High-level production and high-yield recovery of lactobionic acid by the control of pH and temperature in fermentation of Pseudomonas taetrolens

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.

Bioproduction and applications of aldobionic acids with a focus on maltobionic and cellobionic acid

Aldobionic acids are sugar acids which consist of a disaccharide with an anomeric acid group. The most famous is lactobionic acid (LBA). LBA is used in many applications such as food and beverages, pharmaceuticals and medicine, cosmetics or chemical processes. During the last decade, all these industries are observing a shift of consumer preferences towards plant-based options. Thus, the biotechnological industry is trying to replace the animal-derived LBA. Maltobionic acid (MBA) and cellobionic acid (CBA) are two stereoisomers of LBA which have emerged as vegan alternatives. However, MBA and CBA face different obstacles related to their industrial production. While traditionally used electrochemical or chemical catalysis often rely on cost intensive and/or hazardous catalysts, novel production methods with microorganisms are still poorly studied. In the first part, this paper discusses both alternatives in terms of their characteristics and applications. In the second part, it reviews the long-studied chemical production and the novel bioproduction methods, which are based on enzymatic and microbial systems. This review concludes with a discussion of future work needed to bring their production to the industrial scale.

Characterization of putative mannoprotein in Kluyveromyces lactis for lactase production

Lactase is a member of the β-galactosidase family of enzymes that can hydrolyze lactose into galactose and glucose. However, extracellular lactase production was still restricted to the process of cell lysis. In this study, lactase-producing Kluyveromyces lactis JNXR-2101 was obtained using a rapid and sensitive method based on the fluorescent substrate 4-methylumbelliferyl-β-d-galactopyranoside. The purified enzyme was identified as a neutral lactase with an optimum pH of 9. To facilitate extracellular production of lactase, a putative mannoprotein KLLA0_E01057g of K. lactis was knocked out. It could effectively promote cell wall degradation and lactase production after lyticase treatment, which showed potential on other extracellular enzyme preparation. After optimizing the fermentation conditions, the lactase yield from mannoprotein-deficient K. lactis JNXR-2101ΔE01057g reached 159.62 U/mL in a 5-L fed-batch bioreactor.

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.

Efficient isolation of new lactobionic acid-producing microorganisms from environmental samples by colloidal calcium carbonate agar plate-based screening

Lactobionic acid (LBA) has recently emerged as an important substance in various industries, such as cosmetics, foods, and pharmaceuticals. In this study, we developed a simple, efficient, and high-throughput method for screening LBA-producing microorganisms. First, an agar plate was prepared to isolate LBA-producing microorganisms by utilizing the property of LBA to solubilize colloidal calcium carbonate (CaCO₃), resulting in the formation of a clear halo around colonies on a nutrient broth agar plate containing CaCO₃. Subsequently, LBA production from the isolated microorganisms was confirmed using high-performance liquid chromatography (HPLC). Approximately 560 colonies from soil samples in Ulsan, Korea were screened and a clear halo was observed around three colonies on the prepared LBA-screening agar plate. The culture supernatants of these three colonies were analyzed by HPLC and it was found that these strains could produce LBA from lactose. Phylogenetic analysis by comparing their 16S rRNA nucleotide sequences revealed that these strains were Pseudomonas spp. and Alcaligenes faecalis. This is the first report highlighting that A. faecalis can produce LBA. As per the aforementioned results, the LBA-screening method that we devised here is highly effective for isolating and identifying new LBA-producing microorganisms.

Isolation of new lactobionic acid-producing microorganisms and improvement of their production ability by heterologous expression of glucose dehydrogenase from Pseudomonas taetrolens

Lactobionic acid (LBA) is a specialty organic acid that is widely employed in the food, cosmetic, and pharmaceutical industries. In the present study, we screened new LBA-producing bacteria from the soil of a poultry farm. Among the 700 bacterial colonies, five that exhibited LBA-producing ability were successfully isolated. Phylogenetic analysis based on 16 S rRNA sequences identified strain 2-15 as an Acinetobacter sp., strains 3-13 and 3-15 as Pseudomonas spp., and strains 7-7 and 7-8 as Psychrobacter spp. The LBA-producing abilities of the five strains were compared in flask culture, whereupon Psychrobacter sp. 7-8 showed the highest LBA titer (203.7 g/L), LBA yield from lactose (97.3%), and LBA productivity (2.83 g/L/h). To our best knowledge, this is the first study showing that Acinetobacter and Psychrobacter spp. can produce LBA from lactose. Our results would help broaden the spectrum of workhorse bacteria available for the industrially important microbial production of LBA. In addition, we improved the LBA-production ability of the three isolated bacteria, namely Acinetobacter sp. 2-15, Pseudomonas spp. strains 3-13 and 3-15, by heterologously expressing quinoprotein glucose dehydrogenase from Pseudomonas taetrolens. In particular, the LBA-production ability of the recombinant Pseudomonas sp. 3-13 were highly improved that the LBA titer and productivity were 19.2- (205.6 vs. 10.7 g/L, respectively) and 17.8-fold (1.07 vs. 0.06 g/L/h, respectively) higher, respectively, than those of the wild-type strain. These values were almost identical to those of the wild-type Psychrobacter sp. 7-8, which showed the highest LBA productivity among the five isolated strains. This result demonstrated that the expression of lactose-oxidizing enzyme in LBA-producing microorganisms was highly effective to enhance their LBA-production ability. Our study presents a practical method to screen for efficient LBA-producing microorganisms and to improve their production ability by genetic engineering for industrial LBA production.

Metabolic engineering of Pseudomonas putida for production of the natural sweetener 5-ketofructose from fructose or sucrose by periplasmic oxidation with a heterologous fructose dehydrogenase

5-Ketofructose (5-KF) is a promising low-calorie natural sweetener with the potential to reduce health problems caused by excessive sugar consumption. It is formed by periplasmic oxidation of fructose by fructose dehydrogenase (Fdh) of Gluconobacter japonicus, a membrane-bound three-subunit enzyme containing FAD and three haemes c as prosthetic groups. This study aimed at establishing Pseudomonas putida KT2440 as a new cell factory for 5-KF production, as this host offers a number of advantages compared with the established host Gluconobacter oxydans. Genomic expression of the fdhSCL genes from G. japonicus enabled synthesis of functional Fdh in P. putida and successful oxidation of fructose to 5-KF. In a batch fermentation, 129 g l-1 5-KF were formed from 150 g l-1 fructose within 23 h, corresponding to a space-time yield of 5.6 g l-1  h-1 . Besides fructose, also sucrose could be used as substrate for 5-KF production by plasmid-based expression of the invertase gene inv1417 from G. japonicus. In a bioreactor cultivation with pulsed sucrose feeding, 144 g 5-KF were produced from 358 g sucrose within 48 h. These results demonstrate that P. putida is an attractive host for 5-KF production.

High-level production of maltobionic acid from high-maltose corn syrup by genetically engineered Pseudomonas taetrolens

Maltobionic acid (MBA) has recently emerged as an important material in various industries. Here, we showed that quinoprotein glucose dehydrogenase (GDH) from Pseudomonas taetrolens could convert maltose into MBA by heterologously expressing this enzyme in MBA non-producing Escherichia coli. We homologously expressed GDH in P. taetrolens to improve intracellular maltose-oxidizing activity and MBA production. We optimized culture conditions, then applied these conditions to batch fermentation by recombinant P. taetrolens in a 5-L bioreactor. The MBA production, yield, and productivity of batch fermentation using high-maltose corn syrup (HMCS), an inexpensive maltose source, were 200 g/L, 95.6 %, and 6.67 g/L/h, respectively. Although the MBA productivity from HMCS was 70.1 % of that compared with pure maltose as the substrate, HMCS was a better substrate for commercial MBA production, considering the cost was 1.1 % of that of pure maltose. The present findings provide an economically feasible strategy with which to produce MBA.

Purification and Characterization of a Malate:Quinone Oxidoreductase from Pseudomonas taetrolens Capable of Producing Valuable Lactobionic Acid

In this study, we successfully purified a novel lactose-oxidizing enzyme in Pseudomonas taetrolens for the first time. The purified enzyme was identified as malate:quinone oxidoreductase (MQO, EC 1.1.5.4), which showed the malate-oxidizing activity converting malate into oxaloacetate. We characterized the enzymatic properties of this interesting MQO from P. taetrolens, such as the substrate specificity toward various saccharides and the effects of temperature, pH, and metal ions on the activity and stability of MQO. MQO exhibited unique substrate specificity, as it only oxidized disaccharides with reducing-end glucosyl residues, such as lactose, but not monosaccharides. Using the high oxidizing activity of MQO toward lactose, we successfully produced lactobionic acid (LBA), a valuable organic acid used in the cosmetic, food, and pharmaceutical industries, from lactose in Escherichia coli in which the quinoprotein glucose dehydrogenase gene was inactivated, the LBA nonproducing strain, by heterologously expressing MQO with pyrroloquinoline quinone. At 37 h cultivation in a 300 mL flask culture, the LBA production, yield, and productivity of the recombinant E. coli strain were 23 g/L, 100%, and 0.62 g/L/h, respectively. This study is the first to reveal the lactose-oxidizing activity of MQO, which could be used for producing LBA in heterologous bacteria.

Enhancement of Lactobionic Acid Productivity by Homologous Expression of Quinoprotein Glucose Dehydrogenase in Pseudomonas taetrolens

This is the first study on improving lactobionic acid (LBA) production capacity in Pseudomonas taetrolens by genetic engineering. First, quinoprotein glucose dehydrogenase (GDH) was identified as the lactose-oxidizing enzyme of P. taetrolens. Of the two types of GDH genes in P. taetrolens, membrane-bound (GDH1) and soluble (GDH2), only GDH1 showed lactose-oxidizing activity. Next, the genetic tool system for P. taetrolens was developed based on the pDSK519 plasmid for the first time, and GDH1 gene was homologously expressed in P. taetrolens. Recombinant expression of the GDH1 gene enhanced intracellular lactose-oxidizing activity and LBA production of P. taetrolens in flask culture. In batch fermentation of the recombinant P. taetrolens using a 5 L bioreactor, the LBA productivity of the recombinant P. taetrolens was approximately 17% higher (8.70 g/(L h)) than that of the wild type (7.41 g/(L h)). The LBA productivity in this study is the highest ever reported using bacteria as production strains for LBA.