Localization of Glucose Oxidase and Catalase Activities in Aspergillus niger

Catalase from the Antarctic Fungus Aspergillus fumigatus I-9-Biosynthesis and Gene Characterization

Extremely cold habitats are a serious challenge for the existing there organisms. Inhabitants of these conditions are mostly microorganisms and lower mycetae. The mechanisms of microbial adaptation to extreme conditions are still unclear. Low temperatures cause significant physiological and biochemical changes in cells. Recently, there has been increasing interest in the relationship between low-temperature exposure and oxidative stress events, as well as the importance of antioxidant enzymes for survival in such conditions. The catalase is involved in the first line of the cells' antioxidant defense. Published information supports the concept of a key role for catalase in antioxidant defense against cold stress in a wide range of organisms isolated from the Antarctic. Data on representatives of microscopic fungi, however, are rarely found. There is scarce information on the characterization of catalase synthesized by adapted to cold stress organisms. Overall, this study aimed to observe the role of catalase in the survival strategy of filamentous fungi in extremely cold habitats and to identify the gene encoded catalase enzyme. Our results clearly showed that catalase is the main part of antioxidant enzyme defense in fungal cells against oxidative stress caused by low temperature exposure.

An innovative multi-enzymatic system for gluconic acid production from starch using Aspergillus niger whole-cells

The use of multi-enzymatic systems for the industrial production of chemical compounds is currently considered an important green tool in synthetic organic chemistry. Gluconic acid is a multi-functional organic acid widely used in the chemical, pharmaceutical, food, textile, and construction industries. Its industrial production from glucose by fermentation using Aspergillus niger has drawbacks including high costs related to cell growth and maintenance of cell viability. This study presents an innovative one-step multi-enzymatic system for gluconic acid production from starch using Aspergillus niger whole-cells in association with amylolytic enzymes. Using soluble starch as substrate, the following results were achieved for 96 h of reaction: 134.5 ± 4.3 g/L gluconic acid concentration, 98.2 ± 1.3 % gluconic acid yield, and 44.8 ± 1.4 gGA/gwhole-cells biocatalyst yield. Although the process has been developed using starch as raw material, the approach is feasible for any substrate or residue that can be hydrolyzed to glucose.

Histamine Elimination by a Coupling Reaction of Fungal Amine Oxidase and Bacterial Aldehyde Oxidase

Histamine (HIST) and other biogenic amines found in fish and fishery products accumulated by the action of bacterial amino acid decarboxylase cannot be decomposed and eliminated by heating or other chemical methods. A simple method for HIST elimination is proposed by a coupling reaction of the fungal amine oxidase (FAO) and bacterial aldehyde oxidase (ALOX) of acetic acid bacteria. As a model reaction, FAO oxidized benzylamine to benzaldehyde, which in turn was oxidized spontaneously to benzoic acid with ALOX. Likely, in HIST elimination, FAO coupled well with ALOX to produce imidazole 4-acetic acid from HIST with an apparent yield of 100%. Imidazole 4-acetaldehyde was not detected in the reaction mixture. In the absence of ALOX, the coupling reaction was incomplete giving a number of unidentified substances in the reaction mixture. The proposed coupling enzymatic method may be highly effective to eliminate toxic amines from fish and fishery products.

Production of Gluconic Acid and Its Derivatives by Microbial Fermentation: Process Improvement Based on Integrated Routes

Gluconic acid (GA) and its derivatives, as multifunctional biological chassis compounds, have been widely used in the food, medicine, textile, beverage and construction industries. For the past few decades, the favored production means of GA and its derivatives are microbial fermentation using various carbon sources containing glucose hydrolysates due to high-yield GA production and mature fermentation processes. Advancements in improving fermentation process are thriving which enable more efficient and economical industrial fermentation to produce GA and its derivatives, such as the replacement of carbon sources with agro-industrial byproducts and integrated routes involving genetically modified strains, cascade hydrolysis or micro- and nanofiltration in a membrane unit. These efforts pave the way for cheaper industrial fermentation process of GA and its derivatives, which would expand the application and widen the market of them. This review summarizes the recent advances, points out the existing challenges and provides an outlook on future development regarding the production of GA and its derivatives by microbial fermentation, aiming to promote the combination of innovative production of GA and its derivatives with industrial fermentation in practice.

Something old, something new: challenges and developments in Aspergillus niger biotechnology

The filamentous ascomycete fungus Aspergillus niger is a prolific secretor of organic acids, proteins, enzymes and secondary metabolites. Throughout the last century, biotechnologists have developed A. niger into a multipurpose cell factory with a product portfolio worth billions of dollars each year. Recent technological advances, from genome editing to other molecular and omics tools, promise to revolutionize our understanding of A. niger biology, ultimately to increase efficiency of existing industrial applications or even to make entirely new products. However, various challenges to this biotechnological vision, many several decades old, still limit applications of this fungus. These include an inability to tightly control A. niger growth for optimal productivity, and a lack of high-throughput cultivation conditions for mutant screening. In this mini-review, we summarize the current state-of-the-art for A. niger biotechnology with special focus on organic acids (citric acid, malic acid, gluconic acid and itaconic acid), secreted proteins and secondary metabolites, and discuss how new technological developments can be applied to comprehensively address a variety of old and persistent challenges.

Utilization of biosolids for glucose oxidase production: A potential bio-fenton reagent for advanced oxidation process for removal of pharmaceutically active compounds

The current work focuses on the production of glucose oxidase (GOD) in sterilized biosolid (BS) slurries containing BS and municipal wastewater effluent. Various parameters were optimized for maximizing the GOD production and the effects of biostimulation on GOD production was investigated by adding synthetic media components. The studies on inoculum characteristics at an inoculum age of 72 h and inoculum size of 20% (w/v) produced high GOD activities of around 6012 U/L in 25% (dw/v) BS media. Further, the effect of ultrasonication time was determined to release BS-bound GOD in order to maximize enzymes recovery. Using 1000 U/L of the BS-based GOD for 0.55 M glucose oxidation produced the maximum H₂O₂ concentration of 216 ppm. The produced H₂O₂ was utilized for bio-Fenton based advanced oxidation process for the partial removal of 15 pharmaceutically active compounds.

Forward Genetics by Genome Sequencing Uncovers the Central Role of the Aspergillus niger goxB Locus in Hydrogen Peroxide Induced Glucose Oxidase Expression

Aspergillus niger is an industrially important source for gluconic acid and glucose oxidase (GOx), a secreted commercially important flavoprotein which catalyses the oxidation of β-D-glucose by molecular oxygen to D-glucolactone and hydrogen peroxide. Expression of goxC, the GOx encoding gene and the concomitant two step conversion of glucose to gluconic acid requires oxygen and the presence of significant amounts of glucose in the medium and is optimally induced at pH 5.5. The molecular mechanisms underlying regulation of goxC expression are, however, still enigmatic. Genetic studies aimed at understanding GOx induction have indicated the involvement of at least seven complementation groups, for none of which the molecular basis has been resolved. In this study, a mapping-by-sequencing forward genetics approach was used to uncover the molecular role of the goxB locus in goxC expression. Using the Illumina and PacBio sequencing platforms a hybrid high quality draft genome assembly of laboratory strain N402 was obtained and used as a reference for mapping of genomic reads obtained from the derivative NW103:goxB mutant strain. The goxB locus encodes a thioredoxin reductase. A deletion of the encoding gene in the N402 parent strain led to a high constitutive expression level of the GOx and the lactonase encoding genes required for the two-step conversion of glucose in gluconic acid and of the catR gene encoding catalase R. This high constitutive level of expression was observed to be irrespective of the carbon source and oxidative stress applied. A model clarifying the role of GoxB in the regulation of the expression of goxC involving hydrogen peroxide as second messenger is presented.

Ultrastructural and Cytological Studies on Mycosphaerella pinodes Infection of the Model Legume Medicago truncatula

Ascochyta (Mycosphaerella) blight on cultivated peas is primarily caused by infection through asexual spores (pycnospores) of Mycosphaerella pinodes (Berk. et Blox.) Vestergren [recently renamed Peyronellaea pinodes (Berk. & A. Bloxam) Aveskamp, Gruyter & Verkley]. Using a model pathosystem involving Medicago truncatula and Mycosphaerella pinodes strain OMP-1, we examined the histology and ultrastructure of early infection events and fungal development including penetration by appressoria, vegetative growth of infection hyphae, and host responses. On the susceptible ecotype R108-1, pycnospores germinated and grew over the surface of the epidermis, then formed an appressoria and penetrated the cuticle. Beneath the cuticle, the infection peg expanded into a hyphae that grew within the outer wall of the epidermis. Subsequently, the hyphae penetrated down within mesophyll cells and proliferated vigorously, eventually, forming asexual fruiting bodies (pycnidia). In contrast, successful penetration and subsequent growth of infection hyphae were considerably restricted in the ecotype Caliph. Detected by its reaction with cerium chloride (CeCl₃) to generate electron-dense cerium perhydroxides in transmission electron micrographs, hydrogen peroxide (H₂O₂) accumulated in epidermal and mesophyll cells of Caliph challenged with pycnospores of M. pinodes. This intracellular localization was confirmed by energy-dispersive X-ray spectroscopy. Our observations thus indicate that the oxidative burst reaction leading to the generation of reactive oxygen species is associated with a local host defense response in Caliph, since no clear H₂O₂ accumulation was detectable in susceptible R108-1. Indeed, aberrant hyphae such as intrahyphal hyphae and dead hyphae, probably due to a local defense elicited by the fungus, were abundant in Caliph but not in R108-1. Our results on the cellular interactions between the fungus and host cells provide additional insights to understand foliar infection by M. pinodes on cultivated peas.

Improvement Strategies, Cost Effective Production, and Potential Applications of Fungal Glucose Oxidase (GOD): Current Updates

Fungal glucose oxidase (GOD) is widely employed in the different sectors of food industries for use in baking products, dry egg powder, beverages, and gluconic acid production. GOD also has several other novel applications in chemical, pharmaceutical, textile, and other biotechnological industries. The electrochemical suitability of GOD catalyzed reactions has enabled its successful use in bioelectronic devices, particularly biofuel cells, and biosensors. Other crucial aspects of GOD such as improved feeding efficiency in response to GOD supplemental diet, roles in antimicrobial activities, and enhancing pathogen defense response, thereby providing induced resistance in plants have also been reported. Moreover, the medical science, another emerging branch where GOD was recently reported to induce several apoptosis characteristics as well as cellular senescence by downregulating Klotho gene expression. These widespread applications of GOD have led to increased demand for more extensive research to improve its production, characterization, and enhanced stability to enable long term usages. Currently, GOD is mainly produced and purified from Aspergillus niger and Penicillium species, but the yield is relatively low and the purification process is troublesome. It is practical to build an excellent GOD-producing strain. Therefore, the present review describes innovative methods of enhancing fungal GOD production by using genetic and non-genetic approaches in-depth along with purification techniques. The review also highlights current research progress in the cost effective production of GOD, including key advances, potential applications and limitations. Therefore, there is an extensive need to commercialize these processes by developing and optimizing novel strategies for cost effective GOD production.

Structure of a Thermobifida fusca lytic polysaccharide monooxygenase and mutagenesis of key residues

BACKGROUND

Auxiliary activity (AA) enzymes are produced by numerous bacterial and fungal species to assist in the degradation of biomass. These enzymes are abundant but have yet to be fully characterized. Here, we report the X-ray structure of Thermobifida fusca AA10A (TfAA10A), investigate mutational characterization of key surface residues near its active site, and explore the importance of the various domains of Thermobifida fusca AA10B (TfAA10B). The structure of TfAA10A is similar to other bacterial LPMOs (lytic polysaccharide monooxygenases), including signs of photo-reduction and a distorted active site, with mixed features showing both type I and II copper coordination. The point mutation experiments of TfAA10A show that Trp82 and Asn83 are needed for binding, but only Trp82 affects activity. The TfAA10B domain truncation mutants reveal that CBM2 is crucial for the binding of substrate, but that the X1 module does not affect binding or activity.

RESULTS

In TfAA10A, Trp82 and Asn83 are needed for binding, but only Trp82 affects activity. The TfAA10B domain truncation mutants reveal that CBM2 is crucial for substrate binding, but that the X1 module does not affect binding or activity. The structure of TfAA10A is similar to other bacterial lytic polysaccharide monooxygenases with mixed features showing both type I and II copper coordination.

CONCLUSIONS

The role of LPMOs and the variability of abundance in genomes are not fully explored. LPMOs likely perform initial attacks into crystalline cellulose to allow larger processive cellulases to bind and attack, but the precise nature of their synergistic behavior remains to be definitively characterized.

High efficiency cell-recycle continuous sodium gluconate production by Aspergillus niger using on-line physiological parameters association analysis to regulate feed rate rationally

In this paper, a system of cell-recycle continuous fermentation for sodium gluconate (SG) production by Aspergillus niger (A. niger) was established. Based on initial continuous fermentation result (100.0h) with constant feed rate, an automatic feedback strategy to regulate feed rate using on-line physiological parameters (OUR and DO) was proposed and applied successfully for the first time in the improved continuous fermentation (240.5h). Due to less auxiliary time, highest SG production rate (31.05±0.29gL(-1)h(-1)) and highest yield (0.984±0.067molmol(-1)), overall SG production capacity (975.8±5.8gh(-1)) in 50-L fermentor of improved continuous fermentation increased more than 300.0% compared to that of batch fermentation. Improvement of mass transfer and dispersed mycelia morphology were the two major reasons responsible for the high SG production rate. This system had been successfully applied to industrial fermentation and SG production was greatly improved.

A simple novel approach for real-time monitoring of sodium gluconate production by on-line physiological parameters in batch fermentation by Aspergillus niger

In this paper, approach for real-time monitoring of sodium gluconate (SG) fermentation was established for the first time by the equations which can calculate real-time key-parameters by on-line physiological data. Based on this approach, limiting factors were found out in initial fermentation F1 and then step-wise agitation increase and improved medium recipe were proposed in fermentation F2 and F3, respectively. The highest average SG production rate (16.58±0.91 g L(-1) h(-1)) was achieved in fermentation F3, which was 104.2% and 48.0% higher than those in fermentation F1 and F2, respectively. Meanwhile, due to shorter fermentation period (decreased from 34 h to 18.7 h), lower biomass (about 1.5 g L(-1)) and less by-product accumulation, the overall yield of 0.943±0.012 (mol mol(-1)) in fermentation F3 increased more than 16.0% compared to fermentation F1. This approach had been successfully applied to industrial fermentation and greatly improved SG production.

Electrochemical Glucose Biosensor Based on Glucose Oxidase Displayed on Yeast Surface

The conventional enzyme-based biosensor requires chemical or physical immobilization of purified enzymes on electrode surface, which often results in loss of enzyme activity and/or fractions immobilized over time. It is also costly. A major advantage of yeast surface display is that it enables the direct utilization of whole cell catalysts with eukaryote-produced proteins being displayed on the cell surface, providing an economic alternative to traditional production of purified enzymes. Herein, we describe the details of the display of glucose oxidase (GOx) on yeast cell surface and its application in the development of electrochemical glucose sensor. In order to achieve a direct electrochemistry of GOx, the entire cell catalyst (yeast-GOx) was immobilized together with multiwalled carbon nanotubes on the electrode, which allowed sensitive and selective glucose detection.

Production of glucose oxidase and catalase by Aspergillus niger free and immobilized in alginate-polyvinyl alcohol beads

The production of glucose oxidase (GOX) and catalase (CAT) by submerged fermentation of the free and immobilized xerophytic fungus Aspergillus niger under equal conditions was compared. To immobilize fungal spores, entrapment in PVA/alginate beads treated with NaNO3/CaCl2 was performed. The yield of immobilization in the beads with a diameter less than 1mm was equal to 100%. Fungus growth and substrate consumption were evaluated in both fermentation systems, demonstrating the lag-period presence in the case of the first cycle of immobilized fungus use. The enzyme production by immobilized fungus reuse was carried out. In these cases, greater enzymatic GOX activity was detected, while CAT activity decreased. SEM micrographs for the beads with immobilized fungus applied in the first and second fermentation cycles were obtained, presenting fungus spreading inside the sphere, spore presence and branching hyphae. Immobilization of A. niger on PVA/alginate beads is effective for GOX and CAT production at least on 2-3 repeated fermentative cycles. Thus, immobilization enables repeated use of microbial cells.

Utilization of ram horn peptone in the production of glucose oxidase by a local isolate Aspergillus niger OC-3

Glucose oxidase (GO) is an enzyme that is used in many fields. In this study, ram horn peptone (RHP) was utilized as the nitrogen source and compared with other nitrogen sources in the production of GO by Aspergillus niger. To obtain higher GO activity, 14 A. niger strains were isolated from soil samples around Erzurum, Turkey. Among these strains, the isolate that was named A. niger OC-3 achieved the highest GO production. The production of GO was carried out in 100 mL scaled batch culture. The fermentation conditions such as initial pH, temperature, agitation speed, and time were investigated in order to improve GO production. The results showed that the cultivation conditions would significantly affect the formation of GO, and the utilization of the RHP achieved the highest enzyme production (48.6 U/mL) if compared to other nitrogen sources. On the other hand, the maximum biomass was obtained by using the fish peptone (7.2 g/L), while RHP yielded 6.4 g/L. These results suggest that RHP from waste ram horns could effectively be used in the production of GO by A. niger OC-3.

Enigmatic Gratuitous Induction of the Covalent Flavoprotein Vanillyl-Alcohol Oxidase in Penicillium simplicissimum

When Penicillium simplicissimum is grown on veratryl alcohol, anisyl alcohol, or 4-(methoxymethyl)phenol, an intracellular covalent flavin-containing vanillyl-alcohol oxidase is induced. The induction is highest (up to 5% of total protein) during the growth phase. In addition to vanillyl-alcohol oxidase, an intracellular catalase-peroxidase is induced. Induction of vanillyl-alcohol oxidase in P. simplicissimum is prevented by the addition of isoeugenol to veratryl alcohol-containing media, but growth is unaffected. The inhibitory effect of isoeugenol on induction is not observed when anisyl alcohol or 4-(methoxymethyl)phenol is used as the growth substrate. Based on the induction experiments and the degradation pathways for veratryl and anisyl alcohol, we propose that induction of vanillyl-alcohol oxidase is superfluous when P. simplicissimum is grown on these aromatic alcohols. However, the enzyme plays an essential role in the degradation of the methyl ether of p-cresol, 4-(methoxymethyl)phenol.

Gluconic acid production by Aspergillus terreus

AIM

Aspergillus terreus produces itaconic acid at low pH but lovastatin and other secondary metabolites at higher pH in the fermentation. The utilization of glucose as a carbon substrate was investigated for secondary metabolite production by A. terreus.

METHODS AND RESULTS

With a starting pH of 6.5, glucose was rapidly metabolized to gluconic acid by the wild-type strain and by transformants harbouring Aspergillus niger genes encoding 6-phosphofructo-1-kinases with superior kinetic and regulatory properties for bioproduction of metabolites from glucose. On exhaustion of the glucose in batch fermentations, the accumulated gluconic acid was utilized as a carbon source.

CONCLUSIONS

A novel pathway of glucose catabolism was demonstrated in A. terreus, a species whose wild type is, without any strain development, capable of producing gluconic acid at high molar conversion efficiency (up to 0.7 mol mol(-1) glucose consumed).

SIGNIFICANCE AND IMPACT OF THE STUDY

Aspergillus terreus is a potential novel producer organism for gluconic acid, a compound with many uses as a bulk chemical. With a new knowledge of glucose catabolism by A. terreus, fermentation strategies for secondary metabolite production can be devised with glucose feeding using feedback regulation by pH.

Systemic analysis of the response of Aspergillus niger to ambient pH

BACKGROUND

The filamentous fungus Aspergillus niger is an exceptionally efficient producer of organic acids, which is one of the reasons for its relevance to industrial processes and commercial importance. While it is known that the mechanisms regulating this production are tied to the levels of ambient pH, the reasons and mechanisms for this are poorly understood.

METHODS

To cast light on the connection between extracellular pH and acid production, we integrate results from two genome-based strategies: A novel method of genome-scale modeling of the response, and transcriptome analysis across three levels of pH.

RESULTS

With genome scale modeling with an optimization for extracellular proton-production, it was possible to reproduce the preferred pH levels for citrate and oxalate. Transcriptome analysis and clustering expanded upon these results and allowed the identification of 162 clusters with distinct transcription patterns across the different pH-levels examined. New and previously described pH-dependent cis-acting promoter elements were identified. Combining transcriptome data with genomic coordinates identified four pH-regulated secondary metabolite gene clusters. Integration of regulatory profiles with functional genomics led to the identification of candidate genes for all steps of the pal/pacC pH signalling pathway.

CONCLUSIONS

The combination of genome-scale modeling with comparative genomics and transcriptome analysis has provided systems-wide insights into the evolution of highly efficient acidification as well as production process applicable knowledge on the transcriptional regulation of pH response in the industrially important A. niger. It has also made clear that filamentous fungi have evolved to employ several offensive strategies for out-competing rival organisms.

Glucose oxidase--an overview

Glucose oxidase (beta-D-glucose:oxygen 1-oxidoreductase; EC 1.1.2.3.4) catalyzes the oxidation of beta-D-glucose to gluconic acid, by utilizing molecular oxygen as an electron acceptor with simultaneous production of hydrogen peroxide. Microbial glucose oxidase is currently receiving much attention due to its wide applications in chemical, pharmaceutical, food, beverage, clinical chemistry, biotechnology and other industries. Novel applications of glucose oxidase in biosensors have increased the demand in recent years. Present review discusses the production, recovery, characterization, immobilization and applications of glucose oxidase. Production of glucose oxidase by fermentation is detailed, along with recombinant methods. Various purification techniques for higher recovery of glucose oxidase are described here. Issues of enzyme kinetics, stability studies and characterization are addressed. Immobilized preparations of glucose oxidase are also discussed. Applications of glucose oxidase in various industries and as analytical enzymes are having an increasing impact on bioprocessing.

Catalase gene disruptant of the human pathogenic yeast Candida albicans is defective in hyphal growth, and a catalase-specific inhibitor can suppress hyphal growth of wild-type cells

Although the catalase gene (CAT1) disruptant of the human pathogenic yeast Candida albicans was viable under ordinary growth conditions, we previously found that it could not grow on YPD (yeast extract/peptone/dextrose) containing SDS or at higher growth temperatures. To investigate the pleiotrophic nature of the disruptant, we examined the effect of the catalase inhibitor 3-AT on the growth of wild-type strains. Surprisingly, the addition of 3-AT and SDS caused the wild-type cells to be non-viable on YPD plates. We found an additional phenotype of the catalase gene disruptant: it did not produce normal hyphae on Spider medium. Hyphal growth was observed in a CAP1 (Candida AP-1-like protein gene) disruptant, a HOG1 (high-osmolarity glycerol signaling pathway gene) disruptant, and the double CAP1/HOG1 disruptant, suggesting that the defect in hyphal formation by the catalase disruptant was independent of these genes. Addition of 3-AT and SDS to hyphae-inducing media suppressed growth of normal hyphae in the wild-type strain. The potential necessity for catalase action upon exposure to hyphae-inducing conditions was confirmed by the immediate elevation of the catalase gene message. In spite of the requirement for catalase during hyphal growth, the catalase gene disruptant was capable of forming germ tubes in medium containing serum.

Characterization of the distribution of glucose oxidase inPenicillium sp. CBS 120262 andAspergillus niger NRRL-3 cultures and its effect on integrated product recovery

Glucose oxidase (GO) is an important industrial enzyme typically purified from Penicillium and Aspergillus sp. As GO distribution within the cultures influences process design for maximal product recovery, distribution of GO activity in Penicillium sp. CBS 120262 and Aspergillus niger NRRL-3, during mid-exponential and stationary phases, is compared. On progression from mid-exponential to stationary phase, the percentage GO activity in the cytoplasm decreased 1.6- and 1.3-fold in Penicillium sp. and A. niger respectively. In Penicillium sp., a concomitant 1.8- and 1.9-fold decrease in the percentage GO activity in the cell envelope and slime mucilage respectively, translated into a 2.0-fold increase in the extracellular fluid. In A. niger, decreasing cytoplasmic GO activity was accompanied by 1.3-fold increases in the cell envelope and slime mucilage, with a 1.3-fold decrease in the extracellular fluid. Similar trends were observed in specific GO activities. As final GO activity recovered is governed by the purification program, recovery from the extracellular fluid plus cell extract or from the extracellular fluid only were compared through simulating processes of varying complexity. A critical yield for each purification stage was identified above which recovery from the extracellular fluid plus cell extract exceeded that from extracellular fluid alone. These results highlight the influence of microorganism, harvest time and efficiency of downstream process on GO activity delivered. In the systems studied, Penicillium sp. is the organism of choice and should be harvested during stationary phase. The purification process chosen should be informed by both enzyme distribution and individual purification stages yields.

Cell bound and extracellular glucose oxidases from Aspergillus niger BTL: evidence for a secondary glycosylation mechanism

Two glucose oxidase (GOX) isoforms where purified to electrophoretic homogeneity from the mycelium extract (GOXI) and the extracellular medium (GOXII) of Aspergillus niger BTL cultures. Both enzymes were found to be homodimers with nonreduced molecular masses of 148 and 159 kDa and pI values of 3.7 and 3.6 for GOXI and GOXII, respectively. The substrate specificity and the kinetic characteristics of the two GOX forms, as expressed through their apparent K m values on glucose, as well as pH and T activity optima, were almost identical. The only structural difference between the two enzymes was in their degrees of glycosylation, which were determined equal to 14.1 and 20.8% (w/w) of their molecular masses for GOXI and GOXII, respectively. The above difference in the carbohydrate content between the two enzymes seems to influence their pH and thermal stabilities. GOXII proved to be more stable than GOXI at pH values 2.5, 3.0, 8.0, and 9.0. Half-lives of GOXI at pH 3.0 and 8.0 were 8.9 and 17.5 h, respectively, whereas the corresponding values for GOXII were 13.5 and 28.1 h. As far as the thermal stability is concerned, GOXII was also more thermostable than GOXI as judged by the deactivation constants determined at various temperatures. More specifically, the half-lives of GOXI and GOXII, at 45 degrees C, were 12 and 49 h, respectively. These results suggest A. niger BTL probably possesses a secondary glycosylation mechanism that increases the stability of the excreted GOX.

Spores of Aspergillus niger as reservoir of glucose oxidase synthesized during solid-state fermentation and their use as catalyst in gluconic acid production

AIMS

To exploit conidiospores of Aspergillus niger as a vector for glucose oxidase extraction from solid media, and their direct use as biocatalyst in the bioconversion of glucose to gluconic acid.

METHODS AND RESULTS

Spores of A. niger (200 h old) were shown to fully retain all the glucose oxidase synthesized by the mycelium during solid-state fermentation (SSF). They acted as catalyst and carried out the bioconversion reaction effectively, provided they were permeabilized by freezing and thawing. Glucose oxidase activity was found retained in the spores even after repeated washings. Average rate of reaction was 1.5 g l(-1) h(-1) with 102 g l(-1) of gluconic acid produced out of 100 g l(-1) glucose consumed after approx. 100 h reaction, which corresponded to a molar yield close to 93%. These results were obtained with permeabilized spores in the presence of a germination inhibitor, sodium azide.

CONCLUSIONS

Spores of A. niger served as efficient catalyst in the model bioconversion reaction after permeabilization.

SIGNIFICANCE AND IMPACT OF THE STUDY

To our knowledge, this is the first detailed study on the ability of A. niger spores to act as reservoir of enzyme synthesized during SSF without its release into solid media. Use of this material served as an innovative concept for enzyme extraction and purification from a solid medium. Moreover, this approach could compete efficiently with the conventional use of mycelial form of the fungus in gluconic acid production.

Isolation, purification and characterization of a novel glucose oxidase from Penicillium sp. CBS 120262 optimally active at neutral pH

A novel glucose oxidase (GOX), a flavoenzyme, from Penicillium sp. was isolated, purified and partially characterised. Maximum activities of 1.08U mg(-1)dry weight intracellular and 6.9U ml(-1) extracellular GOX were obtained. Isoelectric focussing revealed two isoenzymes present in both intra- and extracellular fractions, having pI's of 4.30 and 4.67. GOX from Penicillium sp. was shown to be dimeric with a molecular weight of 148kDa, consisting of two equal subunits with molecular weight of 70k Da. The enzyme displayed a temperature optimum between 25 and 30 degrees C, and an optimum pH range of 6-8 for the oxidation of beta-d-glucose. The enzyme was stable at 25 degrees C for a minimum of 10h, with a half-life of approximately 30 min at 37 degrees C without any prior stabilisation. The lyophilized enzyme was stable at -20 degrees C for a minimum of 6 months. GOX from Penicillium sp. Tt42 displayed the following kinetic characteristics: Vmax, 240.5U mg(-1); Km, 18.4mM; kcat, 741 s(-1) and kcat/Km, 40 s(-1)mM(-1). Stability at room temperature, good shelf-life without stabilisation and the neutral range for the pH optimum of this GOX contribute to its usefulness in current GOX-based biosensor applications.

Location of glucose oxidase during production by Aspergillus niger

The production of the enzyme glucose oxidase by Aspergillus niger is well documented. However, its distribution within the fungal culture is less well defined. Since the enzyme location impacts significantly on enzyme recovery, this study quantifies the enzyme distribution between the extracellular fluid, cell wall, cytoplasm and slime mucilage fractions in an A. niger NRRL-3. The culture was separated into the individual fractions and the glucose oxidase activity was determined in each. The extracellular fluid contained 38% of the total activity. The remaining 62% was associated with the mycelia and was distributed between the cell wall, cytoplasm and slime mucilage in the proportions of 34, 12 and 16%, respectively. Intracellular cytoplasmic and cell wall sites were confirmed using immunocytochemical labelling of the mycelia. In the non-viable cell, the mycelial-associated enzyme was distributed between these sites, whereas in the viable cell, it was predominantly associated with the cell wall. The distribution of the enzyme activity indicates that recovery from the solids would result in a 38% loss, whereas recovery from the extracellular fluid would result in a 62% loss. The results also suggest, however, that this 62% loss could be reduced to around 34% by disintegrating the solids prior to separation due to the contribution of the enzyme in the cytoplasm and slime mucilage. This was confirmed by independently establishing the percentage activity in the liquid and solid portions of a disintegrated culture as 62 and 38%, respectively.

Production of catalases by Aspergillus niger isolates as a response to pollutant stress by heavy metals

Isolates of Aspergillus niger, selected from the coal dust of a mine containing arsenic (As; 400 mg/kg) and from the river sediment of mine surroundings (As, 1651 mg/kg, Sb, 362 mg/kg), growing in minimal nitrate medium in the phase of hyphal development and spore formation, exhibited much higher levels of total catalase activity than the same species from the culture collection or a culture adapted to soil contaminated with As (5 mg/L). Electrophoretic resolution of catalases in cell-free extracts revealed three isozymes of catalases and production of individual isozymes was not significantly affected by stress environments. Exogenously added stressors (As(5+), Cd(2+), Cu(2+)) at final concentrations of 25 and 50 mg/L and H(2)O(2) (20 or 40 mM) mostly stimulated production of catalases only in isolates from mines surroundings, and H(2)O(2) and Hg(2+) caused the disappearance of the smallest catalase I. Isolates exhibited a higher tolerance of the toxic effects of heavy metals and H(2)O(2), as monitored by growth, than did the strain from the culture collection.

Bacterial catalase in the microsporidian Nosema locustae: implications for microsporidian metabolism and genome evolution

Microsporidia constitute a group of extremely specialized intracellular parasites that infect virtually all animals. They are highly derived, reduced fungi that lack several features typical of other eukaryotes, including canonical mitochondria, flagella, and peroxisomes. Consistent with the absence of peroxisomes in microsporidia, the recently completed genome of the microsporidian Encephalitozoon cuniculi lacks a gene for catalase, the major enzymatic marker for the organelle. We show, however, that the genome of the microsporidian Nosema locustae, in contrast to that of E. cuniculi, encodes a group II large-subunit catalase. Surprisingly, phylogenetic analyses indicate that the N. locustae catalase is not specifically related to fungal homologs, as one would expect, but is instead closely related to proteobacterial sequences. This finding indicates that the N. locustae catalase is derived by lateral gene transfer from a bacterium. The catalase gene is adjacent to a large region of the genome that appears to be far less compact than is typical of microsporidian genomes, a characteristic which may make this region more amenable to the insertion of foreign genes. The N. locustae catalase gene is expressed in spores, and the protein is detectable by Western blotting. This type of catalase is a particularly robust enzyme that has been shown to function in dormant cells, indicating that the N. locustae catalase may play some functional role in the spore. There is no evidence that the N. locustae catalase functions in a cryptic peroxisome.

Improvement of foreign-protein production in Aspergillus niger var. awamori by constitutive induction of the unfolded-protein response

Unfolded-protein response (UPR) denotes the upregulation of endoplasmic reticulum (ER)-resident chaperone and foldase genes and numerous other genes involved in secretory functions during the accumulation of unfolded proteins into the ER. Overexpression of individual foldases and chaperones has been used in attempts to improve protein production in different production systems. We describe here a novel strategy to improve foreign-protein production. We show that the constitutive induction of the UPR pathway in Aspergillus niger var. awamori can be achieved by expressing the activated form of the transcription factor hacA. This induction enhances the production of Trametes versicolor laccase by up to sevenfold and of bovine preprochymosin by up to 2.8-fold in this biotechnically important fungus. The regulatory range of UPR was studied by analyzing the mRNA levels of novel A. niger var. awamori genes involved in different secretory functions. This revealed both similarities and differences to corresponding studies in Saccharomyces cerevisiae.

Autolysis and ageing of Penicillium chrysogenum under carbon starvation: respiration and glucose oxidase production

During the exponential growth phase of Penicillium chrysogenum NCAIM 00237 the effective conversion of glucose and O2 to gluconate and H2O2 by glucose oxidase (GOX) was the most likely source of intracellular ROS measured. In glucose-supplemented autolysing cultures, the increased of intracellular ROS concentration was attributed to respiration in the absence of any significant GOX activity. The induction of GOX and catalase by glucose and H2O2 was clearly age-dependent in P. chrysogenum. In ageing cryptic growth phase cultures, superoxide dismutase and cyanide-resistant respiration were the major elements of antioxidative defence but these activities were insufficient to prevent the progressive accumulation of ROS and the concomitant decrease in cell vitality.

Haemonchus contortus utilises catalase in defence against exogenous hydrogen peroxide in vitro

The toxicity of activated oxygen species towards adult Haemonchus contortus nematodes was examined in in vitro assays using ingestion of [3H]inulin to assess nematode viability. Both glucose/glucose oxidase (generation of hydrogen peroxide) and xanthine/xanthine oxidase (generation of superoxide anion) systems showed concentration-dependent toxicity to the nematodes. Both adult and larval Haemonchus contortus enzyme preparations showed significant catalase activities. Adult nematodes exposed to aminotriazole for 24 h showed catalase activities reduced to less than 20% of controls. Aminotriazole-treated nematodes exposed to a glucose/glucose oxidase system were significantly more susceptible to the toxic effects of the oxidant-generating system than controls (no aminotriazole pre-treatment). The concentration of glucose oxidase required to inhibit feeding by 50% was decreased 33-fold in aminotriazole-treated nematodes compared with controls. The effect of aminotriazole pre-treatment implicates hydrogen peroxide as a significant toxic agent in the glucose/glucose oxidase system. It is apparent that inhibition of Haemonchus contortus catalase increases the susceptibility of the parasite to the toxic effects of hydrogen peroxide, demonstrating a protective role for this enzyme. This suggests that catalase has the potential to play a significant role in the defence of this parasite against hydrogen peroxide produced as part of the respiratory burst of activated phagocytes within the host during its response to nematode infection.