Improving the performance of cell biocatalysis and the productivity of acetoin from 2,3-butanediol using a compressed oxygen supply

Sequential bioaugmentation of the dominant microorganisms to improve the fermentation and flavor of cereal vinegar

Traditional cereal vinegars are fermented by microorganisms that are spontaneously enriched, leading to uncertainty in regulating the fermentation process and flavor. The objective of this study was to elucidate the impact of the predominant microorganisms, provenly Lactobacillus helveticus and Acetobacter pasteurianus, on the solid-state fermentation (SSF) and flavor profile of cereal vinegar by several bioaugmentation strategies. The results indicated that the sequential bioaugmentation of predominant microorganisms improved the utilization of raw material and most key flavor compounds. Through sequential bioaugmentation strategy, bacterial diversity was regulated due to minimizing acetic acid inhibition in the early stages, and the non-volatile acid was targetedly improved by Lactobacillus. Furthermore, the important flavor of non-volatile acid, esters, acetoin, and tetramethyl-pyrazine content was enhanced by sequential bioaugmentation. Therefore, the sensory score on taste and odor were improved. These results provide a reference for the targeted regulation of the SSF and the flavor quality of cereal vinegar.

Design of a synthetic enzyme cascade for the in vitro fixation of formaldehyde to acetoin

Formaldehyde (FALD) has gained prominence as an essential C1 building block in the synthesis of valuable chemicals. However, there are still challenges in converting FALD into commodities. Recently, cell-free biocatalysis has emerged as a popular approach for producing such commodities. Acetoin, also known as 3-hydroxy-2-butanone, has been widely used in food, cosmetic, agricultural and the chemical industry. It is valuable to develop a process to produce acetoin from FALD. In this study, a cell-free multi-enzyme catalytic system for the production of acetoin using FALD as the substrate was designed and constructed. It included three scales: FALD utilization pathway, glycolysis pathway and acetoin synthesis pathway. After the optimization of the reaction system, 20.17 mM acetoin was produced from 122 mM FALD, with a yield of 0.165 mol/mol, reaching 99.0% of the theoretical yield. The pathway provides a new approach for high-yield acetoin production from FALD, which consolidates the foundation for the production of high value-added chemicals using cheap one-carbon compounds.

Efficient production of acetoin from lactate by engineered Escherichia coli whole-cell biocatalyst

Acetoin, an important and high-value added bio-based platform chemical, has been widely applied in fields of foods, cosmetics, chemical synthesis, and agriculture. Lactate is a significant intermediate short-chain carboxylate in the anaerobic breakdown of carbohydrates that comprise ~ 18% and ~ 70% in municipal wastewaters and some food processing wastewaters, respectively. In this work, a series of engineered Escherichia coli strains were constructed for efficient production of acetoin from cheaper and abundant lactate through heterogenous co-expression of fusion protein (α-acetolactate synthetase and α-acetolactate decarboxylase), lactate dehydrogenase and NADH oxidase, and blocking acetate synthesis pathways. After optimization of whole-cell bioconversion conditions, the engineered strain BL-11 produced 251.97 mM (22.20 g/L) acetoin with a yield of 0.434 mol/mol in shake flasks. Moreover, a titer of 648.97mM (57.18 g/L) acetoin was obtained in 30 h with a yield of 0.484 mol/mol lactic acid in a 1-L bioreactor. To the best of our knowledge, this is the first report on the production of acetoin from renewable lactate through whole-cell bioconversion with both high titer and yield, which demonstrates the economy and efficiency of acetoin production from lactate. Key Points • The lactate dehydrogenases from different organisms were expressed, purified, and assayed. • It is the first time that acetoin was produced from lactate by whole-cell biocatalysis. • The highest titer of 57.18 g/L acetoin was obtained with high theoretical yield in a 1-L bioreactor.

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.

Oxidative Fermentation of Acetic Acid Bacteria and Its Products

Acetic acid bacteria (AAB) are a group of Gram-negative, strictly aerobic bacteria, including 19 reported genera until 2021, which are widely found on the surface of flowers and fruits, or in traditionally fermented products. Many AAB strains have the great abilities to incompletely oxidize a large variety of carbohydrates, alcohols and related compounds to the corresponding products mainly including acetic acid, gluconic acid, gulonic acid, galactonic acid, sorbose, dihydroxyacetone and miglitol via the membrane-binding dehydrogenases, which is termed as AAB oxidative fermentation (AOF). Up to now, at least 86 AOF products have been reported in the literatures, but no any monograph or review of them has been published. In this review, at first, we briefly introduce the classification progress of AAB due to the rapid changes of AAB classification in recent years, then systematically describe the enzymes involved in AOF and classify the AOF products. Finally, we summarize the application of molecular biology technologies in AOF researches.

Advances in biological production of acetoin: a comprehensive overview

Acetoin, a high-value-added bio-based platform chemical, is widely used in foods, cosmetics, agriculture, and the chemical industry. It is an important precursor for the synthesis of: 2,3-butanediol, liquid hydrocarbon fuels and heterocyclic compounds. Since the fossil resources are becoming increasingly scarce, biological production of acetoin has received increasing attention as an alternative to chemical synthesis. Although there are excellent reviews on the: application, catabolism and fermentative production of acetoin, little attention has been paid to acetoin production via: electrode-assisted fermentation, whole-cell biocatalysis, and in vitro/cell-free biocatalysis. In this review, acetoin biosynthesis pathways and relevant key enzymes are firstly reviewed. In addition, various strategies for biological acetoin production are summarized including: cell-free biocatalysis, whole-cell biocatalysis, microbial fermentation, and electrode-assisted fermentation. The advantages and disadvantages of the different approaches are discussed and weighed, illustrating the increasing progress toward economical, green and efficient production of acetoin. Additionally, recent advances in acetoin extraction and recovery in downstream processing are also briefly reviewed. Moreover, the current issues and future prospects of diverse strategies for biological acetoin production are discussed, with the hope of realizing the promises of industrial acetoin biomanufacturing in the near future.

Prospects on bio-based 2,3-butanediol and acetoin production: Recent progress and advances

The bio-based platform chemicals 2,3-butanediol (BDO) and acetoin have various applications in chemical, cosmetics, food, agriculture, and pharmaceutical industries, whereas the derivatives of BDO could be used as fuel additives, polymer and synthetic rubber production. This review summarizes the novel technological developments in adapting genetic and metabolic engineering strategies for selection and construction of chassis strains for BDO and acetoin production. The valorization of renewable feedstocks and bioprocess development for the upstream and downstream stages of bio-based BDO and acetoin production are discussed. The techno-economic aspects evaluating the viability and industrial potential of bio-based BDO production are presented. The commercialization of bio-based BDO and acetoin production requires the utilization of crude renewable resources, the chassis strains with high fermentation production efficiencies and development of sustainable purification or conversion technologies.

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%.

Engineering central pathways for industrial-level (3R)-acetoin biosynthesis in Corynebacterium glutamicum

BACKGROUND

Acetoin, especially the optically pure (3S)- or (3R)-enantiomer, is a high-value-added bio-based platform chemical and important potential pharmaceutical intermediate. Over the past decades, intense efforts have been devoted to the production of acetoin through green biotechniques. However, efficient and economical methods for the production of optically pure acetoin enantiomers are rarely reported. Previously, we systematically engineered the GRAS microorganism Corynebacterium glutamicum to efficiently produce (3R)-acetoin from glucose. Nevertheless, its yield and average productivity were still unsatisfactory for industrial bioprocesses.

RESULTS

In this study, cellular carbon fluxes in the acetoin producer CGR6 were further redirected toward acetoin synthesis using several metabolic engineering strategies, including blocking anaplerotic pathways, attenuating key genes of the TCA cycle and integrating additional copies of the alsSD operon into the genome. Among them, the combination of attenuation of citrate synthase and inactivation of phosphoenolpyruvate carboxylase showed a significant synergistic effect on acetoin production. Finally, the optimal engineered strain CGS11 produced a titer of 102.45 g/L acetoin with a yield of 0.419 g/g glucose at a rate of 1.86 g/L/h in a 5 L fermenter. The optical purity of the resulting (3R)-acetoin surpassed 95%.

CONCLUSION

To the best of our knowledge, this is the highest titer of highly enantiomerically enriched (3R)-acetoin, together with a competitive product yield and productivity, achieved in a simple, green processes without expensive additives or substrates. This process therefore opens the possibility to achieve easy, efficient, economical and environmentally-friendly production of (3R)-acetoin via microbial fermentation in the near future.

One-pot efficient biosynthesis of (3R)-acetoin from pyruvate by a two-enzyme cascade

Opening the possibility of sustainable industrial (3R)-acetoin biomanufacturing in vitro.

Integrated process for scalable bioproduction of glycolic acid from cell catalysis of ethylene glycol

Glycolic acid (GA) is presently booming as a versatile raw material in the fields of high-grade cosmetics, polymer degradable materials, and drug production. The biocatalysis of ethylene glycol (EG) to GA is promising, with environmentally friendly benefits, while the effective and straight bioproduction of GA qualified for polymer synthesis purity is a challenge. In this study, we combine whole cell catalysis step and acidification-purification step. A compressed oxygen supply in the sealed aerated stirred tank reaction (COS-SSTR) and a weak basic anion-exchange resins were integrated to develop an efficient process of GA bioproduction from EG. Finally, 110.5 g/L of GA was obtained at the yield of 94.4% and the volume productivity of 2.3 g/L/h in 48 h that presently is the greatest level for GA bioproduction. After 335 resins treatment of 5.0 L catalyzed broth containing 497.2 g EG, we obtained 575.4 g GA at the recovery rate of 98.9%.