Secretion of two beta-fructofuranosidases by Aspergillus niger growing in sucrose

The gold-standard genome of Aspergillus niger NRRL 3 enables a detailed view of the diversity of sugar catabolism in fungi

The fungal kingdom is too large to be discovered exclusively by classical genetics. The access to omics data opens a new opportunity to study the diversity within the fungal kingdom and how adaptation to new environments shapes fungal metabolism. Genomes are the foundation of modern science but their quality is crucial when analysing omics data. In this study, we demonstrate how one gold-standard genome can improve functional prediction across closely related species to be able to identify key enzymes, reactions and pathways with the focus on primary carbon metabolism.

Based on this approach we identified alternative genes encoding various steps of the different sugar catabolic pathways, and as such provided leads for functional studies into this topic. We also revealed significant diversity with respect to genome content, although this did not always correlate to the ability of the species to use the corresponding sugar as a carbon source.

Purification and Characterization of a Novel Intracellular Sucrase Enzyme of Leishmania donovani Promastigotes

The promastigote stage of Leishmania resides in the sand fly gut, enriched with sugar molecules. Recently we reported that Leishmania donovani possesses a sucrose uptake system and a stable pool of intracellular sucrose metabolizing enzyme. In the present study, we purified the intracellular sucrase nearly to its homogeneity and compared it with the purified extracellular sucrase. The estimated size of intracellular sucrase is ~112 kDa by gel filtration chromatography, native PAGE, and substrate staining. However, in SDS-PAGE, the protein is resolved at ~56 kDa, indicating the possibility of a homodimer in its native state. The kinetics of purified intracellular sucrase shows its higher substrate affinity with a K m of 1.61 mM than the extracellular form having a K m of 4.4 mM. The highly specific activity of intracellular sucrase towards sucrose is optimal at pH 6.0 and at 30°C. In this report the purification and characterization of intracellular sucrase provide evidence that sucrase enzyme exists at least in two different forms in Leishmania donovani promastigotes. This intracellular sucrase may support further intracellular utilization of transported sucrose.

Biotechnological production and application of fructooligosaccharides

Currently, prebiotics are all carbohydrates of relatively short chain length. One important group is the fructooligosaccharides (FOS), a special kind of prebiotic associated to the selective stimulation of the activity of certain groups of colonic bacteria. They have a positive and beneficial effect on intestinal microbiota, reducing the incidence of gastrointestinal infections and also possessing a recognized bifidogenic effect. Traditionally, these prebiotic compounds have been obtained through extraction processes from some plants, as well as through enzymatic hydrolysis of sucrose. However, different fermentative methods have also been proposed for the production of FOS, such as solid-state fermentations utilizing various agro-industrial by-products. By optimizing the culture parameters, FOS yields and productivity can be improved. The use of immobilized enzymes and cells has also been proposed as being an effective and economic method for large-scale production of FOS. This article is an overview of the results considering recent studies on FOS biosynthesis, physicochemical properties, sources, biotechnological production and applications.

Mapping the polysaccharide degradation potential of Aspergillus niger

Background

The degradation of plant materials by enzymes is an industry of increasing importance. For sustainable production of second generation biofuels and other products of industrial biotechnology, efficient degradation of non-edible plant polysaccharides such as hemicellulose is required. For each type of hemicellulose, a complex mixture of enzymes is required for complete conversion to fermentable monosaccharides. In plant-biomass degrading fungi, these enzymes are regulated and released by complex regulatory structures. In this study, we present a methodology for evaluating the potential of a given fungus for polysaccharide degradation.

Results

Through the compilation of information from 203 articles, we have systematized knowledge on the structure and degradation of 16 major types of plant polysaccharides to form a graphical overview. As a case example, we have combined this with a list of 188 genes coding for carbohydrate-active enzymes from Aspergillus niger, thus forming an analysis framework, which can be queried. Combination of this information network with gene expression analysis on mono- and polysaccharide substrates has allowed elucidation of concerted gene expression from this organism. One such example is the identification of a full set of extracellular polysaccharide-acting genes for the degradation of oat spelt xylan.

Conclusions

The mapping of plant polysaccharide structures along with the corresponding enzymatic activities is a powerful framework for expression analysis of carbohydrate-active enzymes. Applying this network-based approach, we provide the first genome-scale characterization of all genes coding for carbohydrate-active enzymes identified in A. niger.

Comparative study of two purified inulinases from thermophile Thielavia Terrestris NRRL 8126 and mesophile Aspergillus Foetidus NRRL 337 grown on Cichorium Intybus l

Thirty fungal species grown on Cichorium intybus L. root extract as a sole carbon source, were screened for the production of exo-inulinase activities. The thermophile Thielavia terrestris NRRL 8126 and mesophile Aspergillus foetidus NRRL 337 gave the highest production levels of inulinases I & II at 50 and 24 ºC respectively. Yeast extract and peptone were the best nitrogen sources for highest production of inulinases I & II at five and seven days of incubation respectively. The two inulinases I & II were purified to homogeneity by gel-filtration and ion-exchange chromatography with 66.0 and 42.0 fold of purification respectively. The optimum temperatures of purified inulinases I & II were 75 and 50 ºC respectively. Inulinase I was more thermostable than the other one. The optimum pH for activity was found to be 4.5 and 5.5 for inulinases I & II respectively. A comparatively lower Michaelis–Menten constant (2.15 mg/ml) and higher maximum initial velocity (115 µmol/min/mg of protein) for inulinase I on inulin demonstrated the exoinulinase’s greater affinity for inulin substrate. These findings are significant for its potential industrial application. The molecular mass of the inulinases I & II were estimated to be 72 & 78 kDa respectively by sodium dodecyl sulfate–polyacrylamide gel electrophoresis.

Inhibition of Streptococcus mutans biofilm formation by Streptococcus salivarius FruA

The oral microbial flora consists of many beneficial species of bacteria that are associated with a healthy condition and control the progression of oral disease. Cooperative interactions between oral streptococci and the pathogens play important roles in the development of dental biofilms in the oral cavity. To determine the roles of oral streptococci in multispecies biofilm development and the effects of the streptococci in biofilm formation, the active substances inhibiting Streptococcus mutans biofilm formation were purified from Streptococcus salivarius ATCC 9759 and HT9R culture supernatants using ion exchange and gel filtration chromatography. Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry analysis was performed, and the results were compared to databases. The S. salivarius HT9R genome sequence was determined and used to indentify candidate proteins for inhibition. The candidates inhibiting biofilms were identified as S. salivarius fructosyltransferase (FTF) and exo-beta-d-fructosidase (FruA). The activity of the inhibitors was elevated in the presence of sucrose, and the inhibitory effects were dependent on the sucrose concentration in the biofilm formation assay medium. Purified and commercial FruA from Aspergillus niger (31.6% identity and 59.6% similarity to the amino acid sequence of FruA from S. salivarius HT9R) completely inhibited S. mutans GS-5 biofilm formation on saliva-coated polystyrene and hydroxyapatite surfaces. Inhibition was induced by decreasing polysaccharide production, which is dependent on sucrose digestion rather than fructan digestion. The data indicate that S. salivarius produces large quantities of FruA and that FruA alone may play an important role in multispecies microbial interactions for sucrose-dependent biofilm formation in the oral cavity.

Production of fructooligosaccharides by beta-fructofuranosidases from Aspergillus oryzae KB

Aspergillus oryzae KB produces two types of beta-fructofuranosidases: F1 and F2. F1 produces the fructooligosaccharides (FOSs) 1-kestose, nystose, and fructosyl nystose from sucrose through a transfructosylation action, whereas F2 mainly hydrolyzes sucrose to glucose and fructose. F1 and F2 enzymes were more selectively produced from the KB strain in liquid media with a sucrose concentration>2% and <2%, respectively. Immobilization using an anion-exchange resin (WA-30; polystyrene with tertiary amine) and cross-linking with glutaraldehyde depressed the hydrolysis reaction of F2 (high hydrolyzing enzyme) alone and enhanced the thermal stability of F1 (high transferring enzyme). F1 enzyme produced in the high sucrose medium was immobilized, cross-linked, and packed in a tubular reactor for continuous production of FOSs (24.6% 1-kestose, 21.6% nystose, 5.7% and fructosyl nystose). In a long-term operation in which 60% sucrose was imputed at 55 degrees C, the composition of FOSs produced was 51.9% (transfer ratio: 92%), and production by the immobilized enzyme was maintained for 984 h.

Trends in inulinase production--a review

This article highlights the research work carried out in the production of inulinases from various inulin substrates using strains of bacteria, yeast and fungi. Inulin is one of the numerous polysaccharides of plant origin that contains glucose or fructose. It is used as a substrate in industrial fermentation processes and in food industries due to its relatively cheap and abundant source for the microbiological production of high-fructose syrups, ethanol and acetone-butanol. The various oligosaccharides derived from inulin also find their application in the medical and dietary sector. The inulinase acts on the beta-(2,1)-D-fructoside links in inulin releasing D-fructose. Hence, this article illustrates the capability of various microbes in hydrolyzing the carbon at its optimum nutrient concentration and operating condition towards inulinase production.

Two types of beta-fructofuranosidases from Aspergillus oryzae KB

Aspergillus oryzae KB produces two types of beta-fructofuranosidases, F1 and F2. F1 produces 1-kestose, nystose, and fructosyl nystose from sucrose through its transfructosylation action. F2 hydrolyzes sucrose to glucose and fructose. N-Terminal amino acid sequences of the purified enzymes were DYNAAPPNLST for F1 and YSGDLRPQ for F2. Each enzyme encoding gene was identified in the genome of Aspergillus oryzae. Although the KB strain showed a higher production of F2 than F1 in a low sucrose liquid medium, F2 production gradually decreased, whereas F1 production increased with increasing sucrose concentration in the medium. Synthesis of F1 and F2 mRNAs analyzed on reverse-transcription polymerase chain reaction corresponded to individual enzymatic production. During liquid culture of the KB strain, F1 synthesizes fructooligosaccharides from sucrose through transfructosylation, and F2 gradually hydrolyzes it. In a highly concentrated sucrose medium, intake of sucrose into the KB strain was depressed by F1 through synthesis of transfer products, fructooligosaccharides.

Purification and some properties of β-fructofuranosidase I formed by Aureobasidium pullulans DSM 2404

beta-Fructofuranosidase I (FFase I) formed by Aureobasidium pullulans DSM 2404 was purified. The enzyme had a molecular weight of about 430 kDa, was not affected by various metal ions and showed high transfructosylating activity. The yield of fructooligosaccharides production using purified FFase I was 62%.

Molecular and biochemical characterization of a novel intracellular invertase from Aspergillus niger with transfructosylating activity

A novel subfamily of putative intracellular invertase enzymes (glycoside hydrolase family 32) has previously been identified in fungal genomes. Here, we report phylogenetic, molecular, and biochemical characteristics of SucB, one of two novel intracellular invertases identified in Aspergillus niger. The sucB gene was expressed in Escherichia coli and an invertase-negative strain of Saccharomyces cerevisiae. Enzyme purified from E. coli lysate displayed a molecular mass of 75 kDa, judging from sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. Its optimum pH and temperature for sucrose hydrolysis were determined to be 5.0 and 37 to 40 degrees C, respectively. In addition to sucrose, the enzyme hydrolyzed 1-kestose, nystose, and raffinose but not inulin and levan. SucB produced 1-kestose and nystose from sucrose and 1-kestose, respectively. With nystose as a substrate, products up to a degree of polymerization of 4 were observed. SucB displayed typical Michaelis-Menten kinetics with substrate inhibition on sucrose (apparent K(m), K(i), and V(max) of 2.0 +/- 0.2 mM, 268.1 +/- 18.1 mM, and 6.6 +/- 0.2 mumol min(-1) mg(-1) of protein [total activity], respectively). At sucrose concentrations up to 400 mM, transfructosylation (FTF) activity contributed approximately 20 to 30% to total activity. At higher sucrose concentrations, FTF activity increased to up to 50% of total activity. Disruption of sucB in A. niger resulted in an earlier onset of sporulation on solid medium containing various carbon sources, whereas no alteration of growth in liquid culture medium was observed. SucB thus does not play an essential role in inulin or sucrose catabolism in A. niger but may be needed for the intracellular conversion of sucrose to fructose, glucose, and small oligosaccharides.

Database mining and transcriptional analysis of genes encoding inulin-modifying enzymes of Aspergillus niger

As a soil fungus, Aspergillus niger can metabolize a wide variety of carbon sources, employing sets of enzymes able to degrade plant-derived polysaccharides. In this study the genome sequence of A. niger strain CBS 513.88 was surveyed, to analyse the gene/enzyme network involved in utilization of the plant storage polymer inulin, and of sucrose, the substrate for inulin synthesis in plants. In addition to three known activities, encoded by the genes suc1 (invertase activity; designated sucA), inuE (exo-inulinase activity) and inuA/inuB (endo-inulinase activity), two new putative invertase-like proteins were identified. These two putative proteins lack N-terminal signal sequences and therefore are expected to be intracellular enzymes. One of these two genes, designated sucB, is expressed at a low level, and its expression is up-regulated when A. niger is grown on sucrose- or inulin-containing media. Transcriptional analysis of the genes encoding the sucrose- (sucA) and inulin-hydrolysing enzymes (inuA and inuE) indicated that they are similarly regulated and all strongly induced on sucrose and inulin. Analysis of a DeltacreA mutant strain of A. niger revealed that expression of the extracellular inulinolytic enzymes is under control of the catabolite repressor CreA. Expression of the inulinolytic enzymes was not induced by fructose, not even in the DeltacreA background, indicating that fructose did not act as an inducer. Evidence is provided that sucrose, or a sucrose-derived intermediate, but not fructose, acts as an inducer for the expression of inulinolytic genes in A. niger.

Novel alpha-glucosidase from Aspergillus nidulans with strong transglycosylation activity

Aspergillus nidulans possessed an alpha-glucosidase with strong transglycosylation activity. The enzyme, designated alpha-glucosidase B (AgdB), was purified and characterized. AgdB was a heterodimeric protein comprising 74- and 55-kDa subunits and catalyzed hydrolysis of maltose along with formation of isomaltose and panose. Approximately 50% of maltose was converted to isomaltose, panose, and other minor transglycosylation products by AgdB, even at low maltose concentrations. The agdB gene was cloned and sequenced. The gene comprised 3,055 bp, interrupted by three short introns, and encoded a polypeptide of 955 amino acids. The deduced amino acid sequence contained the chemically determined N-terminal and internal amino acid sequences of the 74- and 55-kDa subunits. This implies that AgdB is synthesized as a single polypeptide precursor. AgdB showed low but overall sequence homology to alpha-glucosidases of glycosyl hydrolase family 31. However, AgdB was phylogenetically distinct from any other alpha-glucosidases. We propose here that AgdB is a novel alpha-glucosidase with unusually strong transglycosylation activity.

The novel non-glycosylated invertase from Candida utilis (the properties and the conditions of production and purification)

The Candida utilis yeast, which is cultivated in liquid media enriched with saccharose, synthesizes the well-known invertase of 300 kDa (EC 3.2.1.26). This enzyme is present both intracellularly in the periplasmic space and extracellularly in the culture broth. However, it was determined that the same C. utilis strain cultured in certain conditions is simultaneously capable of producing another, still unknown form of invertase with a molecular mass of 60 kDa. The presence of the latter enzymatic form was detected in cells as well as in the liquid culture medium. Both invertase forms were purified using a three-step process (ion-exchange chromatography, affinity chromatography, and preparative column electrophoresis) and named, due to their different migration ratio in polyacrylamide gel electrophoresis, F-form (Fast; 60 kDa) and S-form (Slow; 300 kDa). The F-form of invertase was found to be nonglycosylated as opposed to the well-known S-form of invertase from the same source. The physicochemical properties of the F-form of invertase (isoelectric point, substrate specificity, pH, and temperature optima) were determined and compared with those of the S-form of the enzyme.

The effect of pH on glucoamylase production, glycosylation and chemostat evolution of Aspergillus niger

The effect of ambient pH on production and glycosylation of glucoamylase (GAM) and on the generation of a morphological mutant produced by Aspergillus niger strain B1 (a transformant containing an additional 20 copies of the homologous GAM glaA gene) was studied. We have shown that a change in the pH from 4 to 5.4 during continuous cultivation of the A. niger B1 strain instigates or accelerates the spontaneous generation of a morphological mutant (LB). This mutant strain produced approx. 50% less extracellular protein and GAM during both chemostat and batch cultivation compared to another strain with parental-type morphology (PS). The intracellular levels of GAM were also lower in the LB strain. In addition, cultivation of the original parent B1 strain in a batch-pulse bioreactor at pH 5.5 resulted in a 9-fold drop in GAM production and a 5-fold drop in extracellular protein compared to that obtained at pH 4. Glycosylation analysis of the glucoamylases purified from shake-flask cultivation showed that both principal forms of GAM secreted by the LB strain possessed enhanced galactosylation (2-fold), compared to those of the PS. Four diagnostic methods (immunostaining, mild methanolysis, mild acid hydrolysis and beta-galactofuranosidase digestion) provided evidence that the majority of this galactose was of the furanoic conformation. The GAMs produced during batch-pulse cultivation at pH 5.5 similarly showed an approx. 2-fold increase in galactofuranosylation compared to pH 4. Interestingly, in both cases the increased galactofuranosylation appears primarily restricted to the O-linked glycan component. Ambient pH therefore regulates both GAM production and influences its glycosylation.

Galactofuranoic-oligomannose N-linked glycans of alpha-galactosidase A from Aspergillus niger

Extracellular alpha-galactosidase A was purified from the culture filtrate of an over-producing strain of Aspergillus niger containing multiple copies of the encoding aglA gene under the control of the glucoamylase (glaA) promoter. Endoglycosidase digestion followed by SDS/PAGE, lectin and immunoblotting suggested that glycosylation accounted for approximately 25% of the molecular size of the purified protein. Monosaccharide analysis showed that this was composed of N-acetyl glucosamine, mannose and galactose. Mild acid hydrolysis, mild methanolysis, immunoblotting and exoglycosidase digestion indicated that the majority of the galactosyl component was in the furanoic conformation (beta-D-galactofuranose, Galf). At least 20 different N-linked oligosaccharides were fractionated by high-pH anion-exchange chromatography following release from the polypeptide by peptide-N-glycosidase F. The structures of these were subsequently determined by fast atom bombardment mass spectrometry to be a linear series of Hex(7-26)HexHA(c2). Indicating that oligosaccharides from GlcNA(c2)Man(7), increasing in molecular size up to GlcNA(c2)Man(24) were present. Each of these were additionally substituted with up to three beta-Galf residues. Linkage analysis confirmed the presence of mild acid labile terminal hexofuranose residues. These results show that filamentous fungi are capable of producing a heterogeneous mixture of high molecular-size N-linked glycans substituted with galactofuranoic residues, on a secreted glycoprotein.

An extracellular beta-galactofuranosidase from Aspergillus niger and its use as a tool for glycoconjugate analysis

Aspergillus niger produces an extracellular beta-galactofuranosidase, which can specifically hydrolyse beta-D-galactofuranose (Galf) from glycoconjugates. The production of this enzyme can be induced by the addition of a Galf-containing A. niger mycelial wall extract. However, on other carbon sources accumulation occurred only during the starvation conditions of the late stationary phase. Extracellular glucoamylases from this stage of cultivation possessed significantly lower levels of Galf than those from the earlier exponential growth phase when beta-galactofuranosidase is absent, suggesting in situ beta-galactofuranosidic hydrolysis. The beta-galactofuranosidase responsible was subsequently purified to homogeneity and characterised. It is a glycoprotein of 90 kDa (determined by SDS-PAGE) with activity against beta-linked Galf residues, with a Km of 4 mM against p-nitrophenyl-beta-D-galactofuranoside and a pH optimum of 3-4. The preparation did not contain other contaminating glycosidase activities; p-nitrophenyl-beta-D- and -alpha-D-galactopyranose, and alpha-D-methyl-Galf were not hydrolysed. Results are presented to show that this enzyme could be employed as a useful tool for the analysis of glycoconjugates containing biologically important Galf components.

An unambiguous microassay of galactofuranose residues in glycoconjugates using mild methanolysis and high pH anion-exchange chromatography

An original, unambiguous microassay of galactofuranose (Galf) residues in glycoconjugates is described. The method involves mild acid methanolysis (5 mM HCl) for 3 h at 84 degrees C followed by high pH anion-exchange chromatography using a routine monosaccharide system. The methanolysis products Mealpha-Galf and Mebeta-Galf were characterized chromatographically by comparison with the authentic compounds and by their response to treatment with mild acid and with beta-galactofuranosidase. Testing against p-nitrophenyl-beta-Galf and UDPalpha-Galf showed the method to be applicable to both alpha- and beta- galactofuranosides over the range 10-200 pmol. The results of partial mild methanolysis over shorter periods were consistent with initial inversion of anomeric configuration at methylation followed by anomerization to an equilibrium mixture of alpha- and beta-forms. When applied to a sample of invertase from Aspergillus nidulans, the method indicated that all of the mild acid-labile galactose (78% of the total galactose present) was in the form of a galactofuranoside and that much of this was in the beta-configuration. As expected, when applied to asialofetuin (known to contain galactose only in the pyranoside form, Galp), NPalpha-Galp, NPbeta-Galp, or UDPalpha-Galp, mild acid methanolysis failed to produce any galactofuranoside.