Unique glucose oxidation catalysis of Gluconobacter oxydans constitutes an efficient cellulosic gluconic acid fermentation free of inhibitory compounds disturbance

Continuous catalytic production of 1,3-dihydroxyacetone: Sustainable approach combining perfusion cultures and immobilized cells

Currently, the predominant method for the industrial production of 1,3-dihydroxyacetone (DHA) from glycerol involves fed-batch fermentation. However, previous research has revealed that in the biocatalytic synthesis of DHA from glycerol, when the DHA concentration exceeded 50 g·L-1, it significantly inhibited microbial growth and metabolism, posing a challenge in maintaining prolonged and efficient catalytic production of DHA. In this study, a new integrated continuous production and synchronous separation (ICSS) system was constructed using hollow fiber columns and perfusion culture technology. Additionally, a cell reactivation technique was implemented to extend the biocatalytic ability of cells. Compared with fed-batch fermentation, the ICSS system operated for 360 h, yielding a total DHA of 1237.8 ± 15.8 g. The glycerol conversion rate reached 97.7 %, with a productivity of 3.44 g·L-1·h-1, representing 485.0 % increase in DHA production. ICSS system exhibited strong operational characteristics and excellent performance, indicating significant potential for applications in industrial bioprocesses.

Efficient production of 2-keto-L-gulonic acid from D-glucose in Gluconobacter oxydans ATCC9937 by mining key enzyme and transporter

Direct production of 2-keto-L-gulonic acid (2-KLG, the precursor of vitamin C) from D-glucose through 2,5-diketo-D-gluconic acid (2,5-DKG) is a promising alternative route. To explore the pathway of producing 2-KLG from D-glucose, Gluconobacter oxydans ATCC9937 was selected as a chassis strain. It was found that the chassis strain naturally has the ability to synthesize 2-KLG from D-glucose, and a new 2,5-DKG reductase (DKGR) was found on its genome. Several major issues limiting production were identified, including the insufficient catalytic capacity of DKGR, poor transmembrane movement of 2,5-DKG and imbalanced D-glucose consumption flux inside and outside of the host strain cells. By identifying novel DKGR and 2,5-DKG transporter, the whole 2-KLG biosynthesis pathway was systematically enhanced by balancing intracellular and extracellular D-glucose metabolic flux. The engineered strain produced 30.5 g/L 2-KLG with a conversion ratio of 39.0%. The results pave the way for a more economical large-scale fermentation process for vitamin C.

Screening of Gluconobacter oxydans in xylonic acid fermentation for tolerance of the inhibitors formed dilute acid pretreatment

Pre-hydrolysate liquor, as an inevitable by-product, contains a large amount of xylose, and is therefore an inexpensive feedstock that can be upgraded to value-added chemical xylonic acid. However, inhibitors, simultaneously formed in lignocellulose pretreatment process, are regarded as the major obstacle for effectively bio-converting xylose in pre-hydrolysate into xylonic acid. In this study, Gluconobacter oxydans, with highly selective and efficient, was employed for xylonic acid production; the impacts of five typical toxic inhibitory compounds on xylonic acid productivity and the activity of the membrane-bound dehydrogenase were evaluated. The results revealed that the inhibitors showed different degrees of influence toward xylonic acid production, and the order of inhibitory effect for acidic inhibitors was formic acid > acetic acid > levulinic acid; the inhibitory effect of aldehyde inhibitors was furfural > 5-hydroxymethyl-furfural. This study provides an important basis of metabolic modification and detoxification process for enhancing inhibitor tolerance and xylonic acid productivity.

Identification of a Novel Dehydrogenase from Gluconobacter oxydans for Degradation of Inhibitors Derived from Lignocellulosic Biomass

Inhibitors from lignocellulosic biomass have become the bottleneck of biorefinery development. Gluconobacter oxydans DSM2003 showed a high performance of inhibitors degradation, which had a short lag time in non-detoxified corn stover hydrolysate and could convert 90% of aldehyde inhibitors to weaker toxic acids. In this study, an aldehyde dehydrogenase gene W826-RS0111485, which plays an important function in the conversion of aldehyde inhibitors in Gluconobacter oxydans DSM2003, was identified. W826-RS0111485 was found by protein profiling, then a series of enzymatic properties were determined and were heterologously expressed in E. coli. The results indicated that NADP is the most suitable cofactor of the enzyme when aldehyde inhibitor is the substrate, and it had the highest oxidation activity to furfural among several aldehyde inhibitors. Under the optimal reaction conditions (50 °C, pH 7.5), the Km and Vmax of the enzyme under furfural stress were 2.45 and 80.97, respectively, and the Kcat was 232.22 min−1. The biodetoxification performance experiments showed that the recombinant E. coli containing the target gene completely converted 1 g/L furfural to furoic acid within 8 h, while the control E. coli only converted 18% furfural within 8 h. It was further demonstrated that W826-RS0111485 played an important role in the detoxification of furfural. The mining of this inhibitor degradation gene could provide a theoretical basis for rational modification of industrial strains to enhance its capacity of inhibitor degradation in the future.

New perspectives into Gluconobacter-catalysed biotransformations

Different from other aerobic microorganisms that oxidise carbon sources to water and carbon dioxide, Gluconobacter catalyses the incomplete oxidation of various substrates with regio- and stereoselectivity. This ability, as well as its capacity to release the resulting products into the reaction media, place Gluconobacter as a privileged member of a non-model microorganism class that may boost industrial biotechnology. Knowledge of new technologies applied to Gluconobacter has been piling up in recent years. Advancements in its genetic modification, application of immobilisation tools and careful designs of the transformations, have improved productivities and stabilities of Gluconobacter strains or enabled new bioconversions for the production of valuable marketable chemicals. In this work, the latest advancements applied to Gluconobacter-catalysed biotransformations are summarised with a special focus on recent available tools to improve them. From genetic and metabolic engineering to bioreactor design, the most recent works on the topic are analysed in depth to provide a comprehensive resource not only for scientists and technologists working on/with Gluconobacter, but for the general biotechnologist.

Low pH Stress Enhances Gluconic Acid Accumulation with Enzymatic Hydrolysate as Feedstock Using Gluconobacter oxydans

Gluconic acid has been increasingly in demand in recent years due to the wide applications in the food, healthcare and construction industries. Plant-derived biomass is rich in biopolymers that comprise glucose as the monomeric unit, which provide abundant feedstock for gluconic acid production. Gluconobacter oxydans can rapidly and incompletely oxidize glucose to gluconic acid and it is regarded as ideal industrial microorganism. Once glucose is depleted, the gluconic acid will be further bio-oxidized to 2-ketogluconic acid by Gluconobacter oxydans. The endpoint is difficult to be controlled, especially in an industrial fermentation process. In this study, it was found that the low pH environment (2.5~3.5) could limit the further metabolism of gluconic acid and that it resulted in a yield over 95%. Therefore, the low pH stress strategy for efficiently producing gluconic acid from biomass-derived glucose was put forward and investigated with enzymatic hydrolysate. As a result, 98.8 g/L gluconic acid with a yield of 96% could be obtained from concentrated corncob enzymatic hydrolysate that initially contained 100 g/L glucose with 1.4 g/L cells loading of Gluconobacter oxydans. In addition, the low pH stress strategy could effectively control end-point and decrease the risk of microbial contamination. Overall, this strategy provides a potential for industrial gluconic acid production from lignocellulosic materials.

Efficient biosynthesis of (R)-mandelic acid from styrene oxide by an adaptive evolutionary Gluconobacter oxydans STA

BACKGROUND

(R)-mandelic acid (R-MA) is a highly valuable hydroxyl acid in the pharmaceutical industry. However, biosynthesis of optically pure R-MA remains significant challenges, including the lack of suitable catalysts and high toxicity to host strains. Adaptive laboratory evolution (ALE) was a promising and powerful strategy to obtain specially evolved strains.

RESULTS

Herein, we report a new cell factory of the Gluconobacter oxydans to biocatalytic styrene oxide into R-MA by utilizing the G. oxydans endogenous efficiently incomplete oxidization and the epoxide hydrolase (SpEH) heterologous expressed in G. oxydans. With a new screened strong endogenous promoter P₁₂₇₈₀, the production of R-MA was improved to 10.26 g/L compared to 7.36 g/L of using P_lac. As R-MA showed great inhibition for the reaction and toxicity to cell growth, adaptive laboratory evolution (ALE) strategy was introduced to improve the cellular R-MA tolerance. The adapted strain that can tolerate 6 g/L R-MA was isolated (named G. oxydans STA), while the wild-type strain cannot grow under this stress. The conversion rate was increased from 0.366 g/L/h of wild type to 0.703 g/L/h by the recombinant STA, and the final R-MA titer reached 14.06 g/L. Whole-genome sequencing revealed multiple gene-mutations in STA, in combination with transcriptome analysis under R-MA stress condition, we identified five critical genes that were associated with R-MA tolerance, among which AcrA overexpression could further improve R-MA titer to 15.70 g/L, the highest titer reported from bulk styrene oxide substrate.

CONCLUSIONS

The microbial engineering with systematic combination of static regulation, ALE, and transcriptome analysis strategy provides valuable solutions for high-efficient chemical biosynthesis, and our evolved G. oxydans would be better to serve as a chassis cell for hydroxyl acid production.

Oxygen mass transfer enhancement by activated carbon particles in xylose fermentation media

In this work, the effect of activated carbon particles on the production of xylonic acid from xylose by Gluconobacter oxydans in a stirred tank bioreactor was investigated. The enhancement of the oxygen transfer coefficient by activated carbon particles was experimentally evaluated under different solids volume fractions, agitation and aeration rates conditions. The experimental conditions optimized by response surface methodology (agitation speed 800 rpm, aeration rate 7 L min^-1, and activated carbon 0.002%) showed a maximum oxygen transfer coefficient of 520.7 h^-1, 40.4% higher than the control runs without activated carbon particles. Under the maximum oxygen transfer coefficient condition, the xylonic acid titer reached 108.2 g/L with a volumetric productivity of 13.53 g L^-1 h^-1 and a specific productivity of 6.52 g/g_x/h. In conclusion, the addition of activated carbon particles effectively enhanced the oxygen mass transfer rate. These results demonstrate that activated carbon particles enhanced cultivation for xylonic acid production an inexpensive and attractive alternative.

A smart self-balancing biosystem with reversible competitive adsorption of in-situ anion exchange resin for whole-cell catalysis preparation of lignocellulosic xylonic acid

Xylonic acid (XA) bioproduction via whole-cell catalysis of Gluconobacter oxydans is a promising strategy for xylose bioconversion, which is hindered by inhibitor formation during lignocellulosic hydrolysates. Therefore, it is important to develop a catalytic system that can directly utilize hydrolysate and efficiently produce XA. Determination of the dynamic adsorption characteristics of 335 anion exchange resin resulted in a unique and interesting reversible competitive adsorption between acetic acid-like bioinhibitor, fermentable sugar and XA. Xylose in crude lignocellulosic hydrolysates was completely oxidized to 52.52 g/L XA in unprecedented self-balancing biological system through reversible competition. The obtained results showed that in-situ resin adsorption significantly affected the direct utilization of crude lignocellulosic hydrolysate for XA bioproduction (p ≤ 0.05). In addition, the resin adsorbed ca. 90 % of XA during bioconversion. The study achieved a multiple functions and integrated system, "detoxification, neutralization and product separation" for one-pot bioreaction of lignocellulosic hydrolysate.

A consolidated review of commercial-scale high-value products from lignocellulosic biomass

For decades, lignocellulosic biomass has been introduced to the public as the most important raw material for the environmentally and economically sustainable production of high-valued bioproducts by microorganisms. However, due to the strong recalcitrant structure, the lignocellulosic materials have major limitations to obtain fermentable sugars for transformation into value-added products, e.g., bioethanol, biobutanol, biohydrogen, etc. In this review, we analyzed the recent trends in bioenergy production from pretreated lignocellulose, with special attention to the new strategies for overcoming pretreatment barriers. In addition, persistent challenges in developing for low-cost advanced processing technologies are also pointed out, illustrating new approaches to addressing the global energy crisis and climate change caused by the use of fossil fuels. The insights given in this study will enable a better understanding of current processes and facilitate further development on lignocellulosic bioenergy production.

Efficient Production of 2,5-Diketo-D-gluconic Acid by Reducing Browning Levels During Gluconobacter oxydans ATCC 9937 Fermentation

D-Glucose directly generates 2-keto-L-gulonic acid (2-KLG, precursor of vitamin C) through the 2,5-diketo-D-gluconic acid (2,5-DKG) pathway. 2,5-DKG is the main rate-limiting factor of the reaction, and there are few relevant studies on it. In this study, a more accurate quantitative method of 2,5-DKG was developed and used to screen G. oxydans ATCC9937 as the chassis strain for the production of 2,5-DKG. Combining the metabolite profile analysis and knockout and overexpression of production strain, the non-enzymatic browning of 2,5-DKG was identified as the main factor leading to low yield of the target compound. By optimizing the fermentation process, the fermentation time was reduced to 48 h, and 2,5-DKG production peaked at 50.9 g/L, which was 139.02% higher than in the control group. Effectively eliminating browning and reducing the degradation of 2,5-DKG will help increase the conversion of 2,5-DKG to 2-KLG, and finally, establish a one-step D-glucose to 2-KLG fermentation pathway.

The industrial versatility of Gluconobacter oxydans: current applications and future perspectives

Gluconobacter oxydans is a well-known acetic acid bacterium that has long been applied in the biotechnological industry. Its extraordinary capacity to oxidize a variety of sugars, polyols, and alcohols into acids, aldehydes, and ketones is advantageous for the production of valuable compounds. Relevant G. oxydans industrial applications are in the manufacture of L-ascorbic acid (vitamin C), miglitol, gluconic acid and its derivatives, and dihydroxyacetone. Increasing efforts on improving these processes have been made in the last few years, especially by applying metabolic engineering. Thereby, a series of genes have been targeted to construct powerful recombinant strains to be used in optimized fermentation. Furthermore, low-cost feedstocks, mostly agro-industrial wastes or byproducts, have been investigated, to reduce processing costs and improve the sustainability of G. oxydans bioprocess. Nonetheless, further research is required mainly to make these raw materials feasible at the industrial scale. The current shortage of suitable genetic tools for metabolic engineering modifications in G. oxydans is another challenge to be overcome. This paper aims to give an overview of the most relevant industrial G. oxydans processes and the current strategies developed for their improvement.

Improvement of pyrroloquinoline quinone-dependent d-sorbitol dehydrogenase activity from Gluconobacter oxydans via expression of Vitreoscilla hemoglobin and regulation of dissolved oxygen tension for the biosynthesis of 6-(N-hydroxyethyl)-amino-6-deoxy-α-l-sorbofuranose

The miglitol intermediate, 6-(N-hydroxyethyl)-amino-6-deoxy-α-l-sorbofuranose (6NSL), is catalyzed from N-2-hydroxyethyl glucamine (NHEG) by resting cells of Gluconobacter oxydans. One of the key factors limiting 6NSL production was the availability of oxygen during both cell cultivation and biotransformation of NHEG to 6NSL. Based on G. oxydans/pBBR1-sldAB-pqqABCDE-tldD (G. oxydans/AB-PQQ), the Vitreoscilla hemoglobin (VHb) was heterologously expressed in G. oxydans to enhance oxygen transfer efficiency and improve 6NSL production. The recombinant G. oxydans/AB-PQQ-VHb displayed higher biomass and NHEG oxidation activity than the control stain. The transcription levels of respiratory chain-related enzyme genes in G. oxydans/AB-PQQ-VHb exhibited up-regulation, indicating that the presence of VHb promoted the respiration. The dissolved oxygen (DO) concentration for cell cultivation was optimized in a 5-L stirred bioreactor. At a DO concentration of 20%, the maximum volumetric oxidation activity of NHEG of G. oxydans/AB-PQQ-VHb in the stirred bioreactor reached 168.3 ± 3.2 U/L. Furthermore, the biotransformation of NHEG to 6NSL using G. oxydans/AB-PQQ-VHb was carried out under different oxygen tensions to investigate the effect of oxygen on 6NSL production. Finally, up to 87.5 ± 5.9 g/L 6NSL was accumulated in the reaction mixture within 16 h when the DO was controlled at 30%.

Effects of external parameters and mass-transfer on the glucose oxidation process catalyzed by Pd–Bi/Al2O3

This work describes the effect of reaction parameters on glucose oxidation in the presence of a Pd3 : Bi1/Al2O3 catalyst.

Combinational expression of D-sorbitol dehydrogenase and pyrroloquinoline quinone increases 6-(N-hydroxyethyl)-amino-6-deoxy-α-L-sorbofuranose production by Gluconobacter oxydans through cofactor manipulation

6-(N-hydroxyethyl)-amino-6-deoxy-l-sorbofuranose (6NSL), a key precursor in the synthesis of miglitol, is produced from N-2-hydroxyethyl-glucamine (NHEG) by the regioselective oxidation of Gluconobacter oxydans. The limitation of PQQ biosynthesis became a bottleneck for improvement of PQQ-dependent D-sorbitol dehydrogenase (mSLDH) activity. Five expression plasmids were constructed for the co-expression of the pqqABCDE gene cluster and the tldD gene on the basis of pBBR1-gHp0169-sldAB in G. oxydans to increase the biosynthesis of PQQ. The G. oxydans/pGA004, in which pqqABCDE and tldD were expressed as a cluster under the control of gHp0169 promoter, showed the optimal performance. The intracellular PQQ concentration and specific activity of mSLDH in cells increased by 79.3 % and 53.7 %, respectively, compared to that in G. oxydans/pBBR-sldAB. Then, the repeated batch biotransformation of NHEG to 6NSL by G. oxydans/pGA004 was carried out. Up to 75.0 ± 3.0 g/L of 6NSL production with 94.5 ± 3.6 % of average conversion rate of NHEG to 6NSL was achieved after four cycles of run. These results indicated that G. oxydans/pGA004 with high productivity had great potential for 6NSL production in industrial bioprocess.

Adaptive evolution of Gluconobacter oxydans accelerates the conversion rate of non-glucose sugars derived from lignocellulose biomass

Gluconobacter oxydans is capable of oxidizing various lignocellulose derived sugars into the corresponding sugar acids including glucose, xylose, arabinose, galactose and mannose, but simultaneous utilization of these sugars is difficult. This study attempted an adaptive evolution of G. oxydans by alternate transfer in inhibitors containing hydrolysate and inhibitors free hydrolysate for intensifying sugars simultaneous utilization. After 420 days' continuous culture, the conversion rate of all non-glucose sugars significantly improved by several folds and achieved complete conversion of lignocellulose-derived sugars to the corresponding sugar acids. The significant up-regulation of mGDH gene in the adapted G. oxydans strain (more than 40-fold greater than the parental) was considered as the decisive factor for the improvement of strain performance. This evolution adaptation strategy also could be used to accelerate robust sugars utilization for other fermented strains in lignocellulose biorefinery.