Repeated Use of ImmobilizedGluconobacter oxydansCells for Conversion of Glycerol to Dihydroxyacetone

Dihydroxyacetone production via heterogeneous biotransformations of crude glycerol

In this work, several immobilization strategies for Gluconobacter oxydans NBRC 14819 (Gox) were tested in the bioconversion of crude glycerol to dihydroxyacetone (DHA). Agar, agarose and polyacrylamide were evaluated as immobilization matrixes. Glutaraldehyde crosslinked versions of the agar and agarose preparations were also tested. Agar immobilized Gox proved to be the best heterogeneous biocatalyst in the bioconversion of crude glycerol reaching a quantitative production of 50 g/L glycerol into DHA solely in water. Immobilization allowed reutilization for at least eight cycles, reaching four times more DHA than the amount obtained by a single batch of free cells which cannot be reutilized. An increase in scale of 34 times had no impact on DHA productivity. The results obtained herein constitute a contribution to the microbiological production of DHA as they not only attain unprecedented productivities for the reaction with immobilized biocatalysts but also proved that it is feasible to do it in a clean background of solely water that alleviates the cost of downstream processing.

Metabolic integration of azide functionalized glycan on Escherichia coli cell surface for specific covalent immobilization onto magnetic nanoparticles with click chemistry

A method for specific immobilization of whole-cell with covalent bonds was developed through a click reaction between alkyne and azide groups. In this approach, magnetic nanoparticle Fe₃O₄@SiO₂-NH₂-alkyne was synthesized with Fe₃O₄ core preparation, SiO₂ coating, and alkyne functionalization on the surface. The azides were successfully integrated onto the cell surface of the recombinant E. coli harboring glycerol dehydrogenase, which was employed as the model cell. The highest immobilization yield of 83% and activity recovery of 94% were obtained under the conditions of 0.67 mg mg^-1 cell-support ratio, pH 6.0, temperature 45 °C, and 20 mM Cu²⁺ concentration. The immobilized cell showed good reusability, which remained over 50% of initial activity after 10 cycles of utilization. Its activity was 9.7-fold higher than that of the free cell at the condition of pH 8.0 and each optimal temperature. Furthermore, the immobilized cell showed significantly higher activity, operational stability, and reusability.

Repeated production of 6-(N-hydroxyethyl)-amino-6-deoxy-α-L-sorbofuranose by immobilized Gluconobacter oxydans cells with a strategy of in situ exhaustive cell regeneration

The major troubles in 6-(N-hydroxyethyl)-amino-6-deoxy-α-L-sorbofuranose (6NSL) production from N-2-hydroxyethyl glucamine (NHEG) by Gluconobacter oxydans were low cell yield during cell preparation and loss of cells' biocatalytic ability during biotransformation, resulting in high production cost and low 6NSL production. The target of this work was to enhance 6NSL production by reusing cells and improving the cells biocatalytic ability. First, inhibitory effects of substrate and product on 6NSL production, and optimization of cell regeneration condition were investigated, respectively. Then repeated production of 6NSL by immobilized cell using a strategy of in situ exhaustive cell regeneration in a bubble column bioreactor was developed. As a result, the bioprocess underwent nine cycles, the average 6NSL production and conversion rate of NHEG to 6NSL reached 42.6 g L^-1 and 83.1% in each batch was achieved, respectively.

Resolving the formidable barrier of oxygen transferring rate (OTR) in ultrahigh-titer bioconversion/biocatalysis by a sealed-oxygen supply biotechnology (SOS)

BACKGROUND

The critical issue in the competitiveness between bioengineering and chemical engineering is the products titer and the volume productivity. The most direct and effective approach usually employs high-density biocatalyst, while the weakened mass transfer and evoked foam problem accompany ultrahigh-density biocatalyst loading and substrate/product titer. In high-density obligate aerobic bioconversion, oxygen as electron acceptor is a speed-limiting step in bioprocesses, but sufficient oxygen supply will lead to the foaming which results in a significant reduction in oxygen utilization and the use of additional defoamers. In this study, we designed a novel sealed-oxygen supply (SOS) biotechnology to resolve the formidable barrier of oxygen transferring rate (OTR), for bio-based fuels and chemical production process.

RESULTS

Based on systemic analysis of whole-cell catalysis in Gluconobacter oxydans, a novel sealed-oxygen supply technology was smartly designed and experimentally performed for biocatalytic oxidation of alcohols, sugars and so on. By a simple operation skill of automatic online supply of oxygen in a sealed stirring tank bioreactor of SOS, OTR barrier and foaming problem was resolved with great ease. We finally obtained ultrahigh-titer products of xylonic acid (XA), 3-hydroxypropionic acid (3-HPA), and erythrulose at 588.4 g/L, 69.4 g/L, and 364.7 g/L, respectively. Moreover, the volume productivity of three chemical products was improved by 150-250% compared with normal biotechnology. This SOS technology provides a promising approach to promote bioengineering competitiveness and advantages over chemical engineering.

CONCLUSION

SOS technology was demonstrated as an economic and universally applicable approach to bio-based fuels and chemicals production by whole-cell catalysis. The novel technology greatly promotes the competitiveness of bioengineering for chemical engineering, and provides a promising platform for the green and environmental use of biofuels.

Cost-practical of glycolic acid bioproduction by immobilized whole-cell catalysis accompanied with compressed oxygen supplied to enhance mass transfer

Bioprocess for Glycolic acid (GA) production from ethylene glycol by whole-cell catalysis of Gluconobacter oxydans is restrained by various biological impediments and high production costs. In this study, these limitations were subsided through the implementation of immobilized whole-cell bio-catalysis combined with increased oxygen supply. Results indicated that this strategy noticeably enhanced mass transfer efficiency, and prolonged cell life that significantly reduced the cost of biomass. Ultimately, with immobilized whole-cell catalysis in air-open and oxygen-open bioreactor, 41.3 and 66.9 g/L of GA was obtained within 48 h, with an increment of 62.0%. Additionally, in oxygen-compressed bioreactor, 63.3 g/L of GA was accumulated with the yield of 97.2%. Subsequently, 605.7 g of GA was produced after 10 rounds of recovery experiments. Although there was a slight decrease in GA production compared with pure-oxygen supply, production cost was reduced with limited oxygen supply. This strategy commendably demonstrated cost-practical bioprocess for GA production.

Electrodialytic bioproduction of xylonic acid in a bioreactor of supplied-oxygen intensification by using immobilized whole-cell Gluconobacter oxydans as biocatalyst

Immobilized whole-cell fermentation has been proven to be an effective method to improve the performance and cost-effectiveness of Gluconobacter oxydans ATCC 621. In the bio-oxidation of xylose to xylonic acid, the oxygen supply through the immobilized beads is a well-known factor that limits the biocatalytic performance of Gluconobacter oxydans as obligate aerobic bacteria. The activity of immobilized cells could be efficiently improved by execution of pressurized pure oxygen supply strategy. Subsequently, in order to further enhance the production efficiency of xylonic acid and reduce end-product inhibition, online-electrodialysis was employed. Finally, a design of pressurized oxygen supply bioreactor combining with online-electrodialysis was put forward for implementing successive production of xylonic acid. The central features of this a highly integrated design are feasible and thus might enable cost-competitive bacterial xylonic acid production.

Synergistic improvement of PQQ-dependent D-sorbitol dehydrogenase activity from Gluconobacter oxydans for the biosynthesis of miglitol precursor 6-(N-hydroxyethyl)-amino-6-deoxy-α-L-sorbofuranose

6-(N-hydroxyethyl) amino-6-deoxy-l-sorbofuranose (6NSL) is the direct precursor of miglitol for diabetes therapy. The regio- and stereo-selective dehydrogenation offered by the membrane-bound d-sorbitol dehydrogenase (mSLDH) from Gluconobacter oxydans provides an elegant enzymatic method for 6NSL production. In this study, two subunits sldA and sldB of mSLDH were introduced into G. oxydans ZJB-605, and the specific enzyme activity of mSLDH towards NHEG was enhanced by 2.15-fold. However, the endogenous PQQ level was dramatically reduced in the recombinant strain and became a bottleneck to support the holo-enzyme activity. A combined supplementation of four amino acids (Glu, Ile, Ser, Arg) involved in biosynthesis of PQQ in conventional media effectively increased extracellular accumulation of PQQ by 1.49-fold, which further enhanced mSLDH activity by 1.33-fold. The synergic improvement of mSLDH activity provided in this study supports the superior high dehydrogenate activity towards substrate N-2-hydroxyethyl-glucamine, 184.28 g·L^-1 of 6NSL was produced after a repeated bioconversion process catalyzed by the resting cells of G. oxydans/pBB-sldAB, all of which presenting a great potential of their industrial application in 6NSL biosynthesis.

Dihydroxyacetone production from glycerol using Gluconobacter oxydans: Study of medium composition and operational conditions in shaken flasks

The production of dihydroxyacetone from glycerol employing aerobic cultures of Gluconobacter oxydans is studied. Dihydroxyacetone is one of the most important value-added products obtained from glycerol, a by-product of biodiesel production. The effect of organic nitrogen source and initial substrate concentrations has been studied together with the possibility of product inhibition. Afterward, the influence of the main operating conditions (temperature, shaking speed, and initial biomass concentration) on in vivo glycerol dehydrogenase activity has also been considered. The results show no evidence of glycerol inhibition, but an important product inhibition was detected, which has been taken into account in a kinetic model for enzymatic activity description. In terms of operating conditions, pH was found to exert a great impact on glycerol conversion, being necessary to keep it above 4 to ensure complete glycerol conversion. The minimum temperature that maximized enzymatic activity was found to be 30°C. In addition, a surprising decoupling between biomass concentration and dihydroxyacetone production rate was observed when adding increasing nitrogen source concentrations at a fixed shaking speed. Glycerol dehydrogenase activity remains constant despite the increase in biomass concentration, contrary to what would be expected. This fact revealed the existence of a rate limiting factor, identified subsequently as oxygen transfer rate depending on the biomass concentration.

Investigations in sonication-induced intensification of crude glycerol fermentation to dihydroxyacetone by free and immobilized Gluconobacter oxydans

This study reports crude glycerol fermentation by G. oxydans for dihydroxyacetone (DHA) production, and intensification of fermentation with sonication. Fermentation was carried out using both free and immobilized cells (on polyurethane foam support) for initial glycerol concentrations of 20, 30 and 50 g/L. Sonication at 20% duty cycle enhanced glycerol consumption by 60-84% with no significant change in cell morphology. Lesser DHA yield in crude glycerol fermentation was attributed to possible formation of inhibitory products. Slight reduction in DHA yield for initial glycerol concentration of 50 g/L was attributed to substrate inhibition. Higher DHA productivity was obtained for immobilized cells. Circular dichroism analysis of intracellular proteins obtained from ultrasound-treated G. oxydans revealed significant reduction in α-helix and β-sheet content. These conformational changes in protein structure could augment activity of intracellular glycerol dehydrogenase, which is manifested in terms of enhanced metabolism of glycerol by G. oxydans.

Process optimization and analysis of product inhibition kinetics of crude glycerol fermentation for 1,3-Dihydroxyacetone production

In present study, statistical optimization of biodiesel-derived crude glycerol fermentation to DHA by immobilized G. oxydans cells over polyurethane foam is reported. Effect of DHA (product) inhibition on crude glycerol fermentation was analyzed using conventional biokinetic models and new model that accounts for both substrate and product inhibition. Optimum values of fermentation parameters were: pH=4.7, temperature=31°C, initial substrate concentration=20g/L. At optimum conditions, DHA yield was 89% (17.83g/L). Effect of product inhibition on fermentation was trivial for DHA concentrations ≤30g/L. At higher concentrations (≥50g/L), kinetics and yield of fermentation showed marked reduction with sharp drop in V_max and K_S values. Inhibition effect was more pronounced for immobilized cells due to restricted transport of fermentation mixture across polyurethane foam. Retention of fermentation mixture in immobilized matrix resulted in higher localized DHA concentration that possibly enhanced inhibition effect.

Repeated biotransformation of glycerol to 1,3-dihydroxyacetone by immobilized cells of Gluconobacter oxydans with glycerol- and urea-feeding strategy in a bubble column bioreactor

Some inorganic nitrogen sources and amino acids instead of yeast extract, which resulted in trouble of product purification, were introduced for 1,3-dihydroxyacetone (DHA) production by biotransformation with Gluconobacter oxydans. The results showed that urea is an optimal nitrogen source. Furthermore, the effects of glycerol- and urea-feeding strategies for DHA production by immobilized cells in a home-made bubble column bioreactor were optimized. Cells immobilization was prepared by cultivation in the bioreactor packed with porous ceramics, and then the broth was removed. Then, repeated biotransformation by continuous-feeding of glycerol and urea was developed. Up to 96.4±4.1g/L of average DHA concentration with 94.8±2.2% of average conversion rate of glycerol to DHA was achieved after 12 cycles of run. Near colorless DHA solution with few impurities was obtained and the production cost could be decreased.

Structural insights into stereospecific reduction of α, β-unsaturated carbonyl substrates by old yellow enzyme from Gluconobacter oxydans

We report the crystal structure of old yellow enzyme (OYE) family protein Gox0502 (a.a 1-315) in free form at 3.3 Å. Detailed structural analysis revealed the key residues involved in stereospecific determination of Gox0502, such as Trp66 and Trp100. Structure-based computational analysis suggested the bulky side chains of these tryptophan residues may play important roles in product stereoselectivity. The introduction of Ile or Phe or Tyr mutation significantly reduced the product diastereoselectivity. We hypothesized that less bulky side chains at these critical residues could create additional free space to accommodate intermediates with different conformations. Notably, the introduction of Phe mutation at residue Trp100 increased catalytic activity compared to wild-type Gox0502 toward a set of substrates tested, which suggests that a less bulky Phe side chain at residue W100F may facilitate product release. Therefore, Gox0502 structure could provide useful information to generate desirable OYEs suitable for biotechnological applications in industry.

Identification of a novel promoter gHp0169 for gene expression in Gluconobacter oxydans

Gluconobacter oxydans can perform rapid incomplete oxidation of many sugars, sugar polyols and alcohols, and this outstanding ability shows a great potential in industrial bioconversion. Improvements of these industrially important strains would boost their productivities of important metabolites. However, the shortage of molecular tools for homologous and heterologous gene expression has obviously hindered G. oxydans from further application. In this study, a putative promoter sequence (104bp), designated as gHp0169, was isolated and characterized from the chromosome of G. oxydans DSM 2003. Within this promoter sequence, the typical motif, known as -35 and -10 sequences with a 19-bp spacing, was found. The availability and promoter strength of promoter gHp0169 were then evaluated, by insertion into the plasmid pBBR1MCS5 for expression of a green fluorescent protein (GFP) and a membrane-bound type II NADH dehydrogenase (NDH-2) of G. oxydans. In comparison with promoter G. oxydans_tufB, gHp0169 exhibited a stronger promoter activity of NDH-2, indicating its significant value of gene expression in G. oxydans. To promote the production of 2-keto-d-gluconic acid (2-KGA) from gluconic acid (GA) gHp0169 was attempted to equip the flavin-dependent gluconate-2-dehydrogenase (GA2DH) and successfully achieved its overexpression in G. oxydans DSM 2003. As a result, the space-time yield of 2-KGA was boosted up to 29.86mM/h compared with 14.78mM/h for the control, which corresponded to a yield of 98.3% (84% for control).

Combining metabolic engineering and adaptive evolution to enhance the production of dihydroxyacetone from glycerol by Gluconobacter oxydans in a low-cost way

Gluconobacter oxydans can rapidly and effectively transform glycerol to dihydroxyacetone (DHA) by membrane-bound quinoprotein sorbitol dehydrogenase (mSLDH). Two mutant strains of GDHE Δadh pBBR-PtufBsldAB and GDHE Δadh pBBR-sldAB derived from the GDHE strain were constructed for the enhancement of DHA production. Growth performances of both strains were largely improved after adaptively growing in the medium with glucose as the sole carbon source. The resulting GAT and GAN strains exhibited better catalytic property than the GDHE strain in the presence of a high concentration of glycerol. All strains of GDHE, GAT and GAN cultivated on glucose showed enhanced catalytic capacity than those grown on sorbitol, indicating a favorable prospect of using glucose as carbon source to reduce the cost in industrial production. It was also the first time to reveal that the expression level of the sldAB gene in glucose-growing strains were higher than that of the strains cultivated on sorbitol.

Secondary Processing of Plant Oils

Glycerol has an important role to play in advanced oil crop biorefineries. Initially the chapter outlines processes which could be used to purify glycerol that emerges as a by-product from biodisel production. Glycerol in a clean form is in fact a highly valuable chemical and if such purification processes could be optimised, this approach could be a lucrative add-on for many biodisel manufacturers. However in order to conduct any chemical derivitisation, this purification process is a must anyway. There is a vast range of high value chemicals which can be produced and the chapter outlines key processes and products. Interestingly the novel concept of manufacturing biodiesel without any glycerol by-product is also discussed as an ultimate green process.

Use of glycerol for producing 1,3-dihydroxyacetone by Gluconobacter oxydans in an airlift bioreactor

1,3-Dihydroxyacetone can be produced by biotransformation of glycerol with glycerol dehydrogenase from Gluconobacter oxydans cells. Firstly, improvement the activity of glycerol dehydrogenase was carried out by medium optimization. The optimal medium for cell cultivation was composed of 5.6g/l yeast extract, 4.7 g/l glycerol, 42.1g/l mannitol, 0.5 g/l K(2)HPO(4), 0.5 g/l KH(2)PO(4), 0.1g/l MgSO(4)·7H(2)O, and 2.0 g/l CaCO(3) with the initial pH of 4.9. Secondly, an internal loop airlift bioreactor was applied for DHA production from glycerol by resting cells of G. oxydans ZJB09113. Furthermore, the effects of pH, aeration rate and cell content on DHA production and glycerol feeding strategy were investigated. 156.3 ± 7.8 g/l of maximal DHA concentration with 89.8±2.4% of conversion rate of glycerol to DHA was achieved after 72h of biotransformation using 10g/l resting cells at 30°C, pH 5.0 and 1.5vvm of aeration rate.

Biotransformation of glycerol to dihydroxyacetone by recombinant Gluconobacter oxydans DSM 2343

The genus Gluconobacter is well known for its rapid and incomplete oxidation of a wide range of substrates. Therefore, Gluconobacter oxydans especially is used for several biotechnological applications, e.g., the efficient oxidation of glycerol to dihydroxyacetone (DHA). For this reaction, G. oxydans is equipped with a membrane-bound glycerol dehydrogenase that is also described to oxidize sorbitol, gluconate, and arabitol. Here, we demonstrated the impact of sldAB overexpression on glycerol oxidation: Beside a beneficial effect on the transcript level of the sldB gene, the growth on glycerol as a carbon source was significantly improved in the overexpression strains (OD 2.8 to 2.9) compared to the control strains (OD 2.8 to 2.9). Furthermore, the DHA formation rate, as well as the final DHA concentration, was affected so that up to 350 mM of DHA was accumulated by the overexpression strains when 550 mM glycerol was supplied (control strain: 200 to 280 mM DHA). Finally, we investigated the effect on sldAB overexpression on the G. oxydans transcriptome and identified two genes involved in glycerol metabolism, as well as a regulator of the LysR family.