylose-metabolizing genes to provide the xylose-metabolizing capability. In 1995, we achieved another breakthrough by creating the super-stable genetically engineered glucose-xylose-cofermenting Saccharomyces yeasts which contain multiple copies of the same three xylose-metabolizing genes stably integrated on the yeast chromosome. This is another critical development which has made it possible for the genetically engineered yeasts to be effective for cofermenting glucose and xylose by continuous fermentation. It is widely believed that the successful development of the stable glucose-xylose-cofermenting Saccharomyces yeasts has made the biomass-to-ethanol technology a step much closer to commercialization. In this paper, we present an overview of our rationales and strategies as well as our methods and approaches that led to the ingenious design and successful development of our genetically engineered Saccharomyces yeasts for effective cofermentation of glucose and xylose to biofuel ethanol.

Ethanol production from renewable resources

Vast amounts of renewable biomass are available for conversion to liquid fuel, ethanol. In order to convert biomass to ethanol, the efficient utilization of both cellulose-derived and hemicellulose-derived carbohydrates is essential. Six-carbon sugars are readily utilized for this purpose. Pentoses, on the other hand, are more difficult to convert. Several metabolic factors limit the efficient utilization of pentoses (xylose and arabinose). Recent developments in the improvement of microbial cultures provide the versatility of conversion of both hexoses and pentoses to ethanol more efficiently. In addition, novel bioprocess technologies offer a promising prospective for the efficient conversion of biomass and recovery of ethanol.

Simultaneous bioconversion of cellulose and hemicellulose to ethanol

Lignocellulosic materials containing cellulose, hemicellulose, and lignin as their main constituents are the most abundant renewable organic resource present on Earth. The conversion of both cellulose and hemicellulose for production of fuel ethanol is being studied intensively with a view to develop a technically and economically viable bioprocess. The fermentation of glucose, the main constituent of cellulose hydrolyzate, to ethanol can be carried out efficiently. On the other hand, although bioconversion of xylose, the main pentose sugar obtained on hydrolysis of hemicellulose, to ethanol presents a biochemical challenge, especially if it is present along with glucose, it needs to be fermented to make the biomass-to-ethanol process economical. A lot of attention therefore has been focussed on the utilization of both glucose and xylose to ethanol. Accordingly, while describing the advancements that have taken place to get xylose converted efficiently to ethanol by xylose-fermenting organisms, the review deals mainly with the strategies that have been put forward for bioconversion of both the sugars to achieve high ethanol concentration, yield, and productivity. The approaches, which include the use of (1) xylose-fermenting yeasts alone, (2) xylose isomerase enzyme as well as yeast, (3) immobilized enzymes and cells, and (4) sequential fermentation and co-culture process are described with respect to their underlying concepts and major limitations. Genetic improvements in the cultures have been made either to enlarge the range of substrate utilization or to channel metabolic intermediates specifically toward ethanol. These contributions represent real significant advancements in the field and have also been adequately dealt with from the point of view of their impact on utilization of both cellulose and hemicellulose sugars to ethanol.

Genetically engineered Saccharomyces yeast capable of effective cofermentation of glucose and xylose

Xylose is one of the major fermentable sugars present in cellulosic biomass, second only to glucose. However, Saccharomyces spp., the best sugar-fermenting microorganisms, are not able to metabolize xylose. We developed recombinant plasmids that can transform Saccharomyces spp. into xylose-fermenting yeasts. These plasmids, designated pLNH31, -32, -33, and -34, are 2 microns-based high-copy-number yeast-E. coli shuttle plasmids. In addition to the geneticin resistance and ampicillin resistance genes that serve as dominant selectable markers, these plasmids also contain three xylose-metabolizing genes, a xylose reductase gene, a xylitol dehydrogenase gene (both from Pichia stipitis), and a xylulokinase gene (from Saccharomyces cerevisiae). These xylose-metabolizing genes were also fused to signals controlling gene expression from S. cerevisiae glycolytic genes. Transformation of Saccharomyces sp. strain 1400 with each of these plasmids resulted in the conversion of strain 1400 from a non-xylose-metabolizing yeast to a xylose-metabolizing yeast that can effectively ferment xylose to ethanol and also effectively utilizes xylose for aerobic growth. Furthermore, the resulting recombinant yeasts also have additional extraordinary properties. For example, the synthesis of the xylose-metabolizing enzymes directed by the cloned genes in these recombinant yeasts does not require the presence of xylose for induction, nor is the synthesis repressed by the presence of glucose in the medium. These properties make the recombinant yeasts able to efficiently ferment xylose to ethanol and also able to efficiently coferment glucose and xylose present in the same medium to ethanol simultaneously.

Molecular and industrial aspects of glucose isomerase

Glucose isomerase (GI) (D-xylose ketol-isomerase; EC. 5.3.1.5) catalyzes the reversible isomerization of D-glucose and D-xylose to D-fructose and D-xylulose, respectively. The enzyme has the largest market in the food industry because of its application in the production of high-fructose corn syrup (HFCS). HFCS, an equilibrium mixture of glucose and fructose, is 1.3 times sweeter than sucrose and serves as a sweetener for use by diabetics. Interconversion of xylose to xylulose by GI serves a nutritional requirement in saprophytic bacteria and has a potential application in the bioconversion of hemicellulose to ethanol. The enzyme is widely distributed in prokaryotes. Intensive research efforts are directed toward improving its suitability for industrial application. Development of microbial strains capable of utilizing xylan-containing raw materials for growth or screening for constitutive mutants of GI is expected to lead to discontinuation of the use of xylose as an inducer for the production of the enzyme. Elimination of Co2+ from the fermentation medium is desirable for avoiding health problems arising from human consumption of HFCS. Immobilization of GI provides an efficient means for its easy recovery and reuse and lowers the cost of its use. X-ray crystallographic and genetic engineering studies support a hydride shift mechanism for the action of GI. Cloning of GI in homologous as well as heterologous hosts has been carried out, with the prime aim of overproducing the enzyme and deciphering the genetic organization of individual genes (xylA, xylB, and xylR) in the xyl operon of different microorganisms. The organization of xylA and xylB seems to be highly conserved in all bacteria. The two genes are transcribed from the same strand in Escherichia coli and Bacillus and Lactobacillus species, whereas they are transcribed divergently on different strands in Streptomyces species. A comparison of the xylA sequences from several bacterial sources revealed the presence of two signature sequences, VXW(GP)GREG(YSTAE)E and (LIVM)EPKPX(EQ)P. The use of an inexpensive inducer in the fermentation medium devoid of Co2+ and redesigning of a tailor-made GI with increased thermostability, higher affinity for glucose, and lower pH optimum will contribute significantly to the development of an economically feasible commercial process for enzymatic isomerization of glucose to fructose. Manipulation of the GI gene by site-directed mutagenesis holds promise that a GI suitable for biotechnological applications will be produced in the foreseeable future.

Molecular and industrial aspects of glucose isomerase

Glucose isomerase (GI) (D-xylose ketol-isomerase; EC. 5.3.1.5) catalyzes the reversible isomerization of D-glucose and D-xylose to D-fructose and D-xylulose, respectively. The enzyme has the largest market in the food industry because of its application in the production of high-fructose corn syrup (HFCS). HFCS, an equilibrium mixture of glucose and fructose, is 1.3 times sweeter than sucrose and serves as a sweetener for use by diabetics. Interconversion of xylose to xylulose by GI serves a nutritional requirement in saprophytic bacteria and has a potential application in the bioconversion of hemicellulose to ethanol. The enzyme is widely distributed in prokaryotes. Intensive research efforts are directed toward improving its suitability for industrial application. Development of microbial strains capable of utilizing xylan-containing raw materials for growth or screening for constitutive mutants of GI is expected to lead to discontinuation of the use of xylose as an inducer for the production of the enzyme. Elimination of Co2+ from the fermentation medium is desirable for avoiding health problems arising from human consumption of HFCS. Immobilization of GI provides an efficient means for its easy recovery and reuse and lowers the cost of its use. X-ray crystallographic and genetic engineering studies support a hydride shift mechanism for the action of GI. Cloning of GI in homologous as well as heterologous hosts has been carried out, with the prime aim of overproducing the enzyme and deciphering the genetic organization of individual genes (xylA, xylB, and xylR) in the xyl operon of different microorganisms. The organization of xylA and xylB seems to be highly conserved in all bacteria. The two genes are transcribed from the same strand in Escherichia coli and Bacillus and Lactobacillus species, whereas they are transcribed divergently on different strands in Streptomyces species. A comparison of the xylA sequences from several bacterial sources revealed the presence of two signature sequences, VXW(GP)GREG(YSTAE)E and (LIVM)EPKPX(EQ)P. The use of an inexpensive inducer in the fermentation medium devoid of Co2+ and redesigning of a tailor-made GI with increased thermostability, higher affinity for glucose, and lower pH optimum will contribute significantly to the development of an economically feasible commercial process for enzymatic isomerization of glucose to fructose. Manipulation of the GI gene by site-directed mutagenesis holds promise that a GI suitable for biotechnological applications will be produced in the foreseeable future.

Xylulose and glucose fermentation by Saccharomyces cerevisiae in chemostat culture

Saccharomyces cerevisiae ATCC 24860 was cultivated in chemostat culture under anoxic conditions with 111.1 mmol of glucose liter-1 alone or with a mixture of 66.7 mmol of xylulose liter-1 and 111.1 mmol of glucose liter-1. The substrate consumption rate was 5.4 mmol g of cells-1 h-1 for glucose, whereas for xylulose it was 1.0 mmol g of cells-1 h-1. The ethanol yield decreased from 0.52 carbon mole of ethanol produced per carbon mole of sugar consumed during the utilization of glucose alone to 0.49 carbon mole produced per carbon mole consumed during the simultaneous utilization of xylulose and glucose, while cell biomass was maintained at 2.04 to 2.10 g liter-1. Xylulose coutilization was accompanied by a shift in product formation from ethanol to acetate and arabinitol. Xylulokinase activity was absent during glucose metabolism but detectable during simultaneous utilization of xylulose and glucose. Xylulose cometabolism resulted in increased in vitro activity of pyruvate decarboxylase and an increased concentration of the intracellular metabolite fructose 1,6-diphosphate without significant changes in the concentrations of 6-phosphogluconate and pyruvate. The results are discussed in relation to (i) altered enzyme activities and (ii) the redox flux of the cell.

Xylulose fermentation by Saccharomyces cerevisiae and xylose-fermenting yeast strains

Xylulose fermentation by four strains of Saccharomyces cerevisiae and two strains of xylose-fermenting yeasts, Pichia stipitis CBS 6054 and Candida shehatae NJ 23, was compared using a mineral medium at a cell concentration of 10 g (dry weight)/l. When xylulose was the sole carbon source and fermentation was anaerobic, S. cerevisiae ATCC 24860 and CBS 8066 showed a substrate consumption rate of 0.035 g g cells-1 h-1 compared with 0.833 gg cells-1 h-1 for glucose. Bakers' yeast and S. cerevisiae isolate 3 consumed xylulose at a much lower rate although they fermented glucose as rapidly as the ATCC and the CBS strains. While P. stipitis CBS 6054 consumed both xylulose and glucose very slowly under anaerobic conditions, C. shehatae NJ 23 fermented xylulose at a rate of 0.345 gg cells-1 h-1, compared with 0.575 gg cells-1 h-1 for glucose. For all six strains, the addition of glucose to the xylulose medium did not enhance the consumption of xylulose, but increased the cell biomass concentrations. When fermentation was performed under oxygen-limited conditions, less xylulose was consumed by S. cerevisiae ATCC 24860 and C. shehatae NJ 23, and 50%- 65% of the assimilated carbon could not be accounted for in the products determined.

Purification and partial characterization of an aldo-keto reductase from Saccharomyces cerevisiae

A cytosolic aldo-keto reductase was purified from Saccharomyces cerevisiae ATCC 26602 to homogeneity by affinity chromatography, chromatofocusing, and hydroxylapatite chromatography. The relative molecular weights of the aldo-keto reductase as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chromatography were 36,800 and 35,000, respectively, indicating that the enzyme is monomeric. Amino acid composition and N-terminal sequence analysis revealed that the enzyme is closely related to the aldose reductases of xylose-fermenting yeasts and mammalian tissues. The enzyme was apparently immunologically unrelated to the aldose reductases of other xylose-fermenting yeasts. The aldo-keto reductase is NADPH specific and catalyzes the reduction of a variety of aldehydes. The best substrate for the enzyme is the aromatic aldehyde p-nitrobenzaldehyde (Km = 46 microM; kcat/Km = 52,100 s-1 M-1), whereas among the aldoses, DL-glyceraldehyde was the preferred substrate (Km = 1.44 mM; kcat/Km = 1,790 s-1 M-1). The enzyme failed to catalyze the reduction of menadione and p-benzoquinone, substrates for carbonyl reductase. The enzyme was inhibited only slightly by 2 mM sodium valproate and was activated by pyridoxal 5'-phosphate. The optimum pH of the enzyme is 5. These data indicate that the S. cerevisiae aldo-keto reductase is a monomeric NADPH-specific reductase with strong similarities to the aldose reductases.

The influence of cosubstrate and aeration on xylitol formation by recombinant Saccharomyces cerevisiae expressing the XYL1 gene

Xylitol formation by a recombinant Saccharomyces cerevisiae strain containing the XYL1 gene from Pichia stipitis CBS 6054 was investigated under three sets of conditions: (a) with glucose, ethanol, acetate, or glycerol as cosubstrates, (b) with different oxygenation levels, and (c) with different ratios of xylose to cosubstrate. With both glucose and ethanol the conversion yields were close to 1 g xylitol/g consumed xylose. Decreased aeration increased the xylitol yield on the basis of consumed cosubstrate, while the rate of xylitol formation decreased. The xylitol yield based on consumed cosubstrate also increased with increased-xylose:cosubstrate ratios. The transformant utilized the cosubstrate more efficiently than did a reference strain in terms of utilization rate and growth rate, implying that the regeneration of NAD(P)+ during xylitol formation by the transformant balanced the intracellular redox potential.

Role of D-ribose as a cometabolite in D-xylose metabolism by Saccharomyces cerevisiae

The influence of D-ribose as a cosubstrate on the uptake and metabolism of the non-growth substrate D-xylose by Saccharomyces cerevisiae ATCC 26602 was investigated. Xylose was taken up by means of low- and high-affinity glucose transport systems. In cells exposed for 2 days to a mixture of xylose and ribose, only the high-affinity system could be detected. Glucose strongly inhibited the transport of xylose by both systems. Starvation or exposure to either xylose or ribose resulted in inactivation of xylose transport, which did not occur in the presence of a mixture of ribose and xylose. A constitutive non-glucose-repressible NADPH2-dependent xylose reductase with a specific activity of ca. 5 mU/mg of protein that converted xylose to xylitol was present in a glucose-grown culture. No activity converting xylitol to xylulose or vice versa was found in crude extracts. Both xylose and ribose were converted to their corresponding polyols, xylitol and ribitol, as indicated by 13C nuclear magnetic resonance spectroscopy. Furthermore, ethanol was detected, and this implied that pathways for the complete catabolism of xylose and ribose exist. However, the NADPH2 required for the conversion of xylose to xylitol is apparently not supplied by the pentose phosphate pathway since the ethanol produced from D-[1-13C]xylose was labelled only in the C-2 position. Acetic acid was produced from ribose and may assist in the conversion of xylose to xylitol by cycling NADPH2.

Cloning and expression in Saccharomyces cerevisiae of the NAD(P)H-dependent xylose reductase-encoding gene (XYL1) from the xylose-assimilating yeast Pichia stipitis

The XYL1 gene of the yeast Pichia stipitis has been isolated from a genomic library using a specific cDNA probe, and its nucleotide (nt) sequence has been determined. In the 5' noncoding region of the P. stipitis XYL1 gene a TATAAA element (known to be necessary for transcription initiation in most yeast genes) is found at nt -81, and two CCAAT recognition motifs (often referred to as the CCAAT box) are present at nt -146 and -106. The XYL1 encodes a polypeptide of 35,927 Da that constitutes a NADH/NADPH-dependent xylose reductase (XR). The enzyme is part of the xylose-xylulose pathway that is absent or only weakly expressed in Saccharomyces cerevisiae. Extensive homology is found to the N terminus of the XR of Pachysolen tannophilus and Candida shehatae. None of the known cofactor binding domains found in many NAD-dependent dehydrogenases are present in the protein. Transformants of S. cerevisiae containing XYL1 of P. stipitis synthesize an active XR. Fusion of XYL1 with the prokaryotic tac promoter leads to a gene that can be expressed in S. cerevisiae and Escherichia coli.

Effects of increased transaldolase activity on D-xylulose and D-glucose metabolism in Saccharomyces cerevisiae cell extracts

In vitro metabolism of D-xylulose and D-glucose in extracts obtained from D-glucose- and D-xylulose-fermenting Saccharomyces cerevisiae cells was investigated with 10- and 100-fold-increased activity of the enzyme transaldolase (EC 2.2.1.2). The rate of sugar consumption was the same in most cases, whereas the rate of ethanol formation decreased with increased levels of transaldolase. The formation of glycerol, pentitols, and acetic acid was not dependent on added transaldolase but was dependent on the sugar used as the growth substrate and on the sugar used in the in vitro metabolism experiments. The carbon balance showed that the dissimilated carbon could not be accounted for in products when transaldolase was added. The concentration of D-fructose-1,6.-diphosphate in the extracts was not influenced by added transaldolase but was higher with D-xylulose than with D-glucose. Levels of pyruvate, comparable with the two substrates, decreased with increasing levels of transaldolase. Exogenously added transaldolase decreased D-sedoheptulose-7-phosphate levels when D-xylulose was the substrate. The results are discussed in relation to the dissimilation of carbon through the upper part of glycolysis and the pentose phosphate pathway.

Ethanolic fermentation of pentoses in lignocellulose hydrolysates

In the fermentation of lignocellulose hydrolysates to ethanol, two major problems are encountered: the fermentation of the pentose sugar xylose, and the presence of microbial inhibitors. Xylose can be directly fermented with yeasts, such as Pachysolen tannophilus, Candida shehatae, and Pichia stipis, or by isomerization of xylose to xylulose with the enzyme glucose (xylose) isomerase (XI; EC 5.3.1.5), and subsequent fermentation with bakers' yeast, Saccharomyces cerevisiae. The direct fermentation requires low, carefully controlled oxygenation, as well as the removal of inhibitors. Also, the xylose-fermenting yeasts have a limited ethanol tolerance. The combined isomerization and fermentation with XI and S. cerevisiae gives yields and productivities comparable to those obtained in hexose fermentations without oxygenation and removal of inhibitors. However, the enzyme is not very stable in a lignocellulose hydrolysate, and S. cerevisiae has a poorly developed pentose phosphate shunt. Different strategies involving strain adaptation, and protein and genetic engineering adopted to overcome these different obstacles, are discussed.

Fusarium oxysporum: status in bioethanol production

Fermentation of lignocellulosic materials to ethanol and other solvents provides an alternative way of treating wastes and producing chemical feedstocks and fuel additives. Considerable efforts have been made in past 10 years to improve the process based on lignocellulosic biomass and hydrolysate that contains a complex mixture of sugars, decomposition products of sugars, and sometimes the inhibitory levels of soluble lignin. Despite the relative abundance of D-xylose in crop and forest residues it has not been found efficiently fermentable by most of the microorganisms. Recent research has revealed that D-xylose may be fermented to ethanol and organic acids. Recently, several strains of Fusarium oxysporum have been found to have potential for converting not only D-xylose, but also cellulose to ethanol in a one-step process. Distinguishing features of F. oxysporum for ethanol production in comparison to other organisms are identified. These include the advantage of in situ cellulase production and cellulose fermentation, pentose fermentation, and the tolerance of sugars and ethanol. The main disadvantage is the slow conversion rate when compared with yeast.

Isolation and characterization of the Pichia stipitis xylitol dehydrogenase gene, XYL2, and construction of a xylose-utilizing Saccharomyces cerevisiae transformant

A P. stipitis cDNA library in lambda gt11 was screened using antisera against P. stipitis xylose reductase and xylitol dehydrogenase, respectively. The resulting cDNA clones served as probes for screening a P. stipitis genomic library. The genomic XYL2 gene was isolated and the nucleotide sequence of the 1089 bp structural gene, and of adjacent non-coding regions, was determined. The XYL2 open-reading frame codes for a protein of 363 amino acids with a predicted molecular mass of 38.5 kDa. The XYL2 gene is actively expressed in S. cerevisiae transformants. S. cerevisiae cells transformed with a plasmid, pRD1, containing both the xylose reductase gene (XYL1) and the xylitol dehydrogenase gene (XYL2), were able to grow on xylose as a sole carbon source. In contrast to aerobic glucose metabolism, S. cerevisiae XYL1-XYL2 transformants utilize xylose almost entirely oxidatively.

Isolation and Characterization of Pichia heedii Mutants Defective in Xylose Uptake

To investigate the role of xylose uptake in xylose metabolism in yeasts, we isolated a series of mutated strains of the yeast Pichia heedii which are defective in xylose utilization. Four of these demonstrated defects in xylose uptake. Overlaps between the functional or regulatory mechanisms for glucose and xylose uptake may exist in this yeast since some of the mutants defective in xylose uptake were also defective in glucose transport. None of the mutants were defective in xylose reductase or xylitol dehydrogenase activities.

Xylulokinase activity in various yeasts including Saccharomyces cerevisiae containing the cloned xylulokinase gene. Scientific note

D-Xylose is a major constituent of hemicellulose, which makes up 20-30% of renewable biomass in nature. D-Xylose can be fermented by most yeasts, including Saccharomyces cerevisiae, by a two-stage process. In this process, xylose is first converted to xylulose in vitro by the enzyme xylose (glucose) isomerase, and the latter sugar is then fermented by yeast to ethanol. With the availability of an inexpensive source of xylose isomerase produced by recombinant E. coli, this process of fermenting xylose to ethanol can become quite effective. In this paper, we report that yeast xylose and xylulose fermentation can be further improved by cloning and overexpression of the xylulokinase gene. For instance, the level of xylulokinase activity in S. cerevisiae can be increased 230fold by cloning its xylulokinase gene on a high copy-number plasmid, coupled with fusion of the gene with an effective promoter. The resulting genetically-engineered yeast can ferment xylose and xylulose more than twice as fast as the parent yeast.

Conversion of pentoses to ethanol by yeasts and fungi

Fermentation of D-xylose is of interest in enhancing the yield of ethanol obtainable from lignocellulosic hydrolysates. Such hydrolysates can contain both pentoses and hexoses, and while technology to convert hexoses to ethanol is well established, the fermentation of pentoses had been problematical. To overcome the difficulty, yeasts and fungi have been sought and identified in recent years that can convert D-xylose into ethanol. However, operation of their cultures in the presence of the pentose to obtain rapid and efficient ethanol production is somewhat more complex than in the archetype alcoholic fermentation, Saccharomyces cerevisiae on D-glucose. The complexity stems, in part, from the association of ethanol accumulation in cultures where D-xylose is the sole carbon source with conditions that limit growth, by oxygen in particular, although limitation by other nutrients might also be implicated. Aspects of screening for appropriate organisms and of the parameters that play a role in determining culture variables, especially those associated with ethanol productivity, are reviewed. Performance with D-xylose as sole carbon source, in sugar mixtures, and in lignocellulosic hydrolysates is discussed. A model that involves biochemical considerations of D-xylose metabolism is presented that rationalizes the effects of oxygen on cultures where D-xylose is the sole carbon source, notably effects of the specific rate of oxygen use on the rate and extent of ethanol accumulation. Alternate methods to direct fermentation of D-xylose have been developed that depend on its prior isomerization to D-xylose, followed by fermentation of the pentulose by certain yeasts and fungi. Factors involved in the biochemistry, use, and performance of these methods, which with some organisms involves sensitivity to oxygen, are reviewed.

Growth and fermentation of an anaerobic rumen fungus on various carbon sources and effect of temperature on development

An anaerobic fungus (strain R1) resembling Neocallimastix spp. was isolated from sheep rumen. When grown on defined medium, the isolate utilized a wide range of polysaccharides and disaccharides, but of the eight monosaccharides tested only fructose, glucose, and xylose supported growth. The organism had doubling times of 5.56 h on glucose and 6.67 h on xylose, and in each case fermentation resulted in production of formate, acetate, lactate, and ethanol. During active growth, formate was a reliable indicator of fungal biomass. Growth on a medium containing glucose and xylose resulted in a doubling time of 8.70 h, but diauxic growth did not occur since both sugars were utilized simultaneously. The optimum temperature for zoospore and immature plant development was 39 degrees C, and no development occurred below 33 degrees C or above 41 degrees C.

Affinity Purifications of Aldose Reductase and Xylitol Dehydrogenase from the Xylose-Fermenting Yeast Pachysolen tannophilus

Although xylose is a major product of hydrolysis of lignocellulosic materials, few yeasts are able to convert it to ethanol. In Pachysolen tannophilus , one of the few xylose-fermenting yeasts found, aldose reductase and xylitol dehydrogenase were found to be key enzymes in the metabolic pathway for xylose fermentation. This paper presents a method for the rapid and simultaneous purification of both aldose reductase and xylitol dehydrogenase from P. tannophilus. Preliminary studies indicate that this method may be easily adapted to purify similar enzymes from other xylose-fermenting yeasts.

Mechanism of inactivation of UDP-glucose 4-epimerase from Saccharomyces cerevisiae by D-xylose and L-arabinose

In a previous paper (Cármenes et al., 1984) we reported that UDP-glucose 4-epimerase from Saccharomyces was inactivated both in vivo and in vitro (crude extracts) by L-arabinose or D-xylose. In this paper, we report that pure epimerase requires the presence of UMP or UDP to be inactivated by sugars and that the inactivation is due to the reduction of the epimerase NAD+, which is essential for epimerase activity. The inactivation rate is directly proportional to epimerase and sugar concentrations and hyperbolically proportional to UMP concentration. In situ experiments made with permeabilized cells showed that epimerase is inactivated in the same way when it is inside the cell. In vivo studies showed that epimerase is inactivated to a smaller extent when 1% D-galactose is present in the culture medium than when 1% ethanol is the main carbon source.