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PubMed1995

Cloning and characterization of a gene encoding a cell-bound, extracellular beta-glucosidase in the yeast Candida wickerhamii.

The ability of yeasts to ferment cellodextrins is rare. Candida wickerhamii is able to use these sugars for alcohol production because of a cell-bound, extracellular, beta-glucosidase that is unusual by not being inhibited by glucose. A cDNA expression library in lambda phage was prepared with mRNA isolated from cellobiose-grown C. wickerhamii. Immunological screening of the library with polyclonal antibodies against purified C. wickerhamii cell-bound, extracellular beta-glucosidase yielded 12 positive clones. Restriction endonuclease analysis and sequence data revealed that the clones could be divided into two groups, bglA and bglB, which were shown to be genetically distinct by Southern hybridization analyses. Efforts were directed at the study of bglB since it appeared to code for the cell-bound beta-glucosidase. Sequence data from both cDNA and genomic clones showed the absence of introns in bglB. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting of cell lysates from Escherichia coli bglB clones confirmed the presence of an expressed protein with an apparent molecular mass of 72 kDa, which is consistent with that expected for an unglycosylated form of the enzyme. Amino acid comparisons of BglB with other beta-glucosidase sequences suggest that it is a member of family 1 glycosyl hydrolases but is unusual in that it contains an additional 100 to 130 amino acids at the N terminus. This sequence did not have homologies to other known protein sequences and may impart unique properties to this beta-glucosidase.

Amino Acid Sequence↗
PubMed1997

Genes encoding two different beta-glucosidases of Thermoanaerobacter brockii are clustered in a common operon.

A 5.9-kb fragment of chromosomal DNA coding for beta-glucosidase activity of the thermophilic anaerobe Thermoanaerobacter brockii was sequenced. Two genes, cglT and xglS, encoding a cellodextrin-cleaving beta-glucosidase and a xylodextrin-degrading xylo-beta-glucosidase, respectively, were located directly adjacent to each other. The 5' region contained two additional genes, cglF and cglG, whose products exhibited similarity to integral membrane proteins of metabolite transport systems. The two beta-glucosidases, CglT and XglS, with deduced molecular masses of 52 and 81 kDa, belong to different families of glycosyl hydrolases. Both enzymes were overexpressed in Escherichia coli and could be detected after protein gel electrophoresis and activity staining. The enzyme CglT was purified by fast protein liquid chromatography and identified by N-terminal sequencing. The enzyme was thermostable at 60 degrees C for at least 24 h, and the temperature optimum was 75 degrees C. The ki for glucose inhibition was calculated to 200 mM. The enzyme released glucose from the nonreducing end of beta-1,4-cello oligomers as well as from various disaccharides. CglT was active on glucosides, galactosides and on fucosides, while XglS cleaved beta-glucosides and beta-xylosides as well. The cglT gene was also expressed in Bacillus subtilis, and the enzyme was mainly intracellular during exponential growth but was efficiently released into the supernatant after cultures entered the stationary phase.

Amino Acid Sequence↗
PubMed1997

Competition for cellulose among three predominant ruminal cellulolytic bacteria under substrate-excess and substrate-limited conditions.

Three predominant ruminal cellulolytic bacteria (Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD-1, and Ruminococcus albus 7) were grown in different binary combinations to determine the outcome of competition in either cellulose-excess batch culture or in cellulose-limited continuous culture. Relative populations of each species were estimated by using signature membrane-associated fatty acids and/or 16S rRNA-targeted oligonucleotide probes. Both F. succinogenes and R. flavefaciens coexisted in cellulose-excess batch culture with similar population sizes (58 and 42%, respectively; standard error, 12%). By contrast, under cellulose limitation R. flavefaciens predominated (> 96% of total cell mass) in coculture with F. succinogenes, regardless of whether the two strains were inoculated simultaneously or whether R. flavefaciens was inoculated into an established culture of F. succinogenes. The predominance of R. flavefaciens over F. succinogenes under cellulose limitation is in accord with the former's more rapid adherence to cellulose and its higher affinity for cellodextrin products of cellulose hydrolysis. In batch cocultures of F. succinogenes and R. albus, the populations of the two species were similar. However, under cellulose limitation, F. succinogenes was the predominant strain (approximately 80% of cell mass) in cultures simultaneously coinoculated with R. albus. The results from batch cocultures of R. flavefaciens and R. albus were not consistent within or among trials: some experiments yielded monocultures of R. albus (suggesting production of an inhibitory agent by R. albus), while others contained substantial populations of both species. Under cellulose limitation, R. flavefaciens predominated over R. albus (85 and 15%, respectively), as would be expected by the former's greater adherence to cellulose. The retention of R. albus in the cellulose-limited coculture may result from a combination of its ability to utilize glucose (which is not utilizable by R. flavefaciens), its demonstrated ability to adapt under selective pressure in the chemostat to utilization of lower concentrations of cellobiose, a major product of cellulose hydrolysis, and its possible production of an inhibitory agent.

Animals↗
PubMed2001

Kinetics and metabolism of cellulose degradation at high substrate concentrations in steady-state continuous cultures of Clostridium cellulolyticum on a chemically defined medium.

The hydrolysis and fermentation of insoluble cellulose were investigated using continuous cultures of Clostridium cellulolyticum with increasing amounts of carbon substrate. At a dilution rate (D) of 0.048 h(-1), biomass formation increased proportionately to the cellulose concentration provided by the feed reservoir, but at and above 7.6 g of cellulose x liter(-1) the cell density at steady state leveled off. The percentage of cellulose degradation declined from 32.3 to 8.3 with 1.9 and 27.0 g of cellulose x liter(-1), respectively, while cellodextrin accumulation rose and represented up to 4.0% of the original carbon consumed. The shift from cellulose-limited to cellulose-sufficient conditions was accompanied by an increase of both the acetate/ethanol ratio and lactate biosynthesis. A kinetics study of C. cellulolyticum metabolism in cellulose saturation was performed by varying D with 18.1 g of cellulose x liter(-1). Compared to cellulose limitation (M. Desvaux, E. Guedon, and H. Petitdemange, J. Bacteriol. 183:119-130, 2001), in cellulose-sufficient continuous culture (i) the ATP/ADP, NADH/NAD+, and q(NADH produced)/q(NADH used) ratios were higher and were related to a more active catabolism, (ii) the acetate/ethanol ratio increased while the lactate production decreased as D rose, and (iii) the maximum growth yield (Y(max)X/S) (40.6 g of biomass per mol of hexose equivalent) and the maximum energetic yield (Y(max)ATP) (19.4 g of biomass per mol of ATP) were lowered. C. cellulolyticum was then able to regulate and optimize carbon metabolism under cellulose-saturated conditions. However, the facts that some catabolized hexose and hence ATP were no longer associated with biomass production with a cellulose excess and that concomitantly lactate production and pyruvate leakage rose suggest the accumulation of an intracellular inhibitory compound(s), which could further explain the establishment of steady-state continuous cultures under conditions of excesses of all nutrients. The following differences were found between growth on cellulose in this study and growth under cellobiose-sufficient conditions (E. Guedon, S. Payot, M. Desvaux, and H. Petitdemange, Biotechnol. Bioeng. 67:327-335, 2000): (i) while with cellobiose, a carbon flow into the cell of as high as 5.14 mmol of hexose equivalent g of cells(-1) x h(-1) could be reached, the maximum entering carbon flow obtained here on cellulose was 2.91 mmol of hexose equivalent g of cells(-1) x h(-1); (ii) while the NADH/NAD+ ratio could reach 1.51 on cellobiose, it was always lower than 1 on cellulose; and (iii) while a high proportion of cellobiose was directed towards exopolysaccharide, extracellular protein, and free amino acid excretions, these overflows were more limited under cellulose-excess conditions. Such differences were related to the carbon consumption rate, which was higher on cellobiose than on cellulose.

Biodegradation, Environmental↗
PubMed1980

Intermediatry steps in Acetobacter xylinum cellulose synthesis: studies with whole cells and cell-free preparations of the wild type and a celluloseless mutant.

Intermediatry steps in cellulose synthesis in Acetobacter xylinum were studied with resting cells and particulate-membranous preparations of the wild-type strain and of a celluloseless mutant. Exogenously supplied [1-14C]glucose was rapidly converted by resting cells of both types into glucose 6-phosphate, glucose 1-phosphate, and uridine glucose 5'-diphosphate (UDP)-glucose and incorporated into lipid-, water-, and alkali-soluble cellular fractions. The decrease in the level of labeled hexose-phosphates and UDP-glucose upon depletion of the exogenous substrate was accounted for by a continuous incorporation of [14C]glucose into cellulose in the wild type and into the above-mentioned cellular components in the mutant. [14C]glucose retained in the alkali- and water-soluble fractions of pulse-labeled wild-type cells was quantitatively chased into cellulose. Sonic extracts of both strains catalyzed the transfer of glucose from UDP-glucose into lipid-, water-, and alkali-soluble materials, as well as into an alkali-insoluble cellulosic beta-1,4-glucan. The results strongly support the sequence glucose leads to glucose 6-phosphate leads to glucose 1-phosphate leads to UDP-glucose leads to cellulose and indicate that lipid- and protein-linked cellodextrins may function as intermediates between UDP-glucose and cellulose in A. xylinum.

Cell-Free System↗
PubMed1990

Characterization of the extracellular cellulase from a mesophilic clostridium (strain C7).

An extracellular, 700,000-Mr multiprotein complex that catalyzed the hydrolysis of crystalline cellulose (Avicel) was isolated from cultures of Clostridium sp. strain C7, a mesophile from freshwater sediment. In addition to cellulose (Avicel, ball-milled filter paper), the multiprotein complex hydrolyzed carboxymethylcellulose, cellodextrins, xylan, and xylooligosaccharides. Hydrolysis of cellulose or cellotetraose by the complex yielded cellobiose as the main product. Cellopentaose or cellohexaose was hydrolyzed by the complex to cellotriose or cellotetraose, respectively, in addition to cellobiose. Xylobiose was the main product of xylan hydrolysis, and xylobiose and xylotriose were the major products of xylooligosaccharide hydrolysis. Activity (Avicelase) resulting in hydrolysis of crystalline cellulose required Ca2+ and a reducing agent. The multiprotein complex had temperature optima for Avicelase, carboxymethylcellulase, and xylanase activities at 45, 55, and 55 degrees C, respectively, and pH optima at 5.6 to 5.8, 5.5, and 6.55, respectively. Electron microscopy of the 700,000-Mr enzyme complex revealed particles relatively uniform in size (12 to 15 nm wide) and apparently composed of subunit structures. Elution of strain C7 concentrated culture fluid from Sephacryl S-300 columns yielded an A280 peak in the 130,000-Mr region. Pooled fractions from the 130,000-Mr peak had carboxymethylcellulase activity but lacked Avicelase activity. Except for the inability to hydrolyze cellulose, the 130,000-Mr preparation had a substrate specificity identical to that of the 700,000-Mr protein complex. A comparison by immunoblotting techniques of proteins in the 130,000- and 700,000-Mr preparations, indicated that the two enzyme preparations had cross-reacting antigenic determinants.

Bacterial Proteins↗
PubMed1991

Isolation and properties of a major cellobiohydrolase from the cellulosome of Clostridium thermocellum.

In the anaerobic, thermophilic, cellulolytic bacterium Clostridium thermocellum, efficient solubilization of the insoluble cellulose substrate is accomplished largely through the action of a cellulose-binding multienzyme complex, the cellulosome. A major cellobiohydrolase activity from the cellulosome has been traced to its Mr 75,000 S8 subunit, and an active fragment of this subunit was prepared by a novel procedure involving limited proteolytic cleavage. The truncated Mr 68,000 fragment, termed S8-tr, was purified by gel filtration and high-performance ion-exchange chromatography. The purified protein adsorbed weakly to amorphous cellulose, and its enzymatic action yielded cellobiose as the major end product from both amorphous and crystalline cellulose preparations. The high ratio of exo- to endo-beta-glucanase activities was supported by viscosimetric measurements. The use of model substrates showed that the smallest cellodextrin to be degraded was cellotetraose, but cellopentaose was degraded at a much greater rate. Cellobiose dramatically inhibited the cellulolytic activities. In the absence of calcium or other bivalent metal ions, both the truncated cellobiohydrolase activity of S8-tr and the true cellulase activity of the parent cellulosome were relatively unstable at temperatures above 50 degrees C. Cysteine further enhanced the stabilizing effect of calcium. This is the first report of a defined cellobiohydrolase in C. thermocellum. Its association with the cellulosome and the correspondence of several of their major distinctive properties suggest that this cellobiohydrolase plays a key role in the solubilization of cellulose by the intact cellulosomal complex.

Animals↗
PubMed1996

Characterization of CenC, an enzyme from Cellulomonas fimi with both endo- and exoglucanase activities.

The cenC gene, encoding beta-1,4-glucanase C (CenC) from Cellulomonas fimi, was overexpressed in Escherichia coli with a tac-based expression vector. The resulting polypeptide, with an apparent molecular mass of 130 kDa, was purified from the cell extracts by affinity chromatography on cellulose followed by anion-exchange chromatography. N-terminal sequence analysis showed the enzyme to be properly processed. Mature CenC was optimally active at pH 5.0 and 45 degrees C. The enzyme was extremely active on soluble, fluorophoric, and chromophoric glycosides (4-methylumbelliferyl beta-glycosides, 2'-chloro-4'-nitrophenyl-beta-D-cellobioside, and 2'-chloro-4'-nitrophenyl-lactoside) and efficiently hydrolyzed carboxymethyl cellulose, barley beta-glucan, lichenan, and, to a lesser extent, glucomannan. CenC also hydrolyzed acid-swollen cellulose, Avicel, and bacterial microcrystalline cellulose. However, degradation of the latter was slow compared with its degradation by CenB, another C. fimi cellulose belonging to the same enzyme family. CenC acted with inversion of configuration at the anomeric carbon, in accordance with its classification as a family 9 member. The enzyme released mainly cellobiose from soluble cellodextrins and insoluble cellulose. Attack appeared to be from the reducing chain ends. Analysis of carboxymethyl cellulose hydrolysis suggests that CenC is semiprocessive enzyme with both endo- and exoglucanase activities.

Base Sequence↗
PubMed1997

The processive endocellulase CelF, a major component of the Clostridium cellulolyticum cellulosome: purification and characterization of the recombinant form.

The recombinant form of the cellulase CelF of Clostridium cellulolyticum, tagged by a C-terminal histine tail, was overproduced in Escherichia coli. The fusion protein was purified by affinity chromatography on a Ni-nitrilotriacetic acid column. The intact form of CelF (Mr, 79,000) was rapidly degraded at the C terminus, giving a shorter stable form, called truncated CelF (Mr, 71,000). Both the entire and the truncated purified forms degraded amorphous cellulose (kcat = 42 and 30 min(-1), respectively) and microcrystalline cellulose (kcat = 13 and 10 min(-1), respectively). The high ratio of soluble reducing ends to insoluble reducing ends released by truncated CelF from amorphous cellulose showed that CelF is a processive enzyme. Nevertheless, the diversity of the cellodextrins released by truncated CelF from phosphoric acid-swollen cellulose at the beginning of the reaction indicated that the enzyme might randomly hydrolyze beta-1,4 bonds. This hypothesis was supported by viscosimetric measurements and by the finding that CelF and the endoglucanase CelA are able to degrade some of the same cellulose sites. CelF was therefore called a processive endocellulase. The results of immunoblotting analysis showed that CelF was associated with the cellulosome of C. cellulolyticum. It was identified as one of the three major components of cellulosomes. The ability of the entire form of CelF to interact with CipC, the cellulosome integrating protein, or mini-CipC1, a recombinant truncated form of CipC, was monitored by interaction Western blotting (immunoblotting) and by binding assays using a BIAcore biosensor-based analytical system.

Bacterial Proteins↗
PubMed1999

An endoglucanase, EglA, from the hyperthermophilic archaeon Pyrococcus furiosus hydrolyzes beta-1,4 bonds in mixed-linkage (1-->3),(1-->4)-beta-D-glucans and cellulose.

The eglA gene, encoding a thermostable endoglucanase from the hyperthermophilic archaeon Pyrococcus furiosus, was cloned and expressed in Escherichia coli. The nucleotide sequence of the gene predicts a 319-amino-acid protein with a calculated molecular mass of 35.9 kDa. The endoglucanase has a 19-amino-acid signal peptide but not cellulose-binding domain. The P. furiosus endoglucanase has significant amino acid sequence similarities, including the conserved catalytic nucleophile and proton donor, with endoglucanases from glucosyl hydrolase family 12. The purified recombinant enzyme hydrolyzed beta-1,4 but not beta-1,3 glucosidic linkages and had the highest specific activity on cellopentaose (degree of polymerization [DP] = 5) and cellohexaose (DP = 6) oligosaccharides. To a lesser extent, EglA also hydrolyzed shorter cellodextrins (DP < 5) as well as the amorphous portions of polysaccharides which contain only beta-1,4 bonds such as carboxymethyl cellulose, microcrystalline cellulose, Whatman paper, and cotton linter. The highest specific activity toward polysaccharides occurred with mixed-linkage beta-glucans such as barley beta-glucan and lichenan. Kinetics studies with cellooliogsaccharides and p-nitrophenyl-cellooligosaccharides indicated that the enzyme had three glucose binding subsites (-I, -II, and -III) for the nonreducing end and two glucose binding subsites (+I and +II) for the reducing end from the scissile glycosidic linkage. The enzyme had temperature and pH optima of 100 degreesC and 6.0, respectively; a half-life of 40 h at 95 degreesC; and a denaturing temperature of 112 degreesC as determined by differential scanning calorimetry. The discovery of a thermostable enzyme with this substrate specificity has implications for both the evolution of enzymes involved in polysaccharide hydrolysis and the occurrence of growth substrates in hydrothermal vent environments.

Amino Acid Sequence↗
PubMed2001

Carbon flux distribution and kinetics of cellulose fermentation in steady-state continuous cultures of Clostridium cellulolyticum on a chemically defined medium.

The metabolic characteristics of Clostridium cellulolyticum, a mesophilic cellulolytic nonruminal bacterium, were investigated and characterized kinetically for the fermentation of cellulose by using chemostat culture analysis. Since with C. cellulolyticum (i) the ATP/ADP ratio is lower than 1, (ii) the production of lactate at low specific growth rate (mu) is low, and (iii) there is a decrease of the NADH/NAD(+) ratio and q(NADH produced)/ q(NADH used) ratio as the dilution rate (D) increases in carbon-limited conditions, the chemostats used were cellulose-limited continuously fed cultures. Under all conditions, ethanol and acetate were the main end products of catabolism. There was no shift from an acetate-ethanol fermentation to a lactate-ethanol fermentation as previously observed on cellobiose as mu increased (E. Guedon, S. Payot, M. Desvaux, and H. Petitdemange, J. Bacteriol. 181:3262-3269, 1999). The acetate/ethanol ratio was always higher than 1 but decreased with D. On cellulose, glucose 6-phosphate and glucose 1-phosphate are important branch points since the longer the soluble beta-glucan uptake is, the more glucose 1-phosphate will be generated. The proportion of carbon flowing toward phosphoglucomutase remained constant (around 59.0%), while the carbon surplus was dissipated through exopolysaccharide and glycogen synthesis. The percentage of carbon metabolized via pyruvate-ferredoxin oxidoreductase decreased with D. Acetyl coenzyme A was mainly directed toward the acetate formation pathway, which represented a minimum of 27.1% of the carbon substrate. Yet the proportion of carbon directed through biosynthesis (i.e., biomass, extracellular proteins, and free amino acids) and ethanol increased with D, reaching 27.3 and 16.8%, respectively, at 0.083 h(-1). Lactate and extracellular pyruvate remained low, representing up to 1.5 and 0.2%, respectively, of the original carbon uptake. The true growth yield obtained on cellulose was higher, [50.5 g of cells (mol of hexose eq)(-1)] than on cellobiose, a soluble cellodextrin [36.2 g of cells (mol of hexose eq)(-1)]. The rate of cellulose utilization depended on the solid retention time and was first order, with a rate constant of 0.05 h(-1). Compared to cellobiose, substrate hydrolysis by cellulosome when bacteria are grown on cellulose fibers introduces an extra means for regulation of the entering carbon flow. This led to a lower mu, and so metabolism was not as distorted as previously observed with a soluble substrate. From these results, C. cellulolyticum appeared well adapted and even restricted to a cellulolytic lifestyle.

Adenosine Triphosphate↗
PubMed1996

Production of beta-glucosidase and diauxic usage of sugar mixtures by Candida molischiana.

The fermentation of cellobiose is a rare trait among yeasts. Of the 308 yeast species that utilize cellobiose aerobically, only 12 species ferment it, and only 2 species, Candida molischiana and Candida wickerhamii, also ferment cellodextrins. Candida molischiana produced beta-glucosidase activity on all carbon sources tested, except glucose, mannose, and fructose. When these sugars were added to cultures growing on cellobiose, the synthesis of beta-glucosidase ceased. However, the total amount of enzyme activity remained constant, indicating that the C. molischiana beta-glucosidase is catabolite repressed and not catabolite inactivated. When grown in medium initially containing glucose plus xylose, cellobiose, maltose, mannitol, or glucitol, C. molischiana preferentially utilized glucose and produced little beta-glucosidase activity until glucose was nearly depleted from the medium. When grown in medium containing cellobiose plus either fructose or mannose, the yeast preferentially utilized the monosaccharides and produced little beta-glucosidase activity. Candida molischiana produced beta-glucosidase and co-utilized cellobiose and xylose, maltose, or trehalose. Glucose and fructose, mannose, or trehalose were co-utilized; however, no beta-glucosidase activity was detected. Thus, the order of substrate preference groups appeared to be (glucose, trehalose, fructose, mannose) > (cellobiose, maltose, xylose) > (mannitol, glucitol).

Candida↗
PubMed1996

Simultaneous but differential metabolism of glucose and cellobiose in Fibrobacter succinogenes cells, studied by in vivo 13C-NMR.

Kinetics of [1-13C]glucose utilization were monitored by in vivo NMR spectroscopy on resting cells of Fibrobacter succinogenes, in the presence of 32 mM [1-13C]glucose, 32 mM [1-13C]glucose and 64 mM unlabelled glucose, or 32 mM [1-13C]glucose and 32 mM unlabelled cellobiose. A similar production of acetate and succinate and a similar storage of glycogen were observed whatever the exogenous substrate. The presence of cellobiose or that of an equivalent amount of glucose did not reduce [1-13C]glucose incorporation to the same extent. Glucose seemed preferentially used for glycogen storage and energy production, while part of the cellobiose appeared to be used for cellodextrin synthesis. Both cellobiase and cellobiose phosphorylase activities were assayed in cell-free extracts. Finally, the intracellular concentration of glucose 6-phosphate was increased by over threefold when cells metabolized cellobiose (alone or in parallel to glucose) as compared with the metabolism of glucose alone.

Acetates↗
PubMed1996

Why don't ruminal bacteria digest cellulose faster?

The bacteria Fibrobacter succinogenes, Ruminococcus flavefaciens, and Ruminococcus albus generally are regarded as the predominant cellulolytic microbes in the rumen. Comparison of available data from the literature reveals that these bacteria are the most actively cellulolytic of all mesophilic organisms described to date from any habitat. In light of numerous proposals to improve microbial cellulose digestion in ruminants, it is instructive to examine the characteristics of these species that contribute to their superior cellulolytic capabilities and to identify the factors that prevent them from digesting cellulose even more rapidly. As a group, these species have extreme nutritional specialization. They are able to utilize cellulose (or in some cases xylan) and its hydrolytic products as their nearly sole energy sources for growth. Moreover, each species apparently has evolved to similar maximum rates of cellulose digestion (first-order rate constants of 0.05 to 0.08 h-1). Active cellulose digestion involves adherence of cells to the fibers via a glycoprotein glycocalyx, which protects cells from protozoal grazing and cellulolytic enzymes from degradation by ruminal proteases while it retains-at least temporarily-the cellodextrin products for use by the cellulolytic bacteria. These properties result in different ecological roles for the adherent and nonadherent populations of each species, but overall provide an enormous selective advantage to these cellulolytic bacteria in the ruminal environment. However, major constraints to cellulose digestion are caused by cell-wall structure of the plant (matrix interactions among wall biopolymers and low substrate surface area) and by limited penetration of the nonmotile cellulolytic microbes into the cell lumen. Because of these constraints and the highly adapted nature of cellulose digestion by the predominant cellulolytic bacteria in the rumen, transfer of cellulolytic capabilities to noncellulolytic ruminal bacteria (e.g., by genetic engineering) that display other desirable properties offers limited opportunities to improve ruminal digestion of cellulose.

Animals↗
PubMed1998

Use of hemicellulose hydrolysate for beta-glucosidase fermentation.

Hydrolysis of cellulose by Trichoderma cellulases often results in a mixture of glucose, cellobiose, and low-mol-wt cellodextrins. Cellobiose is nonfermentable for most yeasts, and therefore it has to be hydrolyzed to glucose by beta-glucosidase prior to ethanol fermentation. In the present study, the beta-glucosidase production of one Penicillium and three Aspergillus strains, which were previously selected out of 24 strains, was investigated on steam pretreated willow. Both steam-pretreated willow and hemicellulose hydrolysate, released during steam explosion of willow, were used as carbon sources. Reference cultivation runs were performed using prehydrolyzed Solka Floc and glucose. The four strains were compared with Trichoderma reesei regarding sugar consumption and beta-glucosidase production. Aspergillus niger and Aspergillus phoenicis proved to be the best enzyme producers on hemicellulose hydrolysate. The maximum beta-glucosidase activity, 4.60 IU/mL, was obtained when A. phoenicis was cultivated on the mixture of hemicellulose hydrolysate and steam-pretreated willow. The maximum yield of enzyme activity, 502 IU/g total carbohydrate, was obtained when Aspergillus foetidus was cultivated on the hemicellulose hydrolysate.

Aspergillus↗