Identification of a lactose-oxidizing enzyme in Escherichia coli and improvement of lactobionic acid production by recombinant expression of a quinoprotein glucose dehydrogenase from Pseudomonas taetrolens

Quantitative Analysis of Lactobionic Acid in Bioreactor Cultures and Selected Biological Activities

The aim of this study was to quantitatively analyse lactobionic acid obtained from bioreactor cultures using whey as a liquid medium with bacteria of the Pseudomonas taetrolens species. The most important culture parameters affecting the production of the acid are indicated and evaluated. The highest lactobionic acid yield was 37.42 g/L, selecting the appropriate strain (Pseudomonas taetrolens 4') and environmental conditions (2% lactose concentration in the bioreactor). The amount of lactose and lactobionic acid was determined by high-performance liquid chromatography. Microorganism analysis was also carried out using a flow cytometer with imaging to study the metabolic activity of microorganisms during lactobionic acid production. In addition, during the study, Bifidobacteria were microencapsulated with lactobionic acid and their survival was evaluated in an in vitro model of the gastrointestinal tract, checking the prebiotic properties of the acid. The highest number of viable cells in the microcapsules after digestion was obtained using the Bifidobacterium bifidum strain DSM 20082. The antagonistic activity of lactobionic acid was also analysed.

All lactose-oxidizing enzymes of Pseudomonas taetrolens, a highly efficient lactobionic acid-producing microorganism, are pyrroloquinoline quinone-dependent enzymes

In previous and present studies, four enzymes (GCD1, GCD3, GCD4, and MQO1) have been found to act as lactose-oxidizing enzymes of Pseudomonas taetrolens. To investigate whether the four enzymes were the only lactose-oxidizing enzymes of P. taetrolens, we performed the inactivation of gcd1, gcd3, gcd4, and mqo1 genes in P. taetrolens. Compared to the wild-type strain, the lactobionic acid (LBA)-producing ability of P. taetrolens ∆gcd1 ∆gcd3 ∆gcd4 ∆mqo1 was only slightly decreased, implying that P. taetrolens possesses more lactose-oxidizing enzymes. Interestingly, the four lactose-oxidizing enzymes were all pyrroloquinoline quinone (PQQ)-dependent. To identify other unidentified lactose-oxidizing enzymes of P. taetrolens, we prevented the synthesis of PQQ in P. taetrolens by inactivating the genes related to PQQ synthesis such as pqqC, pqqD, and pqqE. Surprisingly, all three knocked-out strains were unable to convert lactose to LBA, indicating that all lactose-oxidizing enzymes in P. taetrolens were inactivated by eliminating PQQ synthesis. In addition, external PQQ supplementation restored the LBA production ability of P. taetrolens ∆pqqC, comparable to the wild-type strain. These results indicate that all lactose-oxidizing enzymes in P. taetrolens are PQQ-dependent.

Production of lactobionic acid at high salt concentrations by Acinetobacter halotolerans isolated from seaside soil

A lactobionic acid (LBA)-producing bacterium isolated from seaside soils was identified as Acinetobacter halotolerans and designated as strain KRICT-1. We determined whether KRICT-1 can produce LBA at high salt concentrations. The KRICT-1 strain grew on a nutrient broth (NB) agar plate with up to 7.0% NaCl, indicating high NaCl tolerance, and 30 °C was the optimum growth temperature for LBA production. We produced LBA using the KRICT-1 strain in NB medium containing various concentrations of NaCl. While Pseudomonas taetrolens, an efficient LBA-producing bacterium, could produce LBA with up to 5.5% NaCl, the KRICT-1 strain could produce LBA at up to 7.0% NaCl and produced more LBA than P. taetrolens with over 5.5% NaCl. We produced LBA using NB medium containing 7.0% NaCl by batch fermentation of the KRICT-1 strain in a 5 L fermenter. The LBA production titer and productivity of the KRICT-1 strain were 32.1 g/L and 0.22 g/L/h, respectively, which were approximately 1.35- and 1.38-fold higher than those (23.7 g/L and 0.16 g/L/h) obtained from flask culture. Additionally, quinoprotein glucose dehydrogenase is an LBA-producing enzyme in A. halotolerans. We demonstrated that the A. halotolerans KRICT-1 strain is appropriate for LBA production at high salt concentrations.

Efficient production of cellobionic acid using whole-cell biocatalyst of genetically modified Pseudomonas taetrolens

Pseudomonas taetrolens has previously been shown to convert cellobiose to cellobionic acid (CBA), which can potentially be used in cosmetics, food, and pharmaceutical industries. The cellobiose-oxidizing activity of the P. taetrolens strain, which expressed the homologous quinoprotein glucose dehydrogenase (GDH), was increased by approximately 50.8% compared to the original strain. Whole-cell biocatalyst (WCB) of the genetically modified P. taetrolens strain [pDSK-GDH] was prepared simply by fermentation and washing processes. Reaction conditions for the proper use of WCB, such as reaction temperature, cell density to be added, and cell harvest time for preparing WCB, were investigated. The highest CBA productivity (18.2 g/L/h) was achieved when WCB prepared in the late-exponential phase of cell culture was used at 35 °C with cell density of 10 at OD_600nm. Under these conditions, 200 g/L of cellobiose was all converted to CBA in 11 h, and the WCB of P. taetrolens [pDSK-GDH] maintained the maximum catalytic activity during at least six cycles without a significant decline in the productivity. Our results suggest that the manufacture of WCB based on genetically engineered P. taetrolens and its optimized use could be further developed as an economically viable option for the large-scale production of CBA.

Whole-cell biocatalysis using genetically modified Pseudomonas taetrolens for efficient production of maltobionic acid from pure maltose and high-maltose corn syrup

Maltobionic acid (MBA) can be applied to various fields such as food, cosmetics, and pharmaceutical industries. In this study, whole-cell biocatalysis for MBA production was performed using recombinant Pseudomonas taetrolens homologously expressing quinoprotein glucose dehydrogenase (GDH). Various reaction parameters such as temperature, cell density, and cell harvest time, were optimized for improving MBA production. Under the optimized reaction conditions using pure maltose as a substrate, the MBA production titer, yield, and productivity of whole-cell biocatalyst (WCB) were 200 g/L, 95.6%, and 18.18 g/L/h, respectively, which were the highest compared to those reported previously. Productivity, a key factor for industrial MBA production, obtained from whole-cell biocatalysis in this study, was enhanced by approximately 1.9-fold compared to that obtained in our previous work (9.52 g/L/h) using the fermentation method. Additionally, the WCB could be reused up to six times without a significant reduction in MBA productivity, indicating that the WCB is very robust. Although MBA productivity (8.33 g/L/h) obtained from high-maltose corn syrup (HMCS) as a substrate was 45.8% of that using pure maltose, HMCS can be a better substrate for commercial MBA production because its price is only 1.1% of that of pure maltose. The results of this study using a WCB to convert maltose into MBA may support the development of a potential industrial process for more economically effective MBA production in the future.

Efficient Production of Lactobionic Acid Using Escherichia coli Capable of Synthesizing Pyrroloquinoline Quinone

Lactobionic acid (LBA) is an emerging chemical that has been widely utilized in food, cosmetic, and pharmaceutical industries. We sought to produce LBA using Escherichia coli. LBA can be produced from lactose in E. coli, which is innately unable to produce LBA, by coexpressing a heterologous quinoprotein glucose dehydrogenase (GDH) and a pyrroloquinoline quinone (PQQ) synthesis gene cluster. Using a recombinant E. coli strain, we successfully produced LBA without additional supplementation of PQQ, and changing the type of heterologous GDH improved the LBA production titer and productivity. To further enhance LBA production, culture conditions, such as growth temperature and isopropyl-β-d-1-thiogalactopyranoside concentration, were optimized. Using optimized culture conditions, batch fermentation of the recombinant E. coli strain was performed using a 5 L bioreactor. After fermentation, this strain produced an LBA titer of 209.3 g/L, a yield of 100%, and a productivity of 1.45 g/L/h. To our best knowledge, this is the first study to produce LBA using heterologous GDH in an E. coli strain without any additional cofactors. Our results provide a simple method to produce LBA from lactose in a naturally non-LBA-producing bacterium and lay the groundwork for highly efficient LBA production in E. coli, which is one of the most versatile metabolite-producing bacterial hosts.