Membrane-bound, 2-keto-D-gluconate-yielding D-gluconate dehydrogenase from "Gluconobacter dioxyacetonicus" IFO 3271: molecular properties and gene disruption

Ketogluconate production by Gluconobacter strains: enzymes and biotechnological applications

Gluconobacter strains perform incomplete oxidation of various sugars and alcohols, employing regio- and stereoselective membrane-bound dehydrogenases oriented toward the periplasmic space. This oxidative fermentation process is utilized industrially. The ketogluconate production pathway, characteristic of these strains, begins with the conversion of d-glucose to d-gluconate, which then diverges and splits into 2 pathways producing 5-keto-d-gluconate and 2-keto-d-gluconate and subsequently 2,5-diketo-d-gluconate. These transformations are facilitated by membrane-bound d-glucose dehydrogenase, glycerol dehydrogenase, d-gluconate dehydrogenase, and 2-keto-d-gluconate dehydrogenase. The variance in end products across Gluconobacter strains stems from the diversity of enzymes and their activities. This review synthesizes biochemical and genetic knowledge with biotechnological applications, highlighting recent advances in metabolic engineering and the development of an efficient production process focusing on enzymes relevant to the ketogluconate production pathway in Gluconobacter strains.

Characteristics of Metabolites in the Development of Atherosclerosis in Tibetan Minipigs Determined Using Untargeted Metabolomics

BACKGROUND

Atherosclerosis (AS) is a chronic progressive disease caused by lipometabolic disorder. However, the pathological characteristics and mechanism of AS have not been fully clarified. Through high-fat and high-cholesterol diet induction, Tibetan minipigs can be used as the AS model animals, as they have a very similar AS pathogenesis to humans.

METHODS

In this study, we built an AS model of Tibetan minipigs and identified the differential abundance metabolites in the development of AS based on untargeted metabolomics.

RESULTS

We found that sphingolipid metabolism and glucose oxidation were obviously higher in the AS group and phenylalanine metabolism was reduced in the AS group. Moreover, in the development of AS, gluconolactone was enriched in the late stage of AS whereas biopterin was enriched in the early stage of AS.

CONCLUSIONS

Our research provides novel clues to investigate the metabolic mechanism of AS from the perspective of metabolomics.

Development of efficient 5-ketogluconate production system by Gluconobacter japonicus

5-Ketogluconate (5KGA) is a precursor for synthesizing tartrate, a valuable compound used in several industries. In a previous study, Gluconobacter japonicus NBRC 3271 mutant strain D2, which lacks two membranous gluconate 2-dehydrogenases, was shown to produce 5KGA but not 2-ketogluconate from a mixture of glucose and gluconate. In this study, we aimed to develop an efficient 5KGA production system using G. japonicus D2 as the parental strain. D2 produced 5KGA from glucose in a jar fermentor culture; however, 5KGA levels were reduced during the late phase of cultivation. To increase the potential of D2 for 5KGA production, the cytoplasmic metabolism related to the utilization of 5KGA and gluconate was modified; the gno and gntK genes encoding 5KGA reductase and gluconokinase, respectively, were deleted from D2, generating D4. Improved 5KGA production was observed in D4 compared to that in D2, but a significant amount of gluconate remained at the end of cultivation, leading to an unsatisfied yield of 0.83 mol (mol glucose)^-1. The conversion of gluconate to 5KGA is catalyzed by pyrroloquinoline quinone (PQQ)-dependent glycerol dehydrogenase (GLDH), which easily forms an apoenzyme by releasing PQQ and calcium ions. Thus, the effects of CaCl₂ addition to the culture medium on 5KGA production by D4 were investigated. We demonstrated that 1 mM CaCl₂ addition positively affected the maintenance of the PQQ-GLDH activity toward gluconate and consequently enhanced 5KGA production, and the yield reached 0.97 mol (mol glucose)^-1. KEY POINTS: • An efficient 5KGA production system was developed with Gluconobacter japonicus. • Deleting the gno and gntK genes blocked the catabolism of 5KGA and gluconate. • The addition of 1 mM CaCl₂ efficiently improved the conversion of glucose to 5KGA.

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.

Identification of the Phosphorus-Solubilizing Bacteria Strain JP233 and Its Effects on Soil Phosphorus Leaching Loss and Crop Growth

Phosphorus (P) is one of the most limiting nutrients in global agricultural ecosystems, and phosphorus-solubilizing bacteria (PSB) can convert insoluble P into soluble P, thereby improving the absorption and use of soil P by plants. Increasing leaching loss of soil P due to PSB that could lead to water eutrophication is a major concern, although no direct experimental evidence is available to evaluate these effects. In this study, a highly efficient PSB strain, Pseudomonas sp. JP233, was isolated from soil and its P-solubilizing agent was identified by metabolomics and HPLC analyses. The effects of JP233 on P contents in soil leachates were also analyzed by microcosm leaching experiments in the absence and presence of maize. JP233 could solubilize insoluble P into soluble forms, and the molybdate reactive phosphorus (MRP) content reached 258.07 mg/L in NBRIP medium containing 5 g/L Ca3(PO4)2 within 48 h. Metabolomics analysis demonstrated that the organic acid involved in JP233 P solubilization was primarily 2-keto gluconic acid (2KGA). Further, HPLC analysis revealed that 2KGA contents rapidly accumulated to 19.33 mg/mL within 48 h. Microcosm leaching experiments showed that MRP and total phosphorus (TP) contents in soil leaching solutions were not significantly higher after JP233 inoculation. However, inoculation with JP233 into maize plant soils significantly decreased MRP and TP contents in the soil leaching solutions on days 14 (P < 0.01), 21 (P < 0.01), and 28 (P < 0.05). Inoculation with strain JP233 also significantly increased the biomass of maize aerial components and that of whole plants (P < 0.05). Thus, strain JP233 exhibited a significant plant-growth-promoting effect on maize development. In conclusion, the application of PSB into soils does not significantly increase P leachate loss. Rather, the application of PSB can help reduce P leachate loss, while significantly promoting plant absorption and use of soil P.

Characterization of 3 phylogenetically distinct membrane-bound d-gluconate dehydrogenases of Gluconobacter spp. and their biotechnological application for efficient 2-keto-d-gluconate production

We identified a novel flavoprotein-cytochrome c complex d-gluconate dehydrogenase (GADH) encoded by gndXYZ of Gluconobacter oxydans NBRC 3293, which is phylogenetically distinct from previously reported GADHs encoded by gndFGH and gndSLC of Gluconobacter spp. To analyze the biochemical properties of respective GADHs, Gluconobacter japonicus NBRC 3271 mutant strain lacking membranous d-gluconate dehydrogenase activity was constructed. All GADHs (GndFGH, GndSLC, and GndXYZ) were successfully overexpressed in the constructed strain. The optimal pH and KM value at that pH of GndFGH, GndSLC, and GndXYZ were 5, 6, and 4, and 8.82 ± 1.15, 22.9 ± 5.0, and 11.3 ± 1.5 m m, respectively. When the mutants overexpressing respective GADHs were cultured in d-glucose-containing medium, all of them produced 2-keto-d-gluconate, revealing that GndXYZ converts d-gluconate to 2-keto-d-gluconate as well as other GADHs. Among the recombinants, the gndXYZ-overexpressing strain accumulated the highest level of 2-keto-d-gluconate, suggesting its potential for 2-keto-d-gluconate production.

Dehydrogenases of acetic acid bacteria

Acetic acid bacteria (AAB) are a group of bacteria that can oxidize many substrates such as alcohols and sugar alcohols and play important roles in industrial biotechnology. A majority of industrial processes that involve AAB are related to their dehydrogenases, including PQQ/FAD-dependent membrane-bound dehydrogenases and NAD(P)⁺-dependent cytoplasmic dehydrogenases. These cofactor-dependent dehydrogenases must effectively regenerate their cofactors in order to function continuously. For PQQ, FAD and NAD(P)⁺ alike, regeneration is directly or indirectly related to the electron transport chain (ETC) of AAB, which plays an important role in energy generation for aerobic cell growth. Furthermore, in changeable natural habitats, ETC components of AAB can be regulated so that the bacteria survive in different environments. Herein, the progressive cascade in an application of AAB, including key dehydrogenases involved in the application, regeneration of dehydrogenase cofactors, ETC coupling with cofactor regeneration and ETC regulation, is systematically reviewed and discussed. As they have great application value, a deep understanding of the mechanisms through which AAB function will not only promote their utilization and development but also provide a reference for engineering of other industrial strains.

A Complex of LaoA and LaoB Acts as a Tat-Dependent Dehydrogenase for Long-Chain Alcohols in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa can utilize unusual carbon sources, like sodium dodecyl sulfate (SDS) and alkanes. Whereas the initiating enzymatic steps of the corresponding degradation pathways have been characterized in detail, the oxidation of the emerging long-chain alcohols has received little attention. Recently, the genes for the Lao (long-chain-alcohol/aldehyde oxidation) system were discovered to be involved in the oxidation of long-chain alcohols derived from SDS and alkane degradation. In the Lao system, LaoA is predicted to be an alcohol dehydrogenase/oxidase; however, according to genetic studies, efficient long-chain-alcohol oxidation additionally required the Tat-dependent protein LaoB. In the present study, the Lao system was further characterized. In vivo analysis revealed that the Lao system complements the substrate spectrum of the well-described Exa system, which is required for growth with ethanol and other short-chain alcohols. Mutational analysis revealed that the Tat site of LaoB was required for long-chain-alcohol oxidation activity, strongly suggesting a periplasmic localization of the complex. Purified LaoA was fully active only when copurified with LaoB. Interestingly, in vitro activity of the purified LaoAB complex also depended on the presence of the Tat site. The copurified LaoAB complex contained a flavin cofactor and preferentially oxidized a range of saturated, unbranched primary alcohols. Furthermore, the LaoAB complex could reduce cytochrome c₅₅₀-type redox carriers like ExaB, a subunit of the Exa alcohol dehydrogenase system. LaoAB complex activity was stimulated by rhamnolipids in vitro. In summary, LaoAB constitutes an unprecedented protein complex with specific properties apparently required for oxidizing long-chain alcohols. IMPORTANCE Pseudomonas aeruginosa is a major threat to public health. Its ability to thrive in clinical settings, water distribution systems, or even jet fuel tanks is linked to detoxification and degradation of diverse hydrophobic substrates that are metabolized via alcohol intermediates. Our study illustrates a novel flavoprotein long-chain-alcohol dehydrogenase consisting of a facultative two-subunit complex, which is unique among related enzymes, while the homologs of the corresponding genes are found in numerous bacterial genomes. Understanding the catalytic and compartmentalization processes involved is of great interest for biotechnological and hygiene research, as it may be a potential starting point for rationally designing novel antibacterial substances with high specificity against this opportunistic pathogen.

FAD dependent glucose dehydrogenases - Discovery and engineering of representative glucose sensing enzymes

The history of the development of glucose sensors goes hand-in-hand with the history of the discovery and the engineering of glucose-sensing enzymes. Glucose oxidase (GOx) has been used for glucose sensing since the development of the first electrochemical glucose sensor. The principle utilizing oxygen as the electron acceptor is designated as the first-generation electrochemical enzyme sensors. With increasing demand for hand-held and cost-effective devices for the "self-monitoring of blood glucose (SMBG)", second-generation electrochemical sensor strips employing electron mediators have become the most popular platform. To overcome the inherent drawback of GOx, namely, the use of oxygen as the electron acceptor, various glucose dehydrogenases (GDHs) have been utilized in second-generation principle-based sensors. Among the various enzymes employed in glucose sensors, GDHs harboring FAD as the redox cofactor, FADGDHs, especially those derived from fungi, fFADGDHs, are currently the most popular enzymes in the sensor strips of second-generation SMBG sensors. In addition, the third-generation principle, employing direct electron transfer (DET), is considered the most elegant approach and is ideal for use in electrochemical enzyme sensors. However, glucose oxidoreductases capable of DET are limited. One of the most prominent GDHs capable of DET is a bacteria-derived FADGDH complex (bFADGDH). bFADGDH has three distinct subunits; the FAD harboring the catalytic subunit, the small subunit, and the electron-transfer subunit, which makes bFADGDH capable of DET. In this review, we focused on the two representative glucose sensing enzymes, fFADGDHs and bFADGDHs, by presenting their discovery, sources, and protein and enzyme properties, and the current engineering strategies to improve their potential in sensor applications.

Efficient biosynthesis of 2-keto-D-gluconic acid by fed-batch culture of metabolically engineered Gluconobacter japonicus

2-keto-d-gluconic acid (2-KGA) is a key precursor for synthesising vitamin C and isovitamin C. However, phage contamination is as constant problem in industrial production of 2-KGA using Pseudomonas fluorescens. Gluconobacter holds promise for producing 2-KGA due to impressive resistance to hypertonicity and acids, and high utilisation of glucose. In this study, the 2-KGA synthesis pathway was regulated to enhance production of 2-KGA and reduce accumulation of the by-products 5-keto-d-gluconic acid (5-KGA) and d-gluconic acid (D-GA) in the 2-KGA producer Gluconobacter japonicus CGMCC 1.49. Knocking out the ga5dh-1 gene from a competitive pathway and overexpressing the ga2dh-A gene from the 2-KGA synthesis pathway via homologous recombination increased the titre of 2-KGA by 63.81% in shake flasks. Additionally, accumulation of 5-KGA was decreased by 63.52% with the resulting G. japonicas-Δga5dh-1-ga2dh-A strain. Using an intermittent fed-batch mode in a 3 L fermenter, 2-KGA reached 235.3 g L⁻¹ with a 91.1% glucose conversion rate. Scaling up in a 15 L fermenter led to stable 2-KGA titre with productivity of 2.99 g L⁻¹ h⁻¹, 11.99% higher than in the 3 L fermenter, and D-GA and 5-KGA by-products were completely converted to 2-KGA.

Comparative genome analysis reveals niche-specific genome expansion in Acinetobacter baumannii strains

The nosocomial pathogen Acinetobacter baumannii acquired clinical significance due to the rapid development of its multi-drug resistant (MDR) phenotype. A. baumannii strains have the ability to colonize several ecological niches including soil, water, and animals, including humans. They also survive under extremely harsh environmental conditions thriving on rare and recalcitrant carbon compounds. However, the molecular basis behind such extreme adaptability of A. baumannii is unknown. We have therefore determined the complete genome sequence of A. baumannii DS002, which was isolated from agricultural soils, and compared it with 78 complete genome sequences of A. baumannii strains having complete information on the source of their isolation. Interestingly, the genome of A. baumannii DS002 showed high similarity to the genome of A. baumannii SDF isolated from the body louse. The environmental and clinical strains, which do not share a monophyletic origin, showed the existence of a strain-specific unique gene pool that supports niche-specific survival. The strains isolated from infected samples contained a genetic repertoire with a unique gene pool coding for iron acquisition machinery, particularly those required for the biosynthesis of acinetobactin. Interestingly, these strains also contained genes required for biofilm formation. However, such gene sets were either partially or completely missing in the environmental isolates, which instead harbored genes required for alternate carbon catabolism and a TonB-dependent transport system involved in the acquisition of iron via siderophores or xenosiderophores.

A Membrane-Bound Gluconate Dehydrogenase from 2-Keto-D-Gluconic Acid Industrial Producing Strain Pseudomonas plecoglossicida JUIM01: Purification, Characterization, and Gene Identification

The membrane-bound gluconate dehydrogenase (mGADH) is a critical enzyme for 2-keto-D-gluconic acid (2KGA) production in Pseudomonas plecoglossicida JUIM01. The purified native flavin adenine dinucleotide-dependent mGADH (FAD-mGADH) was consisted of a gamma subunit, a flavoprotein subunit, and a cytochrome c subunit with molecular mass of ~ 27, 65, and 47 kDa, respectively. The specific activity of FAD-mGADH was determined as 90.71 U/mg at optimum pH and temperature of 6.0 and 35 °C. The K_m and V_max values of calcium D-gluconate were 0.631 mM and 0.734 mM/min. The metal ions Mg²⁺ and Mn²⁺ showed slight positive effects on FAD-mGADH activity. On the other hand, a 3868-bp-length gad gene cluster was amplified and expressed in Escherichia coli BL21(DE3). The recombinant protein showed the same molecular weight and enzyme activity as the native FAD-mGADH, which confirmed it as a FAD-mGADH encoding gene. The flavoprotein subunit and the cytochrome c subunit containing a putative FAD-binding motif and three possible heme-binding motifs concluded from alignment results of mGADHs. This study characterized the native and recombinant FAD-mGADH and would provide the basis for further genetic modification of Pseudomonas plecoglossicida JUIM01 with the intention of 2KGA productivity improvement.

Pyrroloquinoline quinone-dependent dehydrogenases of acetic acid bacteria

Pyrroloquinoline quinone (PQQ)-dependent dehydrogenases (quinoproteins) of acetic acid bacteria (AAB), such as the membrane-bound alcohol dehydrogenase (ADH) and the membrane-bound glucose dehydrogenase, contain PQQ as the prosthetic group. Most of them are located on the periplasmic surface of the cytoplasmic membrane, and function as primary dehydrogenases in cognate substance-oxidizing respiratory chains. Here, we have provided an overview on the function and molecular architecture of AAB quinoproteins, which can be categorized into six groups according to the primary amino acid sequences. Based on the genomic data, we discuss the types of quinoproteins found in AAB genome and how they are distributed. Our analyses indicate that a significant number of uncharacterized orphan quinoproteins are present in AAB. By reviewing recent experimental developments, we discuss how to characterize the as-yet-unknown enzymes. Moreover, our bioinformatics studies also provide insights on how quinoproteins have developed into intricate enzymes. ADH comprises at least two subunits: the quinoprotein dehydrogenase subunit encoded by adhA and the cytochrome subunit encoded by adhB, and the genes are located in a polycistronic transcriptional unit. Findings on stand-alone derivatives of adhA encourage us to speculate on a possible route for ADH development in the evolutional history of AAB. A combination of bioinformatics studies on big genome sequencing data and wet studies assisted with genetic engineering would unravel biochemical functions and physiological role of uncharacterized quinoproteins in AAB, or even in unculturable metagenome.

Determination of Dehydrogenase Activities Involved in D-Glucose Oxidation in Gluconobacter and Acetobacter Strains

Acetic acid bacteria (AAB) are known for rapid and incomplete oxidation of an extensively variety of alcohols and carbohydrates, resulting in the accumulation of organic acids as the final products. These oxidative fermentations in AAB are catalyzed by PQQ- or FAD- dependent membrane-bound dehydrogenases. In the present study, the enzyme activity of the membrane-bound dehydrogenases [membrane-bound PQQ-glucose dehydrogenase (mGDH), D-gluconate dehydrogenase (GADH) and membrane-bound glycerol dehydrogenase (GLDH)] involved in the oxidation of D-glucose and D-gluconic acid (GA) was determined in six strains of three different species of AAB (three natural and three type strains). Moreover, the effect of these activities on the production of related metabolites [GA, 2-keto-D-gluconic acid (2KGA) and 5-keto-D-gluconic acid (5KGA)] was analyzed. The natural strains belonging to Gluconobacter showed a high mGDH activity and low activity in GADH and GLDH, whereas the Acetobacter malorum strain presented low activity in the three enzymes. Nevertheless, no correlation was observed between the activity of these enzymes and the concentration of the corresponding metabolites. In fact, all the tested strains were able to oxidize D-glucose to GA, being maximal at the late exponential phase of the AAB growth (24 h), which coincided with D-glucose exhaustion and the maximum mGDH activity. Instead, only some of the tested strains were capable of producing 2KGA and/or 5KGA. In the case of Gluconobacter oxydans strains, no 2KGA production was detected which is related to the absence of GADH activity after 24 h, while in the remaining strains, detection of GADH activity after 24 h resulted in a high accumulation of 2KGA. Therefore, it is possible to choose the best strain depending on the desired product composition. Moreover, the sequences of these genes were used to construct phylogenetic trees. According to the sequence of gcd, gene coding for mGDH, Acetobacter and Komagataeibacter were phylogenetically more closely related each other than with Gluconobacter.

Combinatorial metabolic engineering of industrial Gluconobacter oxydans DSM2343 for boosting 5-keto-D-gluconic acid accumulation

Background

L-(+)-tartaric acid (L-TA) is an important organic acid, which is produced from the cream of tartar or stereospecific hydrolysis of the cis-epoxysuccinate. The former method is limited by the availability of raw material and the latter is dependent on the petrochemical material. Thus, new processes for the economical preparation of L-TA from carbohydrate or renewable resource would be much more attractive. Production of 5-keto-D-gluconate (5-KGA) from glucose by Gluconobacter oxydans is the first step to produce L-TA. The aim of this work is to enhance 5-KGA accumulation using combinatorial metabolic engineering strategies in G. oxydans. The sldAB gene, encoding sorbitol dehydrogenase, was overexpressed in an industrial strain G. oxydans ZJU2 under a carefully selected promoter, P₀₁₆₉. To enhance the efficiency of the oxidation by sldAB, the coenzyme pyrroloquinoline quinone (PQQ) and respiratory chain were engineered. Besides, the role in sldAB overexpression, coenzyme and respiratory chain engineering and their subsequent effects on 5-KGA production were investigated.

Results

An efficient, stable recombinant strain was constructed, whereas the 5-KGA production could be enhanced. By self-overexpressing the sldAB gene in G. oxydans ZJU2 under the constitutive promoter P₀₁₆₉, the resulting strain, G. oxydans ZJU3, produced 122.48 ± 0.41 g/L of 5-KGA. Furthermore, through the coenzyme and respiratory chain engineering, the titer and productivity of 5-KGA reached 144.52 ± 2.94 g/L and 2.26 g/(L · h), respectively, in a 15 L fermenter. It could be further improved the 5-KGA titer by 12.10 % through the fed-batch fermentation under the pH shift and dissolved oxygen tension (DOT) control condition, obtained 162 ± 2.12 g/L with the productivity of 2.53 g/(L · h) within 64 h.

Conclusions

The 5-KGA production could be significantly enhanced with the combinatorial metabolic engineering strategy in Gluconobacter strain, including sldAB overexpression, coenzyme and respiratory chain engineering. Fed-batch fermentation could further enlarge the positive effect and increase the 5-KGA production. All of these demonstrated that the robust recombinant strain can efficiently produce 5-KGA in larger scale to fulfill the industrial production of L-TA from 5-KGA.

Exploring membrane respiratory chains

Acquisition of energy is central to life. In addition to the synthesis of ATP, organisms need energy for the establishment and maintenance of a transmembrane difference in electrochemical potential, in order to import and export metabolites or to their motility. The membrane potential is established by a variety of membrane bound respiratory complexes. In this work we explored the diversity of membrane respiratory chains and the presence of the different enzyme complexes in the several phyla of life. We performed taxonomic profiles of the several membrane bound respiratory proteins and complexes evaluating the presence of their respective coding genes in all species deposited in KEGG database. We evaluated 26 quinone reductases, 5 quinol:electron carriers oxidoreductases and 18 terminal electron acceptor reductases. We further included in the analyses enzymes performing redox or decarboxylation driven ion translocation, ATP synthase and transhydrogenase and we also investigated the electron carriers that perform functional connection between the membrane complexes, quinones or soluble proteins. Our results bring a novel, broad and integrated perspective of membrane bound respiratory complexes and thus of the several energetic metabolisms of living systems. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

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).

Efficient Production of 2,5-Diketo-d-Gluconate via Heterologous Expression of 2-Ketogluconate Dehydrogenase in Gluconobacter japonicus

2,5-Diketo-d-gluconate (2,5DKG) is a compound that can be the intermediate for d-tartrate and also vitamin C production. Although Gluconobacter oxydans NBRC3293 produces 2,5DKG from d-glucose via d-gluconate and 2-keto-d-gluconate (2KG), with accumulation of the product in the culture medium, the efficiency of 2,5DKG production is unsatisfactory because there is a large amount of residual d-gluconate at the end of the biotransformation process. Oxidation of 2KG to 2,5DKG is catalyzed by a membrane-bound flavoprotein-cytochrome c complex: 2-keto-gluconate dehydrogenase (2KGDH). Here, we studied the kgdSLC genes encoding 2KGDH in G. oxydans NBRC3293 to improve 2,5DKG production by Gluconobacter spp. The kgdS, kgdL, and kgdC genes correspond to the small, large, and cytochrome subunits of 2KGDH, respectively. The kgdSLC genes were cloned into a broad-host-range vector carrying a DNA fragment of the putative promoter region of the membrane-bound alcohol dehydrogenase gene of G. oxydans for expression in Gluconobacter spp. According to our results, 2KGDH that was purified from the recombinant Gluconobacter cells showed characteristics nearly the same as those reported previously. We also expressed the kgdSLC genes in a mutant strain of Gluconobacter japonicus NBRC3271 (formerly Gluconobacter dioxyacetonicus IFO3271) engineered to produce 2KG efficiently from a mixture of d-glucose and d-gluconate. This mutant strain consumed almost all of the starting materials (d-glucose and d-gluconate) to produce 2,5DKG quantitatively as a seemingly unique metabolite. To our knowledge, this is the first report of a Gluconobacter strain that produces 2,5DKG efficiently and homogeneously.

2-Keto-D-gluconate-yielding membrane-bound D-glucose dehydrogenase from Arthrobacter globiformis C224: purification and characterization

Glucose dehydrogenase (GlcDH) is the rate-limiting catalyst for microbial conversion of glucose to the important organic acid 2-ketogluconic acid (2KGlcA). In this study, a d-glucose dehydrogenase was purified from the industrial 2KGlcA producer Arthrobacter globiformis C224. After four purification steps, the GlcDH was successfully purified over 180 folds and specific activity of 88.1 U/mg. A single protein band of 87 kDa was detected by SDS-PAGE. The purified GlcDH had the broad substrate specificity with the K_m values for d-glucose, d-xylose, d-galactose and maltose of 0.21 mM, 0.34 mM, 0.46 mM and 0.59 mM, respectively. The kinetic studies proved that A. globiformis GlcDH followed the ping-pong kinetic mechanism. The GlcDH showed an optimum catalytic activity at pH 5.0 and 45 °C with the stable activity at temperature of 20–40 °C and pH of 6.0–7.0. Organic solvents, metal ions or EDTA could significantly influence the GlcDH activity to different degrees.

2-ketogluconic acid secretion by incorporation of Pseudomonas putida KT 2440 gluconate dehydrogenase (gad) operon in Enterobacter asburiae PSI3 improves mineral phosphate solubilization

Enterobacter asburiae PSI3 is known to efficiently solubilize rock phosphate by secretion of approximately 50 mM gluconic acid in Tris-buffered medium in the presence of 75 mM glucose and in a mixture of seven aldosugars each at 15 mM concentration, mimicking alkaline vertisol soils. Efficacy of this bacterium in the rhizosphere requires P release in the presence of low amount of sugars. To achieve this, E. asburiae PSI3 has been manipulated to express gluconate dehydrogenase (gad) operon of Pseudomonas putida KT 2440 to produce 2-ketogluconic acid. E. asburiae PSI3 harboring gad operon had 438 U of GAD activity, secreted 11.63 mM 2-ketogluconic and 21.65 mM gluconic acids in Tris-rock phosphate-buffered medium containing 45 mM glucose. E. asburiae PSI3 gad transformant solubilized 0.84 mM P from rock phosphate in TRP-buffered liquid medium. In the presence of a mixture of seven sugars each at 12 mM, the transformant brought about a drop in pH to 4.1 and released 0.53 mM P.

Compartmentalized glucose metabolism in Pseudomonas putida is controlled by the PtxS repressor

Metabolic flux analysis revealed that in Pseudomonas putida KT2440 about 50% of glucose taken up by the cells is channeled through the 2-ketogluconate peripheral pathway. This pathway is characterized by being compartmentalized in the cells. In fact, initial metabolism of glucose to 2-ketogluconate takes place in the periplasm through a set of reactions catalyzed by glucose dehydrogenase and gluconate dehydrogenase to yield 2-ketogluconate. This metabolite is subsequently transported to the cytoplasm, where two reactions are carried out, giving rise to 6-phosphogluconate, which enters the Entner-Doudoroff pathway. The genes for the periplasmic and cytoplasmic set of reactions are clustered in the host chromosome and grouped within two independent operons that are under the control of the PtxS regulator, which also modulates its own synthesis. Here, we show that although the two catabolic operons are induced in vivo by glucose, ketogluconate, and 2-ketogluconate, in vitro we found that only 2-ketogluconate binds to the regulator with an apparent K(D) (equilibrium dissociation constant) of 15 muM, as determined using isothermal titration calorimetry assays. PtxS is made of two domains, a helix-turn-helix DNA-binding domain located at the N terminus and a C-terminal domain that binds the effector. Differential scanning calorimetry assays revealed that PtxS unfolds via two events characterized by melting points of 48.1 degrees C and 57.6 degrees C and that, in the presence of 2-ketogluconate, the unfolding of the effector binding domain occurs at a higher temperature, providing further evidence for 2-ketogluconate-PtxS interactions. Purified PtxS is a dimer that binds to the target promoters with affinities in the range of 1 to 3 muM. Footprint analysis revealed that PtxS binds to an almost perfect palindrome that is present within the three promoters and whose consensus sequence is 5'-TGAAACCGGTTTCA-3'. This palindrome overlaps with the RNA polymerase binding site.

Screening of thermotolerant Gluconobacter strains for production of 5-keto-D-gluconic acid and disruption of flavin adenine dinucleotide-containing D-gluconate dehydrogenase

We isolated thermotolerant Gluconobacter strains that are able to produce 5-keto-d-gluconic acid (5KGA) at 37 degrees C, a temperature at which regular mesophilic 5KGA-producing strains showed much less growth and 5KGA production. The thermotolerant strains produced 2KGA as the major product at both 30 and 37 degrees C. The amount of ketogluconates produced at 37 degrees C was slightly less than the amount produced at 30 degrees C. To improve the yield of 5KGA in these strains, we disrupted flavin adenine dinucleotide-gluconate dehydrogenase (FAD-GADH), which is responsible for 2KGA production. Genes for FAD-GADH were cloned by using inverse PCR and an in vitro cloning strategy. The sequences obtained for three thermotolerant strains were identical and showed high levels of identity to the FAD-GADH sequence reported for the genome of Gluconobacter oxydans 621 H. A kanamycin resistance gene cassette was used to disrupt the FAD-GADH genes in the thermotolerant strains. The mutant strains produced 5KGA exclusively, and the final yields were over 90% at 30 degrees C and 50% at 37 degrees C. We found that the activity of pyrroloquinoline quinone (PQQ)-dependent glycerol dehydrogenase, which is responsible for 5KGA production, increased in response to addition of PQQ and CaCl(2) in vitro when cells were grown at 37 degrees C. Addition of 5 mM CaCl(2) to the culture medium of the mutant strains increased 5KGA production to the point where over 90% of the initial substrate was converted. The thermotolerant Gluconobacter strains that we isolated in this study provide a promising new option for industrial 5KGA production.