Microbial Dehydrogenations of Monosaccharides. Advances in Applied Microbiology Volume 34. Elsevier; 1989. pp. 141-82. [DOI: 10.1016/s0065-2164(08)70318-6]

Membrane-bound sorbitol dehydrogenase is responsible for the unique oxidation of D-galactitol to L-xylo-3-hexulose and D-tagatose in Gluconobacter oxydans

BACKGROUND

Gluconobacter oxydans, is used in biotechnology because of its ability to oxidize a wide variety of carbohydrates, alcohols, and polyols in a stereo- and regio-selective manner by membrane-bound dehydrogenases located in periplasmic space. These reactions obey the well-known Bertrand-Hudson's rule. In our previous study (BBA-General Subjects, 2021, 1865:129740), we discovered that Gluconobacter species, including G. oxydans and G. cerinus strain can regio-selectively oxidize the C-3 and C-5 hydroxyl groups of D-galactitol to rare sugars D-tagatose and L-xylo-3-hexulose, which represents an exception to Bertrand Hudson's rule. The enzyme catalyzing this reaction is located in periplasmic space or membrane-bound and is PQQ (pyrroloquinoline quinine) and Ca²⁺-dependent; we were encouraged to determine which type of enzyme(s) catalyze this unique reaction.

METHODS

Enzyme was identified by complementation of multi-deletion strain of Gluconobacter oxydans 621H with all putative membrane-bound dehydrogenase genes.

RESULTS AND CONCLUSIONS

In this study, we identified this gene encoding the membrane-bound PQQ-dependent dehydrogenase that catalyzes the unique galactitol oxidation reaction in its 3'-OH and 5'-OH. Complement experiments in multi-deletion G. oxydans BP.9 strains established that the enzyme mSLDH (encoded by GOX0855-0854, sldB-sldA) is responsible for galactitol's unique oxidation reaction. Additionally, we demonstrated that the small subunit SldB of mSLDH was membrane-bound and served as an anchor protein by fusing it to a red fluorescent protein (mRubby), and heterologously expressed in E. coli and the yeast Yarrowia lipolytica. The SldB subunit was required to maintain the holo-enzymatic activity that catalyzes the conversion of D-galactitol to L-xylo-3-hexulose and D-tagatose. The large subunit SldA encoded by GOX0854 was also characterized, and it was discovered that its 24 amino acids signal peptide is required for the dehydrogenation activity of the mSLDH protein.

GENERAL SIGNIFICANCE

In this study, the main membrane-bound polyol dehydrogenase mSLDH in G. oxydans 621H was proved to catalyze the unique galactitol oxidation, which represents an exception to the Bertrand Hudson's rule, and broadens its substrate ranges of mSLDH. Further deciphering the explicit enzymatic mechanism will prove this theory.

Characterization of a cryptic, pyrroloquinoline quinone-dependent dehydrogenase of Gluconobacter sp. strain CHM43

We characterized the pyrroloquinoline quinone (PQQ)-dependent dehydrogenase 9 (PQQ-DH9) of Gluconobacter sp. strain CHM43, which is a homolog of PQQ-dependent glycerol dehydrogenase (GLDH). We used a plasmid construct to express PQQ-DH9. The expression host was a derivative strain of CHM43, which lacked the genes for GLDH and the membrane-bound alcohol dehydrogenase and consequently had minimal ability to oxidize primary and secondary alcohols. The membranes of the transformant exhibited considerable d-arabitol dehydrogenase activity, whereas the reference strain did not, even if it had PQQ-DH9-encoding genes in the chromosome and harbored the empty vector. This suggests that PQQ-DH9 is not expressed in the genome. The activities of the membranes containing PQQ-DH9 and GLDH suggested that similar to GLDH, PQQ-DH9 oxidized a wide variety of secondary alcohols but had higher Michaelis constants than GLDH with regard to linear substrates such as glycerol. Cyclic substrates such as cis-1,2-cyclohexanediol were readily oxidized by PQQ-DH9.

MEGA X: Molecular Evolutionary Genetics Analysis across Computing Platforms

The Molecular Evolutionary Genetics Analysis (Mega) software implements many analytical methods and tools for phylogenomics and phylomedicine. Here, we report a transformation of Mega to enable cross-platform use on Microsoft Windows and Linux operating systems. Mega X does not require virtualization or emulation software and provides a uniform user experience across platforms. Mega X has additionally been upgraded to use multiple computing cores for many molecular evolutionary analyses. Mega X is available in two interfaces (graphical and command line) and can be downloaded from www.megasoftware.net free of charge.

Biosynthesis of miglitol intermediate 6-(N-hydroxyethyl)-amino-6-deoxy-α-l-sorbofuranose by an improved d-sorbitol dehydrogenase from Gluconobacter oxydans

Adaptable exploitation of the catalytic potential of membrane-bound d-sorbitol dehydrogenase (mSLDH) from Gluconobacter oxydans is desperately needed in the industrial-scale production of miglitol. In the present study, a carbonyl group-dependent colorimetric quantification method was developed for the assay of miglitol key intermediate 6-(N-hydroxyethyl)-amino-6-deoxy-α-l-sorbofuranose (6NSL), and a high-throughput screening process of positive mutants was processed. Combined with several rounds of ultraviolet irradiation mutagenesis and screening procedure, a positive mutant strain G. oxydans ZJB16009 was obtained with significant increase in mSLDH catalytic activity by 1.5-fold, which exhibited an extremely accelerated uptake rate of d-sorbitol, and the fermentation time was significantly shortened from 22 to 11 h. In a 5-L biotransformation system, 60 g/L substrate N-2-hydroxyethyl glucamine (NHEG) was catalyzed by the resting cells of the mutant strain within 36 h and accumulated 53.6 g/L 6NSL, showing a 33.6% increase in the product yield. Therefore, it was indicated that the established high-throughput screening method could provide a highly efficient platform for the breading of G. oxydans strain for the industrial biosynthesis of miglitol intermediate 6NSL.

Aldopentoses as new substrates for the membrane-bound, pyrroloquinoline quinone-dependent glycerol (polyol) dehydrogenase of Gluconobacter sp

Membrane-bound, pyrroloquinoline quinone (PQQ)-dependent glycerol dehydrogenase (GLDH, or polyol dehydrogenase) of Gluconobacter sp. oxidizes various secondary alcohols to produce the corresponding ketones, such as oxidation of D-sorbitol to L-sorbose in vitamin C production. Substrate specificity of GLDH is considered limited to secondary alcohols in the D-erythro configuration at the next to the last carbon. Here, we suggest that L-ribose, D- and L-lyxoses, and L-tagatose are also substrates of GLDH, but these sugars do not meet the substrate specificity rule of GLDH. The oxygen consumption activity of wild-type Gluconobacter frateurii cell membranes depends on several kinds of sugars as compared with that of the membranes of a GLDH-negative variant. Biotransformation of those sugars with the membranes was examined to determine the reaction products. A time course measuring the pH in the reaction mixture and the increase or decrease in substrates and products on TLC suggested that oxidation products of L-lyxose and L-tagatose were ketones with unknown structures, but those of L-ribose and D-lyxose were acids. The oxidation product of L-ribose was purified and revealed to be L-ribonate by HRMS and NMR analysis. Biotransformation of L-ribose with the membranes and also with the whole cells produced L-ribonate in nearly stoichiometric amounts, indicating that the specific oxidation site in L-ribose is recognized by GLDH. Since purified GLDH produced L-ribonate without any intermediate-like compounds, we propose here a reaction model where the first carbon in the pyranose form of L-ribose is oxidized by GLDH to L-ribonolactone, which is further hydrolyzed spontaneously to produce L-ribonate.

Enhancement of 5-keto-d-gluconate production by a recombinant Gluconobacter oxydans using a dissolved oxygen control strategy

The rapid and incomplete oxidation of sugars, alcohols, and polyols by the gram-negative bacterium Gluconobacter oxydans facilitates a wide variety of biological applications. For the conversion of glucose to 5-keto-d-gluconate (5-KGA), a promising precursor of the industrial substance L-(+)-tartaric acid, G. oxydans DSM2343 was genetically engineered to strain ZJU2, in which the GOX1231 and GOX1081 genes were knocked out in a markerless fashion. Then, a secondary alcohol dehydrogenase (GCD) from Xanthomonas campestris DSM3586 was heterologously expressed in G. oxydans ZJU2. The 5-KGA production and cell yield were increased by 10% and 24.5%, respectively. The specific activity of GCD towards gluconate was 1.75±0.02 U/mg protein, which was 7-fold higher than that of the sldAB in G. oxydans. Based on the analysis of kinetic parameters including specific cell growth rate (μ), specific glucose consumption rate (qs) and specific 5-KGA production rate (qp), a dissolved oxygen (DO) control strategy was proposed. Finally, batch fermentation was carried out in a 15-L bioreactor using an initial agitation speed of 600 rpm to obtain a high μ for cell growth. Subsequently, DO was continuously maintained above 20% to achieve a high qp to ensure a high accumulation of 5-KGA. Under these conditions, the maximum concentration of 5-KGA reached 117.75 g/L with a productivity of 2.10 g/(L·h).

Combining metabolic engineering and adaptive evolution to enhance the production of dihydroxyacetone from glycerol by Gluconobacter oxydans in a low-cost way

Gluconobacter oxydans can rapidly and effectively transform glycerol to dihydroxyacetone (DHA) by membrane-bound quinoprotein sorbitol dehydrogenase (mSLDH). Two mutant strains of GDHE Δadh pBBR-PtufBsldAB and GDHE Δadh pBBR-sldAB derived from the GDHE strain were constructed for the enhancement of DHA production. Growth performances of both strains were largely improved after adaptively growing in the medium with glucose as the sole carbon source. The resulting GAT and GAN strains exhibited better catalytic property than the GDHE strain in the presence of a high concentration of glycerol. All strains of GDHE, GAT and GAN cultivated on glucose showed enhanced catalytic capacity than those grown on sorbitol, indicating a favorable prospect of using glucose as carbon source to reduce the cost in industrial production. It was also the first time to reveal that the expression level of the sldAB gene in glucose-growing strains were higher than that of the strains cultivated on sorbitol.

Characterization of enzymes involved in the central metabolism of Gluconobacter oxydans

Gluconobacter oxydans is an industrially important bacterium that lacks a complete Embden-Meyerhof pathway (glycolysis). The organism instead uses the pentose phosphate pathway to oxidize sugars and their phosphorylated intermediates. However, the lack of glycolysis limits the amount of NADH as electron donor for electron transport phosphorylation. It has been suggested that the pentose phosphate pathway contributes to NADH production. Six enzymes predicted to play central roles in intracellular glucose and gluconate flux were heterologously overproduced in Escherichia coli and characterized to investigate the intracellular flow of glucose and gluconates into the pentose phosphate pathway and to explore the contribution of the pentose phosphate pathway to NADH generation. The key pentose phosphate enzymes glucose 6-phosphate dehydrogenase (Gox0145) and 6-phosphogluconate dehydrogenase (Gox1705) had dual cofactor specificities but were physiologically NADP- and NAD-dependent, respectively. Putative glucose dehydrogenase (Gox2015) was NADP-dependent and exhibited a preference for mannose over glucose, whereas a 2-ketogluconate reductase (Gox0417) displayed dual cofactor specificity for NAD(P)H. Furthermore, a putative gluconokinase and a putative glucokinase were identified. The gluconokinase displayed high activities with gluconate and is thought to shuttle intracellular gluconate into the pentose phosphate pathway. A model for the trafficking of glucose and gluconates into the pentose phosphate pathway and its role in NADH generation is presented. The role of NADPH in chemiosmotic energy conservation is also discussed.

Characterization and inactivation of the membrane-bound polyol dehydrogenase in Gluconobacter oxydans DSM 7145 reveals a role in meso-erythritol oxidation

The growth of Gluconobacter oxydans DSM 7145 on meso-erythritol is characterized by two stages: in the first stage, meso-erythritol is oxidized almost stoichiometrically to L-erythrulose according to the Bertrand-Hudson rule. The second phase is distinguished from the first phase by a global metabolic change from membrane-bound meso-erythritol oxidation to L-erythrulose assimilation with concomitant accumulation of acetic acid. The membrane-associated erythritol-oxidizing enzyme was found to be encoded by a gene homologous to sldA known from other species of acetic acid bacteria. Disruption of this gene in the genome of G. oxydans DSM 7145 revealed that the membrane-bound polyol dehydrogenase not only oxidizes meso-erythritol but also has a broader substrate spectrum which includes C3-C6 polyols and D-gluconate and supports growth on these substrates. Cultivation of G. oxydans DSM 7145 on different substrates indicated that expression of the polyol dehydrogenase was not regulated, implying that the production of biomass of G. oxydans to be used as whole-cell biocatalysts in the biotechnological conversion of meso-erythritol to L-erythrulose, which is used as a tanning agent in the cosmetics industry, can be conveniently carried out with glucose as the growth substrate.

The GenusGluconobacter Oxydans: Comprehensive Overview of Biochemistry and Biotechnological Applications

The genus Gluconobacter comprises some of the most frequently used microorganisms when it comes to biotechnological applications. Not only has it been involved in "historical" production processes, such as vinegar production, but in the last decades many bioconversion routes for special and rare sugars involving Gluconobacter have been developed. Among the most recent are the biotransformations involved in the production of L-ribose and miglitol, both very promising pharmaceutical lead molecules. Most of these processes make use of Gluconobacter's membrane-bound polyol dehydrogenases. However, recently other enzymes have also caught the eye of industrial biotechnology. Among them are dextran dextrinase, capable of transglucosylating substrate molecules, and intracellular NAD-dependent polyol dehydrogenases, of interest for co-enzyme regeneration. As such, Gluconobacter is an important industrial microbial strain, but it also finds use in other fields of biotechnology, such as biosensor-technology. This review aims to give an overview of the myriad of applications for Gluconobacter, with a special focus on some recent developments.

Biotechnological applications of acetic acid bacteria

The acetic acid bacteria (AAB) have important roles in food and beverage production, as well as in the bioproduction of industrial chemicals. In recent years, there have been major advances in understanding their taxonomy, molecular biology, and physiology, and in methods for their isolation and identification. AAB are obligate aerobes that oxidize sugars, sugar alcohols, and ethanol with the production of acetic acid as the major end product. This special type of metabolism differentiates them from all other bacteria. Recently, the AAB taxonomy has been strongly rearranged as new techniques using 16S rRNA sequence analysis have been introduced. Currently, the AAB are classified in ten genera in the family Acetobacteriaceae. AAB can not only play a positive role in the production of selected foods and beverages, but they can also spoil other foods and beverages. AAB occur in sugar- and alcohol-enriched environments. The difficulty of cultivation of AAB on semisolid media in the past resulted in poor knowledge of the species present in industrial processes. The first step of acetic acid production is the conversion of ethanol from a carbohydrate carried out by yeasts, and the second step is the oxidation of ethanol to acetic acid carried out by AAB. Vinegar is traditionally the product of acetous fermentation of natural alcoholic substrates. Depending on the substrate, vinegars can be classified as fruit, starch, or spirit substrate vinegars. Although a variety of bacteria can produce acetic acid, mostly members of Acetobacter, Gluconacetobacter, and Gluconobacter are used commercially. Industrial vinegar manufacturing processes fall into three main categories: slow processes, quick processes, and submerged processes. AAB also play an important role in cocoa production, which represents a significant means of income for some countries. Microbial cellulose, produced by AAB, possesses some excellent physical properties and has potential for many applications. Other products of biotransformations by AAB or their enzymes include 2-keto-L-gulonic acid, which is used for the production of vitamin C; D-tagatose, which is used as a bulking agent in food and a noncalorific sweetener; and shikimate, which is a key intermediate for a large number of antibiotics. Recently, for the first time, a pathogenic acetic acid bacterium was described, representing the newest and tenth genus of AAB.

Characterisation of a secondary alcohol dehydrogenase from Xanthomonas campestris DSM 3586

The chromosomal locus NP_636946 of Xanthomonas campestris DSM 3586 (ATCC 33913) which was earlier presumed to encode a quinoprotein glucose dehydrogenase has been cloned, expressed in Escherichia coli and the recombinant enzyme has been characterised. It was found to have no glucose dehydrogenase activity but to be active on many different polyols and diols, aliphatic alcohols, certain aldonic acids and amino-sugars. The product of D: -gluconic acid oxidation was 5-keto-D: -gluconic acid. The enzyme differs from polyol/gluconate dehydrogenases found in Gluconobacter by its single-chain architecture, different substrate specificity and much higher (20- to 30-fold) expression level in E.coli.

The genus Gluconobacter and its applications in biotechnology

Organisms of the genus Gluconobacter have been widely utilized within the biotechnology industry for many decades, due to their unique metabolic characteristics. The metabolic features that render Gluconobacter so useful in biotransformation processes, vitamin synthesis, and, as the biological element in sensor systems, are critically evaluated, and the relevance of recent biochemical genetic studies to current and future industrial Gluconobacter processes is discussed. The impact of recombinant gene technology on the status of Gluconobacter processes and the potential use of such techniques in clarifying aspects of the physiology of Gluconobacter is reviewed.

FET-microbial sensor for xylose detection based on Gluconobacter oxydans cells

A potentiometric biosensor for xylose was devised utilizing Gluconobacter oxydans whole cells. Immobilization methods based on physical adsorption were used for G. oxydans cells and extracellular pH changes resulting from xylose dehydrogenation were monitored by a field effect transistor (FET). The G. oxydans, FET-based sensor detected xylose at a lower limit of 0.5 mM. From 5.0 to 30 mM xylose, the response of the sensor was linear. Expectedly, output signals were significantly suppressed by buffer (Tris-HCl). Responses were essentially stable for at least four weeks of storage and showed only a slight loss of initial xylose sensitivity. Xylitol exerted an insignificant influence on the sensor's response to xylose. However, the response to glucose was 5 times higher in relation to that of xylose at the same concentration (1 mM). For xylose determinations in the presence of glucose, a two-step assay is discussed.

Influence of constant and oscillating dissolved oxygen concentrations on keto acid production by Gluconobacter oxydans subsps. melanogenum

Gluconobacter species are known to oxidise glucose via a direct oxidation pathway which is distinct from the pentose phosphate pathway. In the present communication results of an investigation on the influence of different dissolved oxygen concentrations (DO) on the production of 2,5-diketogluconic acid in batch and chemostat cultures are given. DO of 30% relative to air at 1 bar was found as a threshold level for optimum productivity. The positive influence of continuous availability of dissolved oxygen on the process of rapid glucose oxidation was unambiguously shown as the result of induction of membrane bound dehydrogenases involved in direct glucose oxidation. Furthermore data of scale-down experiments in which the organism was cultivated under oscillations of dissolved oxygen, are given. The influences of such oscillations of DO in the region of the established threshold (30% saturation) were found to result in a prolonged lag phase for growth and product formation. The data obtained in this study revealed critical residence times at low DO that could be employed as a criterion for scale up of this aerobic process.