Nucleotide sequences of Saccharomycopsis fibuligera genes for extracellular beta-glucosidases as expressed in Saccharomyces cerevisiae

Neither 1G nor 2G fuel ethanol: setting the ground for a sugarcane-based biorefinery using an iSUCCELL yeast platform

First-generation (1G) fuel ethanol production in sugarcane-based biorefineries is an established economic enterprise in Brazil. Second-generation (2G) fuel ethanol from lignocellulosic materials, though extensively investigated, is currently facing severe difficulties to become economically viable. Some of the challenges inherent to these processes could be resolved by efficiently separating and partially hydrolysing the cellulosic fraction of the lignocellulosic materials into the disaccharide cellobiose. Here, we propose an alternative biorefinery, where the sucrose-rich stream from the 1G process is mixed with a cellobiose-rich stream in the fermentation step. The advantages of mixing are 3-fold: (i) decreased concentrations of metabolic inhibitors that are typically produced during pretreatment and hydrolysis of lignocellulosic materials; (ii) decreased cooling times after enzymatic hydrolysis prior to fermentation; and (iii) decreased availability of free glucose for contaminating microorganisms and undesired glucose repression effects. The iSUCCELL platform will be built upon the robust Saccharomyces cerevisiae strains currently present in 1G biorefineries, which offer competitive advantage in non-aseptic environments, and into which intracellular hydrolyses of sucrose and cellobiose will be engineered. It is expected that high yields of ethanol can be achieved in a process with cell recycling, lower contamination levels and decreased antibiotic use, when compared to current 2G technologies.

Cellulases from Thermophiles Found by Metagenomics

Cellulases are a heterogeneous group of enzymes that synergistically catalyze the hydrolysis of cellulose, the major component of plant biomass. Such reaction has biotechnological applications in a broad spectrum of industries, where they can provide a more sustainable model of production. As a prerequisite for their implementation, these enzymes need to be able to operate in the conditions the industrial process requires. Thus, cellulases retrieved from extremophiles, and more specifically those of thermophiles, are likely to be more appropriate for industrial needs in which high temperatures are involved. Metagenomics, the study of genes and gene products from the whole community genomic DNA present in an environmental sample, is a powerful tool for bioprospecting in search of novel enzymes. In this review, we describe the cellulolytic systems, we summarize their biotechnological applications, and we discuss the strategies adopted in the field of metagenomics for the discovery of new cellulases, focusing on those of thermophilic microorganisms.

Extra metabolic burden by displaying over secreting: Growth, fermentation and enzymatic activity in cellobiose of recombinant yeast expressing β-glucosidase

β-Glucosidase was selected to be a reporter to study metabolic burden imposed by its expression in yeast. Cell growth, fermentation yield and enzymatic activity were used as indicators of the metabolic burden borne by 14 recombinant yeast strains. Various factors were found to affect metabolic burden, including BGLI gene source, gene dose, trafficking of the enzyme (either cell-surface display or secretion), and oxygen supply. While BGLI gene from Aspergillus aculeatus provided better performance for the host cells than that from Saccharomycopsis fibuligera, displaying β-glucosidase on the cell surface generally led to lower μ_m, total activity and ethanol titer, and longer lag period, lower (aerobic condition) or higher (anaerobic condition) biomass yield than that of secreting β-glucosidase. The negative effect on growth increased with gene dose level until a final failure to grow. This growth difference implies that displaying β-glucosidase on the cell surface imposes an extra metabolic burden. The molecular basis and mechanisms for this phenomenon need to further be investigated in order to develop better strategies for utilizing displayed and secreted enzymes in biotechnology and yeast breeding.

Directed evolution of a fungal β-glucosidase in Saccharomyces cerevisiae

BACKGROUND

β-glucosidases (BGLs) catalyze the hydrolysis of soluble cellodextrins to glucose and are a critical component of cellulase systems. In order to engineer Saccharomyces cerevisiae for the production of ethanol from cellulosic biomass, a BGL tailored to industrial bioconversions is needed.

RESULTS

We applied a directed evolution strategy to a glycosyl hydrolase family 3 (GH3) BGL from Aspergillus niger (BGL1) by expressing a library of mutated bgl1 genes in S. cerevisiae and used a two-step functional screen to identify improved enzymes. Twelve BGL variants that supported growth of S. cerevisiae on cellobiose and showed increased activity on the synthetic substrate p-nitrophenyl-β-D-glucopyranoside were identified and characterized. By performing kinetic experiments, we found that a Tyr → Cys substitution at position 305 of BGL1 dramatically reduced transglycosidation activity that causes inhibition of the hydrolytic reaction at high substrate concentrations. Targeted mutagenesis demonstrated that the position 305 residue is critical in GH3 BGLs and likely determines the extent to which transglycosidation reactions occur. We also found that a substitution at Gln(140) reduced the inhibitory effect of glucose and could be combined with the Y305C substitution to produce a BGL with decreased sensitivity to both the product and substrate. Using the crystal structure of a GH3 BGL from A. aculeatus, we mapped a group of beneficial mutations to the β/α domain of the molecule and postulate that this region modulates activity through subunit interactions. Six BGL variants were identified with substitutions in the MFα pre-sequence that was used to mediate secretion of the protein. Substitutions at Pro(21) or Val(22) of the MFα pre-sequence could produce up to a twofold increase in supernatant hydrolase activity and provides evidence that expression and/or secretion was an additional factor limiting hydrolytic activity.

CONCLUSIONS

Using directed evolution on BGL1, we identified a key residue that controls hydrolytic and transglycosidation reactions in GH3 BGLs. We also found that several beneficial mutations could be combined and increased the hydrolytic activity for both synthetic and natural substrates.

Role of the non-conserved amino acid asparagine 285 in the glycone-binding pocket of Neosartorya fischeri β-glucosidase

Neosartorya fischeriβ-glucosidase (NfBGL595) is distinguished from other BGLs by its high turnover forp-nitrophenyl β-d-glucopyranoside (pNPG) and flavones.

Development of cellobiose-degrading ability in Yarrowia lipolytica strain by overexpression of endogenous genes

BACKGROUND

Yarrowia lipolytica, one of the most widely studied "nonconventional" oleaginous yeast species, is unable to grow on cellobiose. Engineering cellobiose-degrading ability into this yeast is a vital step towards the development of cellulolytic biocatalysts suitable for consolidated bioprocessing.

RESULTS

In the present work, we identified six genes encoding putative β-glucosidases in the Y. lipolytica genome. To study these, homologous expression was attempted in Y. lipolytica JMY1212 Zeta. Two strains overexpressing BGL1 (YALI0F16027g) and BGL2 (YALI0B14289g) produced β-glucosidase activity and were able to degrade cellobiose, while the other four did not display any detectable activity. The two active β-glucosidases, one of which was mainly cell-associated while the other was present in the extracellular medium, were purified and characterized. The two Bgls were most active at 40-45°C and pH 4.0-4.5, and exhibited hydrolytic activity on various β-glycoside substrates. Specifically, Bgl1 displayed 12.5-fold higher catalytic efficiency on cellobiose than Bgl2. Significantly, in experiments where cellobiose or cellulose (performed in the presence of a β-glucosidase-deficient commercial cellulase cocktail produced by Trichoderma reseei) was used as carbon source for aerobic cultivation, Y. lipolytica ∆pox co-expressing BGL1 and BGL2 grew better than the Y. lipolytica strains expressing single BGLs. The specific growth rate and biomass yield of Y. lipolytica JMY1212 co-expressing BGL1 and BGL2 were 0.15 h(-1) and 0.50 g-DCW/g-cellobiose, respectively, similar to that of the control grown on glucose.

CONCLUSIONS

We conclude that the bi-functional Y. lipolytica developed in the current study represents a vital step towards the creation of a cellulolytic yeast strain that can be used for lipid production from lignocellulosic biomass. When used in combination with commercial cellulolytic cocktails, this strain will no doubt reduce enzyme requirements and thus costs.

Conversion of biomass-derived oligosaccharides into lipids

BACKGROUND

Oligocelluloses and oligoxyloses are partially hydrolyzed products from lignocellulosic biomass hydrolysis. Biomass hydrolysates usually contain monosaccharides as well as various amounts of oligosaccharides. To utilize biomass hydrolysates more efficiently, it is important to identify microorganisms capable of converting biomass-derived oligosaccharides into biofuels or biochemicals.

RESULTS

We have demonstrated that the oleaginous yeast Cryptococcus curvatus can utilize either oligocelluloses or oligoxyloses as sole carbon sources for microbial lipid production. When oligocelluloses were used, lipid content and lipid coefficient were 35.9% and 0.20 g/g consumed sugar, respectively. When oligoxyloses were used, lipid coefficient was 0.17 g/g consumed sugar. Ion chromatography analysis showed oligocelluloses with a degree of polymerization from 2 to 9 were assimilated. Our data suggested that these oligosaccharides were transported into cells and then hydrolyzed by cytoplasmic enzymes. Further analysis indicated that these enzymes were inducible by oligocelluloses. Lipid production on cellulose by C. curvatus using the simultaneous saccharification and lipid production process in the absence of cellobiase achieved essentially identical results to that in the presence of cellobiase, suggesting that oligocelluloses generated in situ were utilized with high efficiency. This study has provided inspiring information for oligosaccharides utilization, which should facilitate biorefinery based on lignocellulosic biomass.

CONCLUSIONS

C. curvatus can directly utilize biomass-derived oligosaccharides. Oligocelluloses are transported into the cells and then hydrolyzed by cytoplasmic enzymes. A simultaneous saccharification and lipid production process can be conducted without oligocelluloses accumulation in the absence of cellobiase by C. curvatus, which could reduce the enzyme costs.

Directed evolution of a cellobiose utilization pathway in Saccharomyces cerevisiae by simultaneously engineering multiple proteins

BACKGROUND

The optimization of metabolic pathways is critical for efficient and economical production of biofuels and specialty chemicals. One such significant pathway is the cellobiose utilization pathway, identified as a promising route in biomass utilization. Here we describe the optimization of cellobiose consumption and ethanol productivity by simultaneously engineering both proteins of the pathway, the β-glucosidase (gh1-1) and the cellodextrin transporter (cdt-1), in an example of pathway engineering through directed evolution.

RESULTS

The improved pathway was assessed based on the strain specific growth rate on cellobiose, with the final mutant exhibiting a 47% increase over the wild-type pathway. Metabolite analysis of the engineered pathway identified a 49% increase in cellobiose consumption (1.78 to 2.65 g cellobiose/(L · h)) and a 64% increase in ethanol productivity (0.611 to 1.00 g ethanol/(L · h)).

CONCLUSIONS

By simultaneously engineering multiple proteins in the pathway, cellobiose utilization in S. cerevisiae was improved. This optimization can be generally applied to other metabolic pathways, provided a selection/screening method is available for the desired phenotype. The improved in vivo cellobiose utilization demonstrated here could help to decrease the in vitro enzyme load in biomass pretreatment, ultimately contributing to a reduction in the high cost of biofuel production.

Energetic benefits and rapid cellobiose fermentation by Saccharomyces cerevisiae expressing cellobiose phosphorylase and mutant cellodextrin transporters

Anaerobic bacteria assimilate cellodextrins from plant biomass by using a phosphorolytic pathway to generate glucose intermediates for growth. The yeast Saccharomyces cerevisiae can also be engineered to ferment cellobiose to ethanol using a cellodextrin transporter and a phosphorolytic pathway. However, strains with an intracellular cellobiose phosphorylase initially fermented cellobiose slowly relative to a strain employing an intracellular β-glucosidase. Fermentations by the phosphorolytic strains were greatly improved by using cellodextrin transporters with elevated rates of cellobiose transport. Furthermore under stress conditions, these phosphorolytic strains had higher biomass and ethanol yields compared to hydrolytic strains. These observations suggest that, although cellobiose phosphorolysis has energetic advantages, phosphorolytic strains are limited by the thermodynamics of cellobiose phosphorolysis (ΔG°=+3.6kJmol(-1)). A thermodynamic "push" from the reaction immediately upstream (transport) is therefore likely to be necessary to achieve high fermentation rates and energetic benefits of phosphorolysis pathways in engineered S. cerevisiae.

A highly efficient β-glucosidase from the buffalo rumen fungus Neocallimastix patriciarum W5

BACKGROUND

Cellulose, which is the most abundant renewable biomass on earth, is a potential bio-resource of alternative energy. The hydrolysis of plant polysaccharides is catalyzed by microbial cellulases, including endo-β-1,4-glucanases, cellobiohydrolases, cellodextrinases, and β-glucosidases. Converting cellobiose by β-glucosidases is the key factor for reducing cellobiose inhibition and enhancing the efficiency of cellulolytic enzymes for cellulosic ethanol production.

RESULTS

In this study, a cDNA encoding β-glucosidase was isolated from the buffalo rumen fungus Neocallimastix patriciarum W5 and is named NpaBGS. It has a length of 2,331 bp with an open reading frame coding for a protein of 776 amino acid residues, corresponding to a theoretical molecular mass of 85.1 kDa and isoelectric point of 4.4. Two GH3 catalytic domains were found at the N and C terminals of NpaBGS by sequence analysis. The cDNA was expressed in Pichia pastoris and after protein purification, the enzyme displayed a specific activity of 34.5 U/mg against cellobiose as the substrate. Enzymatic assays showed that NpaBGS was active on short cello-oligosaccharides from various substrates. A weak activity in carboxymethyl cellulose (CMC) digestion indicated that the enzyme might also have the function of an endoglucanase. The optimal activity was detected at 40°C and pH 5 ~ 6, showing that the enzyme prefers a weak acid condition. Moreover, its activity could be enhanced at 50°C by adding Mg2+ or Mn2+ ions. Interestingly, in simultaneous saccharification and fermentation (SSF) experiments using Saccharomyces cerevisiae BY4741 or Kluyveromyces marxianus KY3 as the fermentation yeast, NpaBGS showed advantages in cell growth, glucose production, and ethanol production over the commercial enzyme Novo 188. Moreover, we showed that the KY3 strain engineered with the NpaNGS gene can utilize 2 % dry napiergrass as the sole carbon source to produce 3.32 mg/ml ethanol when Celluclast 1.5 L was added to the SSF system.

CONCLUSION

Our characterizations of the novel β-glucosidase NpaBGS revealed that it has a preference of weak acidity for optimal yeast fermentation and an optimal temperature of ~40°C. Since NpaBGS performs better than Novo 188 under the living conditions of fermentation yeasts, it has the potential to be a suitable enzyme for SSF.

The metabolic burden of cellulase expression by recombinant Saccharomyces cerevisiae Y294 in aerobic batch culture

Two recombinant strains of Saccharomyces cerevisiae Y294 producing cellulase using different expression strategies were compared to a reference strain in aerobic culture to evaluate the potential metabolic burden that cellulase expression imposed on the yeast metabolism. In a chemically defined mineral medium with glucose as carbon source, S. cerevisiae strain Y294[CEL5] with plasmid-borne cellulase genes produced endoglucanase and β-glucosidase activities of 0.038 and 0.30 U mg dry cell weight(-1), respectively. Chromosomal expression of these two cellulases in strain Y294[Y118p] resulted in no detectable activity, although low levels of episomally co-expressed cellobiohydrolase (CBH) activity were detected. Whereas the biomass concentration of strain Y294[CEL5] was slightly greater than that of a reference strain, CBH expression by Y294[Y118p] resulted in a 1.4-fold lower maximum specific growth rate than that of the reference. Supplementation of the growth medium with amino acids significantly improved culture growth and enzyme production, but only partially mitigated the physiological effects and metabolic burden of cellulase expression. Glycerol production was decreased significantly, up to threefold, in amino acid-supplemented cultures, apparently due to redox balancing. Disproportionately higher levels of glycerol production by Y294[CEL5] indicated a potential correlation between the redox balance of anabolism and the physiological stress of cellulase production. With the reliance on cellulase expression in yeast for the development of consolidated bioprocesses for bioethanol production, this work demonstrates the need for development of yeasts that are physiologically robust in response to burdens imposed by heterologous enzyme production.

A new diet for yeast to improve biofuel production

In 2010, our group announced the discovery of two cellodextrin transporter families from the cellulolytic fungus, Neurospora crassa. Furthermore, we demonstrated the utility of these transporters in the production of lignocellulosic biofuels. This discovery was made possible by a decision to systematically study cell wall degradation by N. crassa. The identified transport pathway has opened up a new way of thinking about microbial fermentation of hexoses as well as pentoses derived from plant cell walls. Integrating this pathway with the endogenous metabolism and signaling networks of S. cerevisiae is now a major goal of our group.

High-level expression of barley beta-D-glucan exohydrolase HvExoI from a codon-optimized cDNA in Pichia pastoris

The native beta-d-glucan exohydrolase isoenzyme ExoI from barley seedlings, designated HvExoI, was the first GH3 glycoside hydrolase, for which a crystal structure was determined. A precise understanding of relationships between structure and function in this enzyme has been gained by structural and enzymatic studies. To allow testing of hypotheses gained from these studies, an efficient system for expression of HvExoI in Pichia pastoris was developed using a codon-optimized cDNA. Protein expression at a temperature of 20 degrees C yielded a recombinant enzyme, designated rHvExoI, which had molecular masses of 70-110 kDa due to heavy glycosylation at Asn221, Asn498 and Asn600, the three sites of N-glycosylation in native HvExoI. Most of the N-linked carbohydrate could be removed from rHvExoI, resulting in N-deglycosylated rHvExoI with a substantially decreased molecular mass of 67 kDa. rHvExoI was able to hydrolyse barley (1,3;1,4)-beta-D-glucan, laminarin and lichenans. The catalytic efficiency value k(cat)/K(M) of rHvExoI with barley (1,3;1,4)-beta-D-glucan was similar to that reported for native HvExoI. Further, laminaribiose, cellobiose and gentiobiose were formed through transglycosylation reactions with 4-nitrophenyl beta-D-glucoside and barley (1,3;1,4)-beta-D-glucan. Overall, the biochemical properties of rHvExoI were similar to those reported for native HvExoI, although differences were seen in thermostabilities and hydrolytic rates of certain beta-linked glucosides.

Production of cellulosic ethanol in Saccharomyces cerevisiae heterologous expressing Clostridium thermocellum endoglucanase and Saccharomycopsis fibuligera beta-glucosidase genes

Heterologous secretory expression of endoglucanase E (Clostridium thermocellum) and beta-glucosidase 1 (Saccharomycopsis fibuligera) was achieved in Saccharomyces cerevisiae fermentation cultures as an alpha-mating factor signal peptide fusion, based on the native enzyme coding sequence. Ethanol production depends on simultaneous saccharification of cellulose to glucose and fermentation of glucose to ethanol by a recombinant yeast strain as a microbial biocatalyst. Recombinant yeast strain expressing endoglucanase and beta-glucosidase was able to produce ethanol from beta-glucan, CMC and acid swollen cellulose. This indicates that the resultant yeast strain of this study acts efficiently as a whole cell biocatalyst.

Thiamine increases beta-glucosidase production in the newly isolated strain of Fomitopsis pinicola

AIMS

To isolate a high beta-glucosidase (BGL)-producing strain and to optimize BGL production in the isolated strain.

METHODS AND RESULTS

A high BGL-producing strain was isolated and identified as Fomitopsis pinicola KMJ812 based on its morphology and a comparison of sequence of its internal transcribed spacer rDNA gene. To increase BGL production, F. pinicola was supplemented with various vitamins. Supplementation with thiamine (20 mg l(-1)) improved BGL production in F. pinicola cultures by 3.7-fold to give a specific activity of 114.4 micromol min(-1) mg(-1) protein, one of the highest among BGL-producing micro-organisms. The increased production of BGL in the thiamine-supplemented culture was confirmed by 2D electrophoresis followed by MS/MS sequencing. The BGL purified from F. pinicola culture showed the highest catalytic efficiency ever reported.

CONCLUSION

Supplemental thiamine remarkably increased BGL production by a novel BGL-producing strain, F. pinicola KMJ812.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results provide a high BGL-producing strain and the production media for BGL production, and should contribute to better industrial production of glucose via biological processes.

Breeding of industrial diploid yeast strain with chromosomal integration of multiple beta-glucosidase genes

We constructed a double auxotrophic OC-2 industrial diploid strain of Saccharomyces cerevisiae and introduced 4 copies of cell surface displaying beta-glucosidase (BGL) genes into the chromosome. The engineered OC-2 strain showed 5-fold higher BGL activity compared with the yeast carrying 2 copies of BGL gene and directly produced ethanol from cellobiose.

Simultaneous saccharification and fermentation of acid-pretreated corncobs with a recombinant Saccharomyces cerevisiae expressing beta-glucosidase

To reduce the cellobiose inhibition of exoglucanase and endogulcanase and enhance cellulose hydrolysis during simultaneous saccharification and fermentation (SSF), a beta-glucosidase encoding gene named BGL1 was cloned from Saccharomycopsis fibuligera and integrated into the chromosomal rDNA region of the Saccharomyces cerevisiae industrial strain NAN-27 producing NAN-227. Compared with the parental strain, which had no detectable activity, the beta-glucosidase specific activity in NAN-227 was 1.02 IU/mg of protein. When cellobiose was used as the sole carbon source in a shake-flask, NAN-227 consumed 6.2g/L of cellobiose and produced 3.3g/L of ethanol in 48 h. The yield was 0.532 g/g. The parent strain only consumed 1.92 g/L of cellobiose and no ethanol was detected. During the SSF of acid-pretreated corncobs NAN-227 produced 20 g/L of ethanol at 72 h, which was similar to the parent strain when 20IU of beta-glucosidase/g of substrate was added.

Effect of the cellulose-binding domain on the catalytic activity of a β-glucosidase from Saccharomycopsis fibuligera

Enzyme engineering was performed to link the beta-glucosidase enzyme (BGL1) from Saccharomycopsis fibuligera to the cellulose-binding domain (CBD2) of Trichoderma reesei cellobiohydrolase (CBHII) to investigate the effect of a fungal CBD on the enzymatic characteristics of this non-cellulolytic yeast enzyme. Recombinant enzymes were constructed with single and double copies of CBD2 fused at the N-terminus of BGL1 to mimic the two-domain organization displayed by cellulolytic enzymes in nature. The engineered S. fibuligera beta-glucosidases were expressed in Saccharomyces cerevisiae under the control of phosphoglycerate-kinase-1 promoter (PGK1 ( P )) and terminator (PGK1 ( T )) and yeast mating pheromone alpha-factor secretion signal (MFalpha1 ( S )). The secreted enzymes were purified and characterized using a range of cellulosic and non-cellulosic substrates to illustrate the effect of the CBD on their enzymatic activity. The results indicated that the recombinant enzymes of BGL1 displayed a 2-4-fold increase in their hydrolytic activity toward cellulosic substrates like avicel, amorphous cellulose, bacterial microcrystalline cellulose, and carboxy methyl cellulose in comparison with the native enzyme. The organization of the CBD in these recombinant enzymes also resulted in enhanced substrate affinity, molecular flexibility and synergistic activity, thereby improving the ability of the enzymes to act on and hydrolyze cellulosic substrates, as characterized by adsorption, kinetics, thermal stability, and scanning electron microscopic analyses.

Consolidated bioprocessing for bioethanol production using Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of lignocellulose to bioethanol refers to the combining of the four biological events required for this conversion process (production of saccharolytic enzymes, hydrolysis of the polysaccharides present in pretreated biomass, fermentation of hexose sugars, and fermentation of pentose sugars) in one reactor. CBP is gaining increasing recognition as a potential breakthrough for low-cost biomass processing. Although no natural microorganism exhibits all the features desired for CBP, a number of microorganisms, both bacteria and fungi, possess some of the desirable properties. This review focuses on progress made toward the development of baker's yeast (Saccharomyces cerevisiae) for CBP. The current status of saccharolytic enzyme (cellulases and hemicellulases) expression in S. cerevisiae to complement its natural fermentative ability is highlighted. Attention is also devoted to the challenges ahead to integrate all required enzymatic activities in an industrial S. cerevisiae strain(s) and the need for molecular and selection strategies pursuant to developing a yeast capable of CBP.

Purification and characterization of an intracellular beta-glucosidase from the methylotrophic yeast Pichia pastoris

Pichia pastoris beta-glucosidase was purified to apparent homogeneity by salting out with ammonium sulfate, gel filtration, and ion-exchange chromatography with Q-Sepharose and CM-Sepharose. The enzyme is a tetramer (275 kD) made up of four identical subunits (70 kD). The pH optimum is 7.3, and it is fairly stable in the pH range 5.5-9.5. The temperature optimum is 40 degrees C. The purified beta-glucosidase is effectively active on p-/o-nitrophenyl-beta-D-glucopyranosides (p-/o-NPG) and 4-methylumbelliferyl-beta-D-glucopyranoside (4-MUG) with Km values of 0.12, 0.22, and 0.096 mM and Vmax values of 10.0, 11.7, and 6.2 micromol/min per mg protein, respectively. It also exhibits different levels of activity against p-nitrophenyl-1-thio-beta-D-glucopyranoside, cellobiose, gentiobiose, amygdalin, prunasin, salicin, and linamarin. The enzyme is competitively inhibited by gluconolactone, p-/o-nitrophenyl-beta-D-fucopyranosides (p-/o-NPF), and glucose against p-NPG as substrate. o-NPF is the most effective inhibitor of the enzyme activity with Ki value of 0.41 mM. The enzyme is more tolerant to glucose inhibition with Ki value of 7.2 mM for p-NPG. Pichia pastoris has been employed as a host for the functional expression of heterologous beta-glucosidases and the risk of high background beta-glucosidase activity is discussed.

Cloning and expression of a gene for an alpha-glucosidase from Saccharomycopsis fibuligera homologous to family GH31 of yeast glucoamylases

Cloning of cDNA encoding an alpha-glucosidase from the dimorphous yeast Saccharomycopsis fibuligera and characterization of the gene product were performed. The cDNA of the putative alpha-glucosidase gene consists of 2,886 bp, which includes an open reading frame encoding a 19 amino acid signal peptide at the N-terminal end and a 944 amino acid mature protein with a predicted molecular mass of 105.4 kDa and pI value of 4.52. The deduced amino acid sequence shows a high degree of identity (70%) with two yeast glucoamylases, namely, the extracellular glucoamylase Gam from Schwanniomyces occidentalis and the cell surface glucoamylase Gca from Candida albicans. The recombinant product, synthesized in Saccharomyces cerevisiae, is localized on the cell surface and hydrolyses maltooligosaccharides exclusively without the ability to digest soluble starch, which is consistent with the specificity characteristic of alpha-glucosidase, EC. 3.2.1.20.

Purification and characterization of an extracellular β-glucosidase with high transglucosylation activity and stability from Aspergillus niger No. 5.1

An extracellular beta-glucosidase was extracted from the culture filtrate of Aspergillus niger No. 5.1 and purified to homogeneity by using ammonium sulfate precipitation, Chitopearl-DEAE chromatography, and Sephadex G-100 chromatography. The specific activity of the enzyme was enriched 6.33-fold, with a recovery of 11.67%. The enzyme was a monomer and the molecular mass was 67.5 kDa by sodium dodecyl sulfate polyacrylamide gel electrophoresis and 66.5 kDa by gel-filtration chromatography. The enzyme had optimum activity at pH 6.0 and 60 degrees C and was stable over the pH range of 3.0-9.0. It showed specificity of hydrolysis for p-nitrophenyl-beta-D-glucoside and cellobiose. The Km and Vmax values of the enzyme for cellobiose and salicin were 5.34 mM, 2.57 micromol/(mL.s), and 3.09 mM, 1.34 micromol/(mL.s), respectively. Both amino acid composition and N-terminal amino acid sequence of the enzyme were determined, which provides useful information for cloning of this enzyme.

Purification and characterization of recombinant Escherichia coli-expressed Pichia etchellsii ?-glucosidase II with high hydrolytic activity on sophorose

Beta-glucosidase II (Bgl II), encoded by the betaglu2 gene of the thermo-tolerant yeast Pichia etchellsii, was purified from recombinant Escherichia coli pBG22:JM109. The enzyme had a molecular mass of 176 kDa and was a dimer with an apparent subunit mass of 83 kDa. It exhibited broad substrate specificity and hydrolyzed beta-linked gluco-disaccharides and oligosaccharides, salicin, and cyanogenic glucoside amygladin. The unusually high hydrolytic activity of 7,680 units min(-1) g(-1) protein was obtained on sophorose. Competition experiments performed using differently linked beta-disaccharides indicated these to be hydrolyzed at the same active site. Transglycosylation activity leading to the biosynthesis of several disaccharides and oligosaccharides was observed. The enzyme was placed in glycosyl hydrolase family 3, based on a statistical approach using amino acid composition data. The involvement of His as a catalytically important residue was confirmed by diethylpyrocarbonate modification. Pre-incubation of the purified enzyme with 5 mM p-nitrophenyl-beta-D-glucoside offered 2.5-fold higher residual activity compared with unbound enzyme, indicating protection at the active site. The feasibility of this enzyme as a biocatalyst of choice for the synthesis of glyco-conjugates is discussed.

Microbial beta-glucosidases: cloning, properties, and applications

Beta-glucosidases constitute a major group among glycosylhydrolase enzymes. Out of the 82 families classified under glycosylhydrolase category, these belong to family 1 and family 3 and catalyze the selective cleavage of glucosidic bonds. This function is pivotal in many crucial biological pathways, such as degradation of structural and storage polysaccharides, cellular signaling, oncogenesis, host-pathogen interactions, as well as in a number of biotechnological applications. In recent years, interest in these enzymes has gained momentum owing to their biosynthetic abilities. The enzymes exhibit utility in syntheses of diverse oligosaccharides, glycoconjugates, alkyl- and aminoglucosides. Attempts are being made to understand the structure-function relationship of these versatile biocatalysts. Earlier reviews described the sources and properties of microbial beta-glucosidases, yeast beta-glucosidases, thermostable fungal beta-glucosidase, and the physiological functions, characteristics, and catalytic action of native beta-glucosidases from various plant, animal, and microbial sources. Recent efforts have been directed towards molecular cloning, sequencing, mutagenesis, and crystallography of the enzymes. The aim of the present article is to describe the sources and properties of recombinant beta-glucosidases, their classification schemes based on similarity at the structural and molecular levels, elucidation of structure-function relationships, directed evolution of existing enzymes toward enhanced thermostability, substrate range, biosynthetic properties, and applications.

Production and characterization of recombinant Phanerochaete chrysosporium beta-glucosidase in the methylotrophic yeast Pichia pastoris

The extracellular beta-glucosidase from the white-rot fungus Phanerochaete chrysosporium was expressed heterologously in the methylotrophic yeast Pichia pastoris. After 7 days' cultivation in an induction medium containing 1% (v/v) methanol, the expression level of the recombinant enzyme was 28,500 U/l, 38 times that of the wild-type enzyme. The specific activity of the crude recombinant enzyme for p-nitrophenyl-beta-D-glucoside was 52 U/mg, 37 times that of the wild-type enzyme; this difference made the purification of the enzyme simple. On a SDS-PAGE, the molecular mass of the recombinant enzyme was 133 kDa, and that of the wild-type enzyme was 116 kDa, but the difference had no effect on the hydrolysis of cellobiose or p-nitrophenyl-beta-D-glucoside. We concluded that the recombinant enzyme produced by Pichia pastoris retains the catalytic properties of the wild-type enzyme from Phanerochaete chrysosporium.

Microbial cellulose utilization: fundamentals and biotechnology

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

Microbial cellulose utilization: fundamentals and biotechnology

Fundamental features of microbial cellulose utilization are examined at successively higher levels of aggregation encompassing the structure and composition of cellulosic biomass, taxonomic diversity, cellulase enzyme systems, molecular biology of cellulase enzymes, physiology of cellulolytic microorganisms, ecological aspects of cellulase-degrading communities, and rate-limiting factors in nature. The methodological basis for studying microbial cellulose utilization is considered relative to quantification of cells and enzymes in the presence of solid substrates as well as apparatus and analysis for cellulose-grown continuous cultures. Quantitative description of cellulose hydrolysis is addressed with respect to adsorption of cellulase enzymes, rates of enzymatic hydrolysis, bioenergetics of microbial cellulose utilization, kinetics of microbial cellulose utilization, and contrasting features compared to soluble substrate kinetics. A biological perspective on processing cellulosic biomass is presented, including features of pretreated substrates and alternative process configurations. Organism development is considered for "consolidated bioprocessing" (CBP), in which the production of cellulolytic enzymes, hydrolysis of biomass, and fermentation of resulting sugars to desired products occur in one step. Two organism development strategies for CBP are examined: (i) improve product yield and tolerance in microorganisms able to utilize cellulose, or (ii) express a heterologous system for cellulose hydrolysis and utilization in microorganisms that exhibit high product yield and tolerance. A concluding discussion identifies unresolved issues pertaining to microbial cellulose utilization, suggests approaches by which such issues might be resolved, and contrasts a microbially oriented cellulose hydrolysis paradigm to the more conventional enzymatically oriented paradigm in both fundamental and applied contexts.

Metabolic engineering of Saccharomyces cerevisiae for xylose utilization

Metabolic engineering of Saccharomyces cerevisiae for ethanolic fermentation of xylose is summarized with emphasis on progress made during the last decade. Advances in xylose transport, initial xylose metabolism, selection of host strains, transformation and classical breeding techniques applied to industrial polyploid strains as well as modeling of xylose metabolism are discussed. The production and composition of the substrates--lignocellulosic hydrolysates--is briefly summarized. In a future outlook iterative strategies involving the techniques of classical breeding, quantitative physiology, proteomics, DNA micro arrays, and genetic engineering are proposed for the development of efficient xylose-fermenting recombinant strains of S. cerevisiae.

Degradation of xylan to D-xylose by recombinant Saccharomyces cerevisiae coexpressing the Aspergillus niger beta-xylosidase (xlnD) and the Trichoderma reesei xylanase II (xyn2) genes

The beta-xylosidase-encoding xlnD gene of Aspergillus niger 90196 was amplified by the PCR technique from first-strand cDNA synthesized on mRNA isolated from the fungus. The nucleotide sequence of the cDNA fragment was verified to contain a 2,412-bp open reading frame that encodes a 804-amino-acid propeptide. The 778-amino-acid mature protein, with a putative molecular mass of 85.1 kDa, was fused in frame with the Saccharomyces cerevisiae mating factor alpha1 signal peptide (MFalpha1(s)) to ensure correct posttranslational processing in yeast. The fusion protein was designated Xlo2. The recombinant beta-xylosidase showed optimum activity at 60 degrees C and pH 3.2 and optimum stability at 50 degrees C. The K(i(app)) value for D-xylose and xylobiose for the recombinant beta-xylosidase was determined to be 8.33 and 6.41 mM, respectively. The XLO2 fusion gene and the XYN2 beta-xylanase gene from Trichoderma reesei, located on URA3-based multicopy shuttle vectors, were successfully expressed and coexpressed in the yeast Saccharomyces cerevisiae under the control of the alcohol dehydrogenase II gene (ADH2) promoter and terminator. These recombinant S. cerevisiae strains produced 1,577 nkat/ml of beta-xylanase activity when expressing only the beta-xylanase and 860 nkat/ml when coexpressing the beta-xylanase with the beta-xylosidase. The maximum beta-xylosidase activity was 5.3 nkat/ml when expressed on its own and 3.5 nkat/ml when coexpressed with the beta-xylanase. Coproduction of the beta-xylanase and beta-xylosidase enabled S. cerevisiae to degrade birchwood xylan to D-xylose.

The bglA gene of Aspergillus kawachii encodes both extracellular and cell wall-bound beta-glucosidases

We cloned the genomic DNA and cDNA of bglA, which encodes beta-glucosidase in Aspergillus kawachii, based on a partial amino acid sequence of purified cell wall-bound beta-glucosidase CB-1. The nucleotide sequence of the cloned bglA gene revealed a 2,933-bp open reading frame with six introns that encodes an 860-amino-acid protein. Based on the deduced amino acid sequence, we concluded that the bglA gene encodes cell wall-bound beta-glucosidase CB-1. The amino acid sequence exhibited high levels of homology with the amino acid sequences of fungal beta-glucosidases classified in subfamily B. We expressed the bglA cDNA in Saccharomyces cerevisiae and detected the recombinant beta-glucosidase in the periplasm fraction of the recombinant yeast. A. kawachii can produce two extracellular beta-glucosidases (EX-1 and EX-2) in addition to the cell wall-bound beta-glucosidase. A. kawachii in which the bglA gene was disrupted produced none of the three beta-glucosidases, as determined by enzyme assays and a Western blot analysis. Thus, we concluded that the bglA gene encodes both extracellular and cell wall-bound beta-glucosidases in A. kawachii.

Region specificity of chromosome III on gene expression in the yeast Saccharomyces cerevisiae

A single copy of a reporter gene cassette, such as PGKP-lacZ-LEU2 (promoter-reporter-marker gene) cassette, was inserted into one of 32 positions along chromosome III in Saccharomyces cerevisiae with an interval of approximately 10 kb. The amounts of translational gene product (beta-galactosidase) synthesized by the cassette-transformed cells were then determined. The region specificity in chromosome III could be demonstrated in gene expression: two higher-expressed regions (hot regions), 133 and 199 (MAT) regions, and seven lower-expressed regions (cold regions). For the steady and high production of polypeptide, foreign gene products, by yeast, we would like to state that we hope for an insertion of the artificially prepared gene cassette [(promoter)-(foreign gene)-(marker gene) ] into a hot region, such as 199 (MAT) region of chromosome III.

Gene cloning and characterization of a novel cellulose-binding beta-glucosidase from Phanerochaete chrysosporium

Analysis of a 2.4-kb cDNA of the cellulose-binding extracellular beta-glucosidase (CBGL) from Phanerochaete chrysosporium suggested that CBGL is organized into two domains, an N-terminal cellulose-binding domain and a C-terminal catalytic domain. Genomic sequence analysis suggested that cbgl is encoded by 30 exons. Southern analysis of DNA from homokaryotic cultures indicated that CBGL is encoded by two alleles, cbgl-1 and cbgl-2, of a single gene.

Purification and characterization of an extracellular beta-glucosidase from the filamentous fungus Acremonium persicinum and its probable role in beta-glucan degradation

A beta-glucosidase from the culture filtrates of the filamentous fungus Acremonium persicinum has been purified by (NH4)2SO4 precipitation followed by anion-exchange and gel filtration chromatography. SDS-PAGE of the purified enzyme gave a single band with an apparent molecular mass of 128 kDa. The enzyme is a monomeric protein with an isoelectric point of 4.3 and a pH optimum of 5.5. Comparison of the N-terminal amino acid sequence revealed similarities between the A. persicinum enzyme and several other extracellular fungal beta-glucosidases including those from Trichoderma reesei, Aspergillus aculeatus, Saccharomycopsis fibuligera, and Pichia anomala. In addition to the hydrolysis of p-nitrophenyl-beta-glucoside, the enzyme was also active against several other aryl-beta-glucosides as well as a range of beta-linked oligoglucosides including laminaribiose, gentiobiose, cellobiose, and sophorose. D-Glucono-1,5-lactone and glucose are competitive inhibitors while the enzyme was also inhibited by N-bromosuccinimide, N-acetylimidazole, dicyclohexyl carbodiimide, Woodward's Reagent K, 2-hydroxy-5-nitrobenzyl bromide, KMnO4, and some metal ions. Possible roles for this enzyme in the noncellulolytic fungus A. persicinum are discussed in light of the increase in the rate of reducing sugar release from beta-glucans by (1-->3)- and (1-->6)-beta-glucanases when the beta-glucosidase is also present in the reaction mixtures.