Effect of different nitrogen sources on activities of UDPG-pyrophosphorylase involved in pullulan synthesis and pullulan production by Aureobasidium pullulans

Recent insights into glucans biosynthesis and engineering strategies in edible fungi

Fungal α/β-glucans have significant importance in cellular functions including cell wall structure, host-pathogen interactions and energy storage, and wide application in high-profile fields, including food, nutrition, and pharmaceuticals. Fungal species and their growth/developmental stages result in a diversity of glucan contents, structures and bioactivities. Substantial progresses have been made to elucidate the fine structures and functions, and reveal the potential molecular synthesis pathway of fungal α/β-glucans. Herein, we review the current knowledge about the biosynthetic machineries, including: precursor UDP-glucose synthesis, initiation, elongation/termination and remodeling of α/β-glucan chains, and molecular regulation to maximally produce glucans in edible fungi. This review would provide future perspectives to biosynthesize the targeted glucans and reveal the catalytic mechanism of enzymes associated with glucan synthesis, including: UDP-glucose pyrophosphate phosphorylases (UGP), glucan synthases, and glucanosyltransferases in edible fungi.

Metabolomics analysis based on UHPLC-QTOF-MS/MS to explore the synthesis mechanism and culture conditions optimization of Penicillium EF-2 exopolysaccharide

Penicillium exopolysaccharide (EPS) inhibits galactose lectins and enhances immunity. However, EPS production is low and its synthesis mechanism remains unclear. Penicillium EF-2 strains with high EPS production were selected for this study, and Penicillium fermentation conditions were subsequently improved. The optimal culture conditions were 30 g/L lactose, 6 g/L yeast extract powder, 4 d seed age, 10 % inoculation amount, 3 d of secondary fermentation time, and the final EPS yield was 3.97 g/L. UHPLC-Q-TOF-MS/MS was used to explore the mechanism of EPS synthesis at the metabolic level. Optimal carbon source: lactose and optimal nitrogen source: yeast extract can provide precursors for EPS synthesis through related metabolic pathways. Moreover, regulating the energy, vitamin, and lipid metabolic pathways created favourable conditions for EPS synthesis and secretion. These findings explain the mechanism of EPS synthesis at the metabolic level and provide a theoretical basis for optimising and industrialising EPS production.

Enhanced Effects of Iron on Mycelial Growth, Metabolism and In Vitro Antioxidant Activity of Polysaccharides from Lentinula edodes

The polysaccharides found in Lentinula edodes have a variety of medicinal properties, such as anti-tumor and anti-viral effects, but their content in L. edodes sporophores is very low. In this study, Fe2+ was added to the liquid fermentation medium of L. edodes to analyze its effects on mycelial growth, polysaccharide and enzyme production, gene expression, and the activities of enzymes involved in polysaccharide biosynthesis, and in vitro antioxidation of polysaccharides. The results showed that when 200 mg/L of Fe2+ was added, with 7 days of shaking at 150 rpm and 3 days of static culture, the biomass reached its highest value (0.28 mg/50 mL) 50 days after the addition of Fe2+. Besides, Fe2+ addition also enhanced intracellular polysaccharide (IPS) and exopolysaccharide (EPS) productions, the levels of which were 2.98- and 1.79-fold higher than the control. The activities of the enzymes involved in polysaccharides biosynthesis, including phosphoglucomutase (PGM), phosphoglucose isomerase (PGI), and UDPG-pyrophosphorylase (UGP) were also increased under Fe2+ addition. Maximum PGI activity reached 1525.20 U/mg 30 days after Fe2+ addition, whereas PGM and UGP activities reached 3607.05 U/mg and 3823.27 U/mg 60 days after Fe2+ addition, respectively. The Pearson correlation coefficient showed a strong correlation (p < 0.01) between IPS production and PGM and UGP activities. The corresponding coding genes of the three enzymes were also upregulated. When evaluating the in vitro antioxidant activities of polysaccharides, EPS from all Fe2+-treated cultures exhibited significantly better capacity (p < 0.05) for scavenging -OH radicals. The results of the two-way ANOVA indicated that the abilities of polysaccharides to scavenge O2− radicals were significantly (p < 0.01) affected by Fe2+ concentration and incubation time. These results indicated that the addition of iron provided a good way to achieve desirable biomass, polysaccharide production, and the in vitro antioxidation of polysaccharides from L. edodes.

Bacterial Extracellular Polymers: A Review

Prokaryotic microbial cells especially bacteria are highly emphases for their exopolysaccharides (EPS) production. EPS are the higher molecular weight natural extracellular compounds observe at the surface of the bacterial cells. Nowadays bacterial EPS represent rapidly emerging as new and industrially important biomaterials because it having tremendous physical and chemical properties with novel functionality. Due to its industrial demand as well as research studies the different extraction processes have been discovered to remove the EPS from the microbial biofilm. The novelties of EPS are also based on the microbial habitat conditions such as higher temperature, lower temperature, acidic, alkaliphilic, saline, etc. Based on its chemical structure they can be homopolysaccharide or heteropolysaccharide. EPSs have a wide range of applications in various industries such as food, textile, pharmaceutical, heavy metal recovery, agriculture, etc. So, this review focus on the understanding of the structure, different extraction processes, biosynthesis and genetic engineering of EPS as well as their desirable biotechnological applications.

Triton X-100 improves co-production of β-1,3-D-glucan and pullulan by Aureobasidium pullulans

The effects of several surfactants on the biosynthesis of β-1,3-D-glucan (β-glucan) and pullulan by Aureobasidium pullulans CCTCC M 2012259 were investigated, and Triton X-100 was found to decrease biomass formation but increase β-glucan and pullulan production. The addition of 5 g/L Triton X-100 to the fermentation medium and bioconversion broth significantly increased β-glucan production by 76.6% and 69.9%, respectively, when compared to the control without surfactant addition. To reveal the physiological mechanism underlying the effect of Triton X-100 on polysaccharides production, the cell morphology and viability, membrane permeability, key enzyme activities, and intracellular levels of UDPG, NADH, and ATP were determined. The results indicated that Triton X-100 increased the activities of key enzymes involved in β-glucan and pullulan biosynthesis, improved intracellular UDPG and energy supply, and accelerated the transportation rate of precursors across the cell membrane, all of which contributed to the enhanced production of β-glucan and pullulan. Moreover, a two-stage culture strategy with combined processes of batch fermentation and bioconversion was applied, and co-production of β-glucan and pullulan in the presence of 5 g/L Triton X-100 additions was further improved. The present study not only provides insights into the effect of surfactant on β-glucan and pullulan production but also presents a feasible approach for efficient production of analogue exopolysaccharides. KEY POINTS: • Triton X-100 increased β-glucan and pullulan production under either batch fermentation or bioconversion. • Triton X-100 increased the permeability of cell membrane and accelerated the transportation rate of precursors across cell membrane. • Activities of key enzymes involved in β-glucan and pullulan biosynthesis were increased in the presence of Triton X-100. • Intracellular UDPG levels and energy supply were improved by Triton X-100 addition.

Effect of sodium and calcium on polysaccharide production and the activities of enzymes involved in the polysaccharide synthesis of Lentinus edodes

Lentinan is a Lentinus edodes secondary metabolite that can regulate human immune function, but yields are low. Here, the effects of Ca²⁺ and Na⁺ on L. edodes lentinan content were investigated. Metal ion concentrations and induction times were optimized according to mycelial biomass, and intracellular polysaccharide (IPS), extracellular polysaccharide (EPS), and total polysaccharide (TPS) content. The activities and gene expression of phospho-glucose isomerase (PGI), phosphoglucomutase (PGM), and UDP-glcpyrophosphorylase (UGP) were also measured. Ca²⁺ and Na⁺ concentration and induction time affected biomass, IPS, and EPS concentrations. Na⁺ increased EPS, IPS and TPS, while Ca²⁺ increased biomass, IPS, and TPS. During fermentation, mycelial biomass varied greatly under Ca²⁺ induction, while IPS, EPS and TPS varied greatly under Na⁺ induction. PGM and UGP activities increased in the presence of Na⁺, while PGI increased with Ca²⁺. Compared to control samples, pgi and pgm expression under Na⁺ was greater at days 45 and 60, respectively, while under Ca²⁺, ugp expression was greater at day 45. IPS content correlated significantly with enzyme activity, while EPS correlated with PGM activity. Our data contributes to better understanding how Na⁺ and Ca²⁺ affect mycelial growth and secondary metabolite production, and of polysaccharide biosynthesis mechanisms of L. edodes.

Discovering the role of the apolipoprotein gene and the genes in the putative pullulan biosynthesis pathway on the synthesis of pullulan, heavy oil and melanin in Aureobasidium pullulans

Pullulan produced by Aureobasidium pullulans presents various applications in food manufacturing and pharmaceutical industry. However, the pullulan biosynthesis mechanism remains unclear. This work proposed a pathway suggesting that heavy oil and melanin may correlate with pullulan production. The effects of overexpression or deletion of genes encoding apolipoprotein, UDPG-pyrophosphorylase, glucosyltransferase, and α-phosphoglucose mutase on the production of pullulan, heavy oil, and melanin were examined. Pullulan production increased by 16.93 and 8.52% with the overexpression of UDPG-pyrophosphorylase and apolipoprotein genes, respectively. Nevertheless, the overexpression or deletion of other genes exerted little effect on pullulan biosynthesis. Heavy oil production increased by 146.30, 64.81, and 33.33% with the overexpression of UDPG-pyrophosphorylase, α-phosphoglucose mutase, and apolipoprotein genes, respectively. Furthermore, the syntheses of pullulan, heavy oil, and melanin can compete with one another. This work may provide new guidance to improve the production of pullulan, heavy oil, and melanin through genetic approach.

Low-melanin containing pullulan production from sugarcane bagasse hydrolysate by Aureobasidium pullulans in fermentations assisted by light-emitting diode

Pullulan is a polymer produced by Aureobasidium pullulans and the main bottleneck for its industrial production is the presence of melanin pigment. In this study, light-emitting diodes (LEDs) of different wavelengths were used to assist the fermentation process aiming to produce low-melanin containing pullulan by wild strain of A. pullulans LB83 with different carbon sources. Under white light using glucose-based medium, 11.75g.L^-1 of pullulan with high melanin content (45.70UA_540nm.g^-1) was obtained, this production improved in process assisted by blue LED light, that resulted in 15.77g.L^-1 of pullulan with reduced content of melanin (4.46UA_540nm.g^-1). By using sugarcane bagasse (SCB) hydrolysate as carbon source, similar concentration of pullulan (about 20g.L^-1) was achieved using white and blue LED lights, with lower melanin contents in last. Use of LED light was found as a promising approach to assist biotechnological process for low-melanin containing pullulan production.

Efficient production of pullulan by Aureobasidium pullulans grown on mixtures of potato starch hydrolysate and sucrose

Pullulan is a natural exopolysaccharide with many useful characteristics. However, pullulan is more costly than other exopolysaccharides, which limits its effective application. The purpose of this study was to adopt a novel mixed-sugar strategy for maximizing pullulan production, mainly using potato starch hydrolysate as a low-cost substrate for liquid-state fermentation by Aureobasidium pullulans. Based on fermentation kinetics evaluation of pullulan production by A. pullulans 201253, the pullulan production rate of A. pullulans with mixtures of potato starch hydrolysate and sucrose (potato starch hydrolysate:sucrose = 80:20) was 0.212 h⁻¹, which was significantly higher than those of potato starch hydrolysate alone (0.146 h⁻¹) and mixtures of potato starch hydrolysate, glucose, and fructose (potato starch hydrolysate:glucose:fructose = 80:10:10, 0.166 h⁻¹) with 100 g L⁻¹ total carbon source. The results suggest that mixtures of potato starch hydrolysate and sucrose could promote pullulan synthesis and possibly that a small amount of sucrose stimulated the enzyme responsible for pullulan synthesis and promoted effective potato starch hydrolysate conversion effectively. Thus, mixed sugars in potato starch hydrolysate and sucrose fermentation might be a promising alternative for the economical production of pullulan.

Why sucrose is the most suitable substrate for pullulan fermentation by Aureobasidium pullulans CGMCC1234?

This paper studies the metabolic pathway of sucrose in pullulan fermentation by Aureobasidium pullulans. Because of its high pullulan production, sucrose proved to be the best carbon source for pullulan synthesis by A. pullulans CGMCC1234 (36.3g/L). Compared to other carbon sources, A. pullulans cells reached the stationary phase more quickly in the presence of sucrose. The specific sugar types and concentrations occurring during pullulan fermentation were detected using High Performance Liquid Chromatography (HPLC). HPLC results revealed that sucrose did not simply break down into glucose and fructose in the medium employed. Kestose (22.69g/L) also accumulated during early stages of fermentation (24h), which reduced the osmotic pressure of the extracellular fluid and diminished the inhibition of cell growth and pullulan production. β-Fructofuranosidase activity strongly depended on the carbon source. Sucrose was the best inducer of β-fructofuranosidase production. However, β-fructofuranosidase production did not directly and/or proportionally correlate with the growth of A. pullulans cells.

Central metabolic pathways of Aureobasidium pullulans CGMCC1234 for pullulan production

With the purpose of understanding the metabolic network of Aureobasidium pullulans, the central metabolic pathways were confirmed by the activities of the key enzymes involved in different pathways. The effect of different iodoacetic acid concentrations on pullulan fermentation was also investigated in this paper. The activities of phosphofructokinases and glucose-6-phosphate dehydrogenase existed in A. pullulans CGMCC1234, whereas 2-keto-3-deoxy-6-phosphogluconate aldolase activity was not detected. We proposed that the central metabolic pathways of A. pullulans CGMCC1234 included EMP and PPP, but no ED. Pullulan production declined fast as the iodoacetic acid increased, while cell growth offered upgrade firstly than descending latter tendency. Compared to the control group, the ratio of ATP/ADP of 0.60 mM iodoacetic acid group was lower at different stages of pullulan fermentation. The findings revealed that low concentration of iodoacetic acid might impel carbon flux flow toward the PPP, but reduce the flux of the EMP.

The mechanism of improved pullulan production by nitrogen limitation in batch culture of Aureobasidium pullulans

Batch culture of Aureobasidium pullulans CCTCC M 2012259 for pullulan production at different concentrations of ammonium sulfate and yeast extract was investigated. Increased pullulan production was obtained under nitrogen-limiting conditions, as compared to that without nitrogen limitation. The mechanism of nitrogen limitation favoring to pullulan overproduction was revealed by determining the activity as well as gene expression of key enzymes, and energy supply for pullulan biosynthesis. Results indicated that nitrogen limitation increased the activities of α-phosphoglucose mutase and glucosyltransferase, up-regulated the transcriptional levels of pgm1 and fks genes, and supplied more ATP intracellularly, which were propitious to further pullulan biosynthesis. The economic analysis of batch pullulan production indicated that nitrogen limitation could reduce more than one third of the cost of raw materials when glucose was supplemented to a total concentration of 70 g/L. This study also helps to understand the mechanism of other polysaccharide overproduction by nitrogen limitation.

Comparative proteomic analysis of Aureobasidium pullulans in the presence of high and low levels of nitrogen source

Pullulan, produced by Aureobasidium pullulans strain, has been broadly used in the food and medical industries. However, relatively little is known concerning the molecular basis of pullulan biosynthesis of this strain. In this paper, the effect of different concentrations of (NH4)2SO4 on pullulan fermentation was studied. Proteomics containing two-dimensional gel electrophoresis (2-DE) and matrix-assisted laser desorption/ionization time-of-flight/time-of-flight mass spectrometry (MALDI-TOF/TOF MS) were used to analyze the protein with different expressions of A. pullulans cells between the nitrogen limitation and nitrogen repletion. Maximum pullulan production reached 37.72 g/L when 0.6 g/L of initial (NH4)2SO4 was added. Excess nitrogen source would impel carbon flux flow toward biomass production, but decreased the pullulan production. Nitrogen limitation in A. pullulans seemed to influence the flux change of carbon flux flow toward exopolysaccharide accumulation. The findings indicated that 12 identified protein spots were involved in energy-generating enzymes, antioxidant-related enzymes, amino acid biosynthesis, glycogen biosynthesis, glycolysis, protein transport, and transcriptional regulation. These results presented more evidence of pullulan biosynthesis under nitrogen-limited environment, which would provide a molecular understanding of the physiological response of A. pullulans for optimizing the performance of industrial pullulan fermentation.

Influence of controlled pH on the activity of UDPG-pyrophosphorylase in Aureobasidium pullulans

UDPG-pyrophosphorylase is the key enzyme involved in pullulan biosynthesis and pullulan production by Aureobasidium pullulans. In this study, effect of controlled pH on fermentation time, pullulan production, biomass, and UDPG-pyrophosphorylase activity was investigated. Pullulan yield increased to reach a maximum within 4 days, and maximum UDPG-pyrophosphorylase activity was observed at day 3, while the biomass continued to increase until the end of the experimental period. The A. pullulans isolated from sea mud grew well at relatively low pH. UDPG-pyrophosphorylase activity was affected by the controlled pH and reached a maximum at pH 5.5. Results indicated that UDPG-pyrophosphorylase activity was highly correlated with controlled pH and pullulan biosynthesis rate.