Product yield and by-product formation in glycerol conversion to dihydroxyacetone by Gluconobacter oxydans

Ketogluconate production by Gluconobacter strains: enzymes and biotechnological applications

Gluconobacter strains perform incomplete oxidation of various sugars and alcohols, employing regio- and stereoselective membrane-bound dehydrogenases oriented toward the periplasmic space. This oxidative fermentation process is utilized industrially. The ketogluconate production pathway, characteristic of these strains, begins with the conversion of d-glucose to d-gluconate, which then diverges and splits into 2 pathways producing 5-keto-d-gluconate and 2-keto-d-gluconate and subsequently 2,5-diketo-d-gluconate. These transformations are facilitated by membrane-bound d-glucose dehydrogenase, glycerol dehydrogenase, d-gluconate dehydrogenase, and 2-keto-d-gluconate dehydrogenase. The variance in end products across Gluconobacter strains stems from the diversity of enzymes and their activities. This review synthesizes biochemical and genetic knowledge with biotechnological applications, highlighting recent advances in metabolic engineering and the development of an efficient production process focusing on enzymes relevant to the ketogluconate production pathway in Gluconobacter strains.

Dihydroxyacetone production by glycerol oxidation under moderate condition using Pt loaded on La1-xBixOF solids

Pt/La_1-xBi_xOF/SBA-16 (SBA-16: Santa Barbara Amorphous no. 16) catalysts were prepared to produce dihydroxyacetone (DHA) from glycerol under moderate conditions. By using 7 wt% Pt/16 wt% La_0.95Bi_0.05OF/SBA-16, the DHA yield reached up to 78.4% (glycerol conversion: 100%) after reacting for 6 h at 30 °C in an atmospheric open-air system.

Production of glycerate from glucose using engineered Escherichiacoli

In this study, glycerate was produced from glucose using engineered Escherichia coli BW25113. Plasmid pSR3 carrying gpd1 and gpp2 encoding two isoforms of glycerol-3-phosphate dehydrogenase from Saccharomyces cerevisiae and plasmid pLB2 carrying aldO encoding alditol oxidase from Streptomyces violaceoruber were introduced into E. coli to enable the production of glycerate from glucose via glycerol. Disruptions of garK and glxK genes in the E. coli genome were performed to minimize the consumption of glycerate produced. As a result, E. coli carrying these plasmids could produce nearly three times higher concentration of glycerate (0.50 ± 0.01 g/L) from 10 g/L glucose compared to E. coli EG_2 (0.14 ± 0.02 g/L). In M9 medium, disruption of garK and glxK resulted in an impaired growth rate with low production of glycerate, while supplementation of 0.5 g/L casamino acids and 0.5 g/L manganese sulfate to the medium replenished the growth rate and elevated the glycerate titer. Further disruption of glpF, encoding a glycerol transporter, increased the glycerate production to 0.80 ± 0.00 g/L. MR2 medium improved the glycerate production titers and specific productivities of E. coli EG_4, EG_5, and EG_6. Upscale production of glycerate was carried out in a jar fermentor with MR2 medium using E. coli EG_6, resulting in an improvement in glycerate production up to 2.37 ± 0.46 g/L with specific productivity at 0.34 ± 0.11 g-glycerate/g-cells. These results indicate that E. coli is an appropriate host for glycerate production from glucose.

Heterologous expression of membrane-bound alcohol dehydrogenase-encoding genes for glyceric acid production using Gluconobacter sp. CHM43 and its derivatives

In contrast to D-glyceric acid (D-GA) production with 99% enantiomeric excess (ee) by Acetobacter tropicalis NBRC 16470, Gluconobacter sp. CHM43 produced 19.6 g L^-1 of D-GA with 73.7% ee over 4 days of incubation in flask culture. To investigate the reason for this enantiomeric composition of GA, the genes encoding membrane-bound alcohol dehydrogenase (mADH) of A. tropicalis NBRC 16470, composed of three subunits (adhA, adhB, and adhS), were cloned using the broad-host-range vector pBBR1MCS-2 and heterologously expressed in Gluconobacter sp. CHM43 and its ΔadhAB ΔsldBA derivative TORI4. Reverse-transcription quantitative real-time polymerase chain reaction demonstrated that adhABS genes from A. tropicalis were expressed in TORI4 transformants, and their membrane fraction exhibited mADH activities of 0.13 and 0.31 U/mg with or without AdhS, respectively. Compared with the GA production of TORI4-harboring pBBR1MCS-2 (1.23 g L^-1), TORI4 transformants expressing adhABS and adhAB showed elevated GA production of 2.46 and 3.67 g L^-1, respectively, suggesting a negative effect of adhS gene expression on GA production as well as mADH activity in TORI4. Although TORI4 was found to produce primarily L-GA with 42.5% ee, TORI4 transformants expressing adhABS and adhAB produced D-GA with 27.6% and 49.0% ee, respectively, demonstrating that mADH of A. tropicalis causes a sharp increase in the enantiomeric composition of D-GA. These results suggest that one reason for D-GA production with 73.7% ee in Gluconobacter spp. might be a property of the host, which possibly produces L-GA intracellularly. KEY POINTS: • Membrane-bound ADH from Acetobacter tropicalis showed activity in Gluconobacter sp. • D-GA production from glycerol was performed using recombinant Gluconobacter sp. • Enantiomeric excess of D-GA was affected by both membrane and intracellular ADHs.

Tuning product selectivity in the catalytic oxidation of glycerol by employing metal-ZSM-11 materials

Selective oxidation of glycerol towards dihidoxyacetone and lactic acid, employing micro/mesoporous zeolites with copper(ii) and chromium(iii).

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.

Effective improvement of the activity of membrane-bound alcohol dehydrogenase by overexpression of adhS in Gluconobacter oxydans

OBJECTIVES

To investigate the roles of adhS, which encodes the AdhS subunit of membrane-bound alcohol dehydrogenase (mADH) in Gluconobacter oxydans DSM2003, and to rationally improve mADH activity.

RESULTS

adhS was identified and overexpressed in G. oxydans DSM2003. Its overexpression promoted the AdhA subunit which serves as the primary dehydrogenase transfer from the periplasmic space to the periplasmic surface of the membrane thereby increasing the amount of active mADH and thus enhancing mADH activity up to 1.96-fold. The increased mADH activity significantly altered product selectivity (glyceric acid/dihydroxyacetone) during glycerol oxidation and increased the glyceric acid production by 7.6-fold. By comparison, overexpression of adhS and adhABS was equally effective in increasing the mADH activity and glyceric acid production.

CONCLUSIONS

adhS overexpression effectively improved mADH activity, indicating that for mADH, adhS might be a limiting component. The findings provide a guide for the efficient application of Gluconobacter spp. in hydroxy acid production.

Simultaneous Bioconversion of Xylose and Glycerol to Xylonic Acid and 1,3-Dihydroxyacetone from the Mixture of Pre-Hydrolysates and Ethanol-Fermented Waste Liquid by Gluconobacter oxydans

Simultaneous bioconversion of xylose and glycerol to xylonic acid and 1,3-dihydroxyacetone (DHA) was realized by using Gluconobacter oxydans (G. oxydans). Currently, the enzymatic hydrolysate to ethanol-fermented waste liquid and the inorganic acid pre-hydrolysate that contain abundant glycerol and xylose were difficult to be utilized or disposed. Based on the method of compressed oxygen supply-sealed and stirred tank reactor system (COS-SSTR), the xylonic acid and 1,3-dihydroxyacetone could be co-produced rapidly with the mixture of the dilute sulfuric acid pre-hydrolysate and ethanol-fermented waste liquid of enzymatic hydrolysate (MPEW) as material. By means of the system, we finally produced 102.3 ± 3.2 g/L xylonic acid and 40.6 ± 1.8 g/L 1,3-dihydroxyacetone at yield of 92.4 ± 2.8 % and 80.6 ± 3.5 % directly and simultaneously from the mixed solution. The central features of this bioprocess application would enable cost-competitive bacterial xylonic acid and 1,3-dihydroxyacetone production from lignocellulosic materials.

Draft Genome Sequence of Gluconobacter frateurii NBRC 103465, a Glyceric Acid-Producing Strain

Gluconobacter frateurii strain NBRC 103465 can efficiently produce glyceric acid (GA) from raw glycerol feedstock derived from biodiesel fuel production processes. Here, we report the 3.4-Mb draft genome sequence of G. frateurii NBRC 103465. The draft genome sequence can be applied to examine the enzymes and electron transport system involved in GA production.

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.

Disruption of the membrane-bound alcohol dehydrogenase-encoding gene improved glycerol use and dihydroxyacetone productivity in Gluconobacter oxydans

Dihydroxyacetone (DHA) production from glycerol by Gluconobacter oxydans is an industrial form of fermentation, but some problems exist related to microbial DHA production. For example, glycerol inhibits DHA production and affects its biological activity. G. oxydans produces both DHA and glyceric acid (GA) from glycerol simultaneously, and membrane-bound glycerol dehydrogenase and membrane-bound alcohol dehydrogenases are involved in the two reactions, respectively. We discovered that the G. oxydans mutant DeltaadhA, in which the membrane-bound alcohol dehydrogenase-encoding gene (adhA) was disrupted, significantly improved its ability to grow in a higher concentration of glycerol and to produce DHA compared to a wild-type strain. DeltaadhA grew on 220 g/l of initial glycerol and produced 125 g/l of DHA during a 3-d incubation, whereas the wild-type did not. Resting DeltaadhA cells converted 230 g/l of glycerol aqueous solution to 139.7 g/l of DHA during a 3-d incubation. The inhibitory effect of glycerate sodium salt on DeltaadhA was investigated. An increase in the glycerate concentration at the beginning of growth resulted in decreases in both growth and DHA production.

Microbial production of glyceric acid, an organic acid that can be mass produced from glycerol

Glyceric acid (GA), an unfamiliar biotechnological product, is currently produced as a small by-product of dihydroxyacetone production from glycerol by Gluconobacter oxydans. We developed a method for the efficient biotechnological production of GA as a target compound for new surplus glycerol applications in the biodiesel and oleochemical industries. We investigated the ability of 162 acetic acid bacterial strains to produce GA from glycerol and found that the patterns of productivity and enantiomeric GA compositions obtained from several strains differed significantly. The growth parameters of two different strain types, Gluconobacter frateurii NBRC103465 and Acetobacter tropicalis NBRC16470, were optimized using a jar fermentor. G. frateurii accumulated 136.5 g/liter of GA with a 72% d-GA enantiomeric excess (ee) in the culture broth, whereas A. tropicalis produced 101.8 g/liter of d-GA with a 99% ee. The 136.5 g/liter of glycerate in the culture broth was concentrated to 236.5 g/liter by desalting electrodialysis during the 140-min operating time, and then, from 50 ml of the concentrated solution, 9.35 g of GA calcium salt was obtained by crystallization. Gene disruption analysis using G. oxydans IFO12528 revealed that the membrane-bound alcohol dehydrogenase (mADH)-encoding gene (adhA) is required for GA production, and purified mADH from G. oxydans IFO12528 catalyzed the oxidation of glycerol. These results strongly suggest that mADH is involved in GA production by acetic acid bacteria. We propose that GA is potentially mass producible from glycerol feedstock by a biotechnological process.

Production of glyceric acid by Gluconobacter sp. NBRC3259 using raw glycerol

Gluconobacter sp. NBRC3259 converted glycerol to glyceric acid (GA). The enantiomeric composition of the GA produced was a mixture of DL-forms with a 77% enantiomeric excess of D-GA. After culture conditions, such as initial glycerol concentration, types and amounts of nitrogen sources, and initial pH, were optimized, Gluconobacter sp. NBRC3259 produced 54.7 g/l of GA as well as 33.7 g/l of dihydroxyacetone (DHA) from 167 g/l of glycerol during 4 d of incubation in a jar fermentor with pH control. GA production from raw glycerol samples, the main by-product of the transesterification process in the biodiesel production and oleochemical industries, was also evaluated after proper pretreatment of the samples. Using a raw glycerol sample with activated charcoal pretreatment, 45.9 g/l of GA and 28.2 g/l of DHA were produced from 174 g/l of glycerol.

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.

Study of the inhibitory effect of the product dihydroxyacetone on Gluconobacter oxydans in a semi-continuous two-stage repeated-fed-batch process

The influence of the product inhibition by dihydroxyacetone (DHA) on Gluconobacter oxydans for a novel semi-continuous two-stage repeated-fed-batch process was examined quantitatively. It was shown that the culture was able to grow up to a DHA concentration of 80 kg m(-3) without any influence of product inhibition. The regeneration capability of the reversibly product inhibited culture from a laboratory-scale bioreactor system was observed up to a DHA concentration of about 160 kg m(-3). At higher DHA concentrations, the culture was irreversibly product inhibited. However, due to the robust membrane-bound glycerol dehydrogenase of G. oxydans, product formation was still active for a prolonged period of time. The reachable maximum final DHA concentration was as high as 220 kg m(-3). The lag phases for growth increased exponentially with increasing DHA threshold values of the first reactor stage. These results correlated well with fluorescence in situ hybridization (FISH) measurements confirming that the number of active cells decreased exponentially with increasing DHA concentrations.

Monitoring of dihydroxyacetone production during oxidation of glycerol by immobilized Gluconobacter oxydans cells with an enzyme biosensor

A bi-enzymatic biosensor for monitoring of dihydroxyacetone production during oxidation of glycerol by bacterial cells of Gluconobacter oxydans is presented. Galactose oxidase oxidizes dihydroxyacetone efficiently producing hydrogen peroxide, which reacts with co-immobilized peroxidase and ferrocene pre-adsorbed on graphite electrode. This mediator-based bi-enzymatic biosensor possesses very high sensitivity (4.7 µA/mM in phosphate buffer), low detection limit (0.8 µM, signal/noise = 3), short response time (22 s, 95% of steady-state) and broad linear range (0.002-0.55 mM in phosphate buffer). The effect of pH, temperature, type of buffer, as well as different stabilizers (combinations of a polyelectrolyte and a polyol) on the sensor performance were carefully optimized and discussed. Dihydroxyacetone produced during a batch conversion of glycerol by the pectate-immobilized bacteria in an air-lift reactor was determined by the biosensor and by reference spectrophotometric method. Both methods were compared and were in a very good correlation. The main advantage of the biosensor is a very short time needed for sample analysis (less than 1 min).