Factors that affect the biosynthesis of xylitol by xylose-fermenting yeasts. A review

Conversion of L-arabinose to L-ribose by genetically engineered Candida tropicalis

l-Ribose, a starting material for the synthesis of l-nucleoside, has attracted lots of attention since l-nucleoside is responsible for the antiviral activities of the racemic mixtures of nucleoside enantiomers. In this study, the l-ribulose-producing Candida tropicalis strain was engineered for the conversion of l-arabinose to l-ribose. For the construction of a uracil auxotroph, the URA3 gene was excised by homologous recombination. The expression cassette of codon-optimized l-ribose isomerase gene from Acinetobacter calcoaceticus DL-28 under the control of the GAPDH promoter was integrated to the uracil auxotroph. The resulting strain, K1 CoSTP2 LsaAraA AcLRI, was cultivated with the glucose/l-arabinose mixture. At 45.5 h of fermentation, 6.0 g/L of l-ribose and 3.2 g/L of l-ribulose were produced from 30 g/L of l-arabinose. The proportion between l-ribose and l-ribulose was approximately 2:1 and the conversion yield of l-arabinose to l-ribose was about 20% (w/w). The l-ribose-producing yeast strain was successfully constructed for the first time and could convert l-arabinose to l-ribose in one-pot fermentation using the mixture of glucose and l-arabinose.

Comparison of Areal Productivity of Nannochloropsis oceanica Between Lab-Scale and Industrial-Scale Raceway Pond

The areal biomass productivities (g^-1 m^-2 day^-1) of Nannochloropsis oceanica between different sizes of way ponds were compared. Sequential batch cultivation using 2-m², 20-m², and 200-m² raceway ponds with an industrial scale 4000-m² raceway as the main culture was conducted in summer and autumn during 2017 at Whyalla, Australia. Areal productivities of sequential batch cultivation during the same culture period were 8.4 g ± 0.9 g^-1 m^-1 day^-1 in the 2-m² ponds, 9.3 g^-1 m^-1 day^-1 in the 20-m² ponds, and 8.0 g^-1 m^-1 day^-1 in the 200-m² ponds respectively. In parallel with the operation of the main 4000-m² pond, some smaller scale ponds of 2, 20, and 200 m² were operated at the same site under the same conditions. Areal productivity data of dry biomass of Nannochloropsis oceanica in each pond are very similar between industrial 4000-m² pond and other smaller ponds. In this work, the authors demonstrate that using the growth rate and productivities of Nannochloropsis from smaller scale open ponds with the same depth is valid to estimate for large-scale ponds in excess of 4000 m².

Construction of genetically engineered Candida tropicalis for conversion of l-arabinose to l-ribulose

For the biological production of l-ribulose, conversion by enzymes or resting cells has been investigated. However, expensive or concentrated substrates, an additional purification step to remove borate and the requirement for cell cultivation and harvest steps before utilization of resting cells make the production process complex and unfavorable. Microbial fermentation may help overcome these limitations. In this study, we constructed a genetically engineered Candida tropicalis strain to produce l-ribulose by fermentation with a glucose/l-arabinose mixture. For the uptake of l-arabinose as a substrate and conversion of l-arabinose to l-ribulose, two heterologous genes coding for l-arabinose transporter and l-arabinose isomerase, were constitutively expressed in C. tropicalis under the GAPDH promoter. The Arabidopsis thaliana-originated l-arabinose transporter gene (STP2)-expressing strain exhibited a high l-arabinose uptake rate of 0.103 g/g cell/h and the expression of l-arabinose isomerase from Lactobacillus sakei 23 K showed 30% of conversion (9 g/L) from 30 g/L of l-arabinose. This genetically engineered strain can be used for l-ribulose production by fermentation using mixed sugars of glucose and l-arabinose.

Multi response optimization for enhanced xylitol production by Debaryomyces nepalensis in bioreactor

In this study, the optimization of different process variables—pH (4–6), aeration rate (200–550 rpm) and agitation rate (0.6–1.8 vvm) were investigated using rotating simplex method and uniform design method to enhance xylitol production from xylose by D. nepalensis in a batch stirred tank bioreactor. Maximum xylitol productivity (0.576 g L⁻¹ h⁻¹) was obtained at pH 4.0, agitation 300 rpm and aeration 1.5 vvm by rotating simplex method. Individual optimum values of pH, agitation and aeration are 4.2, 370 rpm and 1.2 vvm, respectively, for productivity, 4.3, 350 rpm and 1.0 vvm, respectively for xylitol concentration and 4.4, 360 rpm and 0.8 vvm, respectively for yield. Using generalized distance approach, the simultaneous optimal values were found to be—pH 4.3, 370 rpm and 0.9 vvm. After multi-response analysis, batch fermentation at optimal operating conditions resulted in enhanced productivity (0.76 g L⁻¹ h⁻¹), xylitol concentration (59.4 g L⁻¹) and yield (0.58 g g⁻¹) with an increase of 76.74 % of xylitol productivity.

Modeling and simulation of xylitol production in bioreactor by Debaryomyces nepalensis NCYC 3413 using unstructured and artificial neural network models

This study examines the use of unstructured kinetic model and artificial neural networks as predictive tools for xylitol production by Debaryomyces nepalensis NCYC 3413 in bioreactor. An unstructured kinetic model was proposed in order to assess the influence of pH (4, 5 and 6), temperature (25°C, 30°C and 35°C) and volumetric oxygen transfer coefficient kLa (0.14h(-1), 0.28h(-1) and 0.56h(-1)) on growth and xylitol production. A feed-forward back-propagation artificial neural network (ANN) has been developed to investigate the effect of process condition on xylitol production. ANN configuration of 6-10-3 layers was selected and trained with 339 experimental data points from bioreactor studies. Results showed that simulation and prediction accuracy of ANN was apparently higher when compared to unstructured mechanistic model under varying operational conditions. ANN was found to be an efficient data-driven tool to predict the optimal harvest time in xylitol production.

Sugarcane straw as a feedstock for xylitol production by Candida guilliermondii FTI 20037

Sugarcane straw has become an available lignocellulosic biomass since the progressive introduction of the non-burning harvest in Brazil. Besides keeping this biomass in the field, it can be used as a feedstock in thermochemical or biochemical conversion processes. This makes feasible its incorporation in a biorefinery, whose economic profitability could be supported by integrated production of low-value biofuels and high-value chemicals, e.g., xylitol, which has important industrial and clinical applications. Herein, biotechnological production of xylitol is presented as a possible route for the valorization of sugarcane straw and its incorporation in a biorefinery. Nutritional supplementation of the sugarcane straw hemicellulosic hydrolyzate as a function of initial oxygen availability was studied in batch fermentation of Candida guilliermondii FTI 20037. The nutritional supplementation conditions evaluated were: no supplementation; supplementation with (NH4)2SO4, and full supplementation with (NH4)2SO4, rice bran extract and CaCl2·2H2O. Experiments were performed at pH 5.5, 30°C, 200rpm, for 48h in 125mL Erlenmeyer flasks containing either 25 or 50mL of medium in order to vary initial oxygen availability. Without supplementation, complete consumption of glucose and partial consumption of xylose were observed. In this condition the maximum xylitol yield (0.67gg(-1)) was obtained under reduced initial oxygen availability. Nutritional supplementation increased xylose consumption and xylitol production by up to 200% and 240%, respectively. The maximum xylitol volumetric productivity (0.34gL(-1)h(-1)) was reached at full supplementation and increased initial oxygen availability. The results demonstrated a combined effect of nutritional supplementation and initial oxygen availability on xylitol production from sugarcane straw hemicellulosic hydrolyzate.

Genome sequencing of four Aureobasidium pullulans varieties: biotechnological potential, stress tolerance, and description of new species

BACKGROUND

Aureobasidium pullulans is a black-yeast-like fungus used for production of the polysaccharide pullulan and the antimycotic aureobasidin A, and as a biocontrol agent in agriculture. It can cause opportunistic human infections, and it inhabits various extreme environments. To promote the understanding of these traits, we performed de-novo genome sequencing of the four varieties of A. pullulans.

RESULTS

The 25.43-29.62 Mb genomes of these four varieties of A. pullulans encode between 10266 and 11866 predicted proteins. Their genomes encode most of the enzyme families involved in degradation of plant material and many sugar transporters, and they have genes possibly associated with degradation of plastic and aromatic compounds. Proteins believed to be involved in the synthesis of pullulan and siderophores, but not of aureobasidin A, are predicted. Putative stress-tolerance genes include several aquaporins and aquaglyceroporins, large numbers of alkali-metal cation transporters, genes for the synthesis of compatible solutes and melanin, all of the components of the high-osmolarity glycerol pathway, and bacteriorhodopsin-like proteins. All of these genomes contain a homothallic mating-type locus.

CONCLUSIONS

The differences between these four varieties of A. pullulans are large enough to justify their redefinition as separate species: A. pullulans, A. melanogenum, A. subglaciale and A. namibiae. The redundancy observed in several gene families can be linked to the nutritional versatility of these species and their particular stress tolerance. The availability of the genome sequences of the four Aureobasidium species should improve their biotechnological exploitation and promote our understanding of their stress-tolerance mechanisms, diverse lifestyles, and pathogenic potential.

Statistical approach to study the interactive effects of process parameters for enhanced xylitol production by Candida tropicalis and its potential for the synthesis of xylitol monoesters

Previous results showed that an indigenously isolated yeast strain of Candida tropicalis was found to produce 12.11 g/L of xylitol under unoptimized conditions in presence of 50 g/L of xylose. In the present study, optimizing the process using one-variable at-a-time resulted in the production of 59.07 g/L of xylitol in 96 h in presence of 100 g/L xylose. Further optimization using response surface methodology led to the production of 65.45 g/L in medium containing 100 g/L xylose, 0.5% yeast extract, 0.03% MgSO4.7H2O and 0.2% KH2PO4, pH-4.5, 30 °C, 200 r/min for 96 h with 4% inoculum level. Addition of 1% methanol in response surface methodology optimized–medium led to the production of 67.12 g/L. Scaling up in 10 L fermentor resulted in productivity of 0.80 g/Lh with yield of 0.68 g/g. Efficient synthesis of xylitol esters was achieved with butyric acid (50.32%) and caproic acid (38.36%) in 4 h using Pseudomonas aeruginosa lipase in t-butanol: tetrahydrofuran (1:1 v/v).

Diversity and physiological characterization of D-xylose-fermenting yeasts isolated from the Brazilian Amazonian Forest

Background

This study is the first to investigate the Brazilian Amazonian Forest to identify new D-xylose-fermenting yeasts that might potentially be used in the production of ethanol from sugarcane bagasse hemicellulosic hydrolysates.

Methodology/Principal Findings

A total of 224 yeast strains were isolated from rotting wood samples collected in two Amazonian forest reserve sites. These samples were cultured in yeast nitrogen base (YNB)-D-xylose or YNB-xylan media. Candida tropicalis, Asterotremella humicola, Candida boidinii and Debaryomyces hansenii were the most frequently isolated yeasts. Among D-xylose-fermenting yeasts, six strains of Spathaspora passalidarum, two of Scheffersomyces stipitis, and representatives of five new species were identified. The new species included Candida amazonensis of the Scheffersomyces clade and Spathaspora sp. 1, Spathaspora sp. 2, Spathaspora sp. 3, and Candida sp. 1 of the Spathaspora clade. In fermentation assays using D-xylose (50 g/L) culture medium, S. passalidarum strains showed the highest ethanol yields (0.31 g/g to 0.37 g/g) and productivities (0.62 g/L·h to 0.75 g/L·h). Candida amazonensis exhibited a virtually complete D-xylose consumption and the highest xylitol yields (0.55 g/g to 0.59 g/g), with concentrations up to 25.2 g/L. The new Spathaspora species produced ethanol and/or xylitol in different concentrations as the main fermentation products. In sugarcane bagasse hemicellulosic fermentation assays, S. stipitis UFMG-XMD-15.2 generated the highest ethanol yield (0.34 g/g) and productivity (0.2 g/L·h), while the new species Spathaspora sp. 1 UFMG-XMD-16.2 and Spathaspora sp. 2 UFMG-XMD-23.2 were very good xylitol producers.

Conclusions/Significance

This study demonstrates the promise of using new D-xylose-fermenting yeast strains from the Brazilian Amazonian Forest for ethanol or xylitol production from sugarcane bagasse hemicellulosic hydrolysates.

Genetic and protein engineering of diagnostic enzymes, cholesterol oxidase and xylitol oxidase

For a long time, clinical diagnosis has been made mainly using chemical methods. Recently, several excellent substrate-specific enzymes have been developed and these enzymes are used as diagnostic catalysts. Using enzymes, it is possible to assay for a specific substance from specimens of serum or urine without the need for isolation of the substance which simplifies the process and shortens the assay time. Furthermore, the use of enzymatic assay methods for diagnosis has been facilitated by the developments in genetic engineering which made it possible to overproduce enzymes inexpensively. Here, we review the diagnostic enzymes, cholesterol oxidase and xylitol oxidase, which were successfully overproduced in our laboratory. In particular, the catalytic activity and pH and thermal stabilities of cholesterol oxidase were improved.