Computer-Aided Semi-Rational Design to Enhance the Activity of l-Sorbosone Dehydrogenase from Gluconobacter oxidans WSH-004

The titer of the microbial fermentation products can be increased by enzyme engineering. l-Sorbosone dehydrogenase (SNDH) is a key enzyme in the production of 2-keto-l-gulonic acid (2-KLG), which is the precursor of vitamin C. Enhancing the activity of SNDH may have a positive impact on 2-KLG production. In this study, a computer-aided semirational design of SNDH was conducted. Based on the analysis of SNDH's substrate pocket and multiple sequence alignment, three modification strategies were established: (1) expanding the entrance of SNDH's substrate pocket, (2) engineering the residues within the substrate pocket, and (3) enhancing the electron transfer of SNDH. Finally, mutants S453A, L460V, and E471D were obtained, whose specific activity was increased by 20, 100, and 10%, respectively. In addition, the ability of Gluconobacter oxidans WSH-004 to synthesize 2-KLG was improved by eliminating H2O2. This study provides mutant enzymes and metabolic engineering strategies for the microbial-fermentation-based production of 2-KLG.

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.

Identification of Pathways for Production of D-Glucaric Acid by Pseudogluconobacter saccharoketogenes

Pseudogluconobacter saccharoketogenes produces glucaric acid from D-glucose via two pathways, i.e., through D-glucuronic acid or D-gluconic acid. These pathways are catalyzed by alcohol dehydrogenase, aldehyde dehydrogenase, and gluconate dehydrogenase. Although D-glucaraldehyde and L-guluronic acid are also theorized to be produced in pathways throsugh D-glucuronic acid and D-gluconic acid, respectively, no direct data to identify these intermediates have been reported. In this study, the intermediates were purified and identified as D-glucaraldehyde and L-guluronic acid. The substrate specificities of the three enzymes on these intermediates and their oxidation products were studied, and the roles of alcohol, aldehyde, and gluconate dehydrogenases in D-glucaric acid-producing pathways were elucidated using the intermediates. Additionally, the substrate specificities of alcohol and aldehyde dehydrogenases on some alcohols, aldehydes, and aldoses were determined. Alcohol dehydrogenase showed wide substrate specificities, whereas the substrates oxidized by aldehyde dehydrogenase were limited. A 30-L scale reaction using the resting cells of Rh47-3 revealed that D-glucaric acid was produced from D-glucose and D-gluconic acid in 60.3 mol% (7.0 g/L) and 78.6 mol% (22.5 g/L) yields, respectively.

Insights into the pH-dependent, extracellular sucrose utilization and concomitant levan formation by Gluconobacter albidus TMW 2.1191

Many bacteria and archaea produce the polydisperse fructose polymer levan from sucrose upon biofilm formation via extracellular levansucrases (EC 2.4.1.10). We have investigated levansucrase-release and -activities as well as molecular size of the levan formed by the acetic acid bacterium Gluconobacter albidus TMW 2.1191 at varying environmental pH conditions to obtain insight in the ecological role of its constitutively expressed levansucrase and the produced levan. A buffer system was established enabling the recovery of levansucrase-containing supernatants from preincubated cell suspensions at pH 4.3-pH 5.7. The enzyme solutions were used to produce levans at different pH values and sucrose concentrations. Finally, the amounts and size distributions of the produced levans as well as the corresponding levansucrase activities were determined and correlated with each other. The data revealed that the levansucrase was released into the environment independently of its substrate sucrose, and that more levansucrase was released at pH ≥ 5.0. The glucose release and formation of high molecular weight levans (> 3.5 kDa) from 0.1 M initial sucrose was comparable between pH ~ 4.3-5.7 using equal amounts of released levansucrase. Hence, this type of levansucrase appears to be structurally adapted to changes in the extracellular pH and to exhibit a similar total activity over a wide acidic pH range, while it produced higher amounts of larger levan molecules at higher production pH and sucrose concentrations. These findings indicate the physiological adaptation of G. albidus TMW 2.1191 to efficient colonisation of sucrose-rich habitats via released levansucrases despite changing extracellular pH conditions in course of acid formation.

Molecular characterization of a novel arylesterase from the wine-associated acetic acid bacterium Gluconobacter oxidans 621H

An arylesterase from the wine-making acetic acid bacterium, Gluconobacter oxidans, was cloned and expressed into Escherichia coli. The soluble 76.8 kDa dimeric enzyme obtained, Est0881, was purified in only two steps with a 3.1-fold purification, 43% recovery, and a specific activity of 214 U/mg for the hydrolysis of p-nitrophenyl acetate. The optimum pH and temperature were 7.0 and 40 °C, respectively. The substrate specificity of this arylesterase was higher toward short chain p-nitrophenyl esters (C(2) to C(4)) and also toward aromatic esters, such as phenyl acetate. The deduced amino acid sequence shares high identity with esterases of the HSL family. The inhibition results obtained showed that the enzyme was a serine esterase, belonging to the A-esterases (arylesterases) and contains a catalytic triad composed of Ser163, Asp263, and His293 in the active site. Est0881 retained significant activity under conditions simulating those of wine-making (75% activity at 20% ethanol), making it a promising biocatalyst for modulating the final aroma of wine.

Bioelectrocatalysis of ethanol via PQQ-dependent dehydrogenases utilizing carbon nanomaterial supports

In bioelectrocatalysis, nanomaterials are typically used as a conductive bridge for the gap between the site of oxidation/reduction (i.e., enzymatic biocatalyst) and the current collector (electrode). In this paper, carbon nanomaterial supports have been employed in conjunction with heme-c containing pyrroloquinoline quinone-dependent alcohol dehydrogenase (PQQ-ADH) and aldehyde dehydrogenase (PQQ-AldDH) oxidoreductase enzymes as oxidation catalysts to produce stable high surface area catalyst supports for the bioelectrocatalysis of ethanol in biofuel cells. The structure of PQQ-ADH and PQQ-AldDH allow for direct electron transfer (DET) between the enzymes and carbon nanomaterial support without the use of additional charge carrying chemical mediators. In this paper, the employment of nanomaterials are used to produce stable, high surface area catalyst supports which aid in enzyme adsorption and direct electron transfer. Fundamental DET studies were performed on both PQQ-ADH and PQQ-AldDH in order to understand the processes occurring at the electrode surface. Data shows a direct correlation between concentration of substrate and peak potential and peak current. Incorporating nanotubes into this technology has allowed an increase in the current density of ethanol/air biofuel cells by up to 14.5 fold and increased the power density by up to 18.0 fold.

Purification and characterization of alcohol dehydrogenase from Gluconobacter suboxydans

Purification and characterization of alcohol dehydrogenase (ADH) from Gluconobacter suboxydans was done in order to biotechnological and industrial application. Solubilization of enzyme from bacterial membrane fraction by Triton X-100 and subsequent fractionation on DEAE-Sephadex A-50 and Hydroxyapatite was successful in enzyme purification. Enzyme assay reaction mixture contained potassium ferricyanide 0.1 M, McIlvaine buffer 0.1 M (pH 5.5), Triton X-100 10%, ethanol 1 M and enzyme solution. The purified ADH Optimum pH activity was 5.5. The enzyme was in maximum stability in pH 5.8. The substrate specificity of the enzyme was determined using the same enzyme assay method as described above, except that various substrates (100 mM) were used instead of ethanol. The relative activity of the ADH for ethanol was higher than the others. The effects of metal ions and inhibitors on the activity of the enzyme were examined by measuring the activity using the same assay method as described above. Activity of purified enzyme was increased in the presence of Ca(+2) and was decreased in presence the of ethylenediamine tetra acetic acid (EDTA). Because the proper structure and function of the enzyme is related to structural Ca(+2) and EDTA can chelate Ca(+2). An apparent Michaelis constant for ethanol were examined to be 1.7 x 10(-3) M for ethanol as substrate.

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

Most Gluconobacter species produce and accumulate 2-keto-d-gluconate (2KGA) and 5KGA simultaneously from d-glucose via GA in culture medium. 2KGA is produced by membrane-bound flavin adenine dinucleotide-containing GA 2-dehydrogenase (FAD-GADH). FAD-GADH was purified from "Gluconobacter dioxyacetonicus" IFO 3271, and N-terminal sequences of the three subunits were analyzed. PCR primers were designed from the N-terminal sequences, and part of the FAD-GADH genes was cloned as a PCR product. Using this PCR product, gene fragments containing whole FAD-GADH genes were obtained, and finally the nucleotide sequence of 9,696 bp was determined. The cloned sequence had three open reading frames (ORFs), gndS, gndL, and gndC, corresponding to small, large, and cytochrome c subunits of FAD-GADH, respectively. Seven other ORFs were also found, one of which showed identity to glucono-delta-lactonase, which might be involved directly in 2KGA production. Three mutant strains defective in either gndL or sldA (the gene responsible for 5KGA production) or both were constructed. Ferricyanide-reductase activity with GA in the membrane fraction of the gndL-defective strain decreased by about 60% of that of the wild-type strain, while in the sldA-defective strain, activity with GA did not decrease and activities with glycerol, d-arabitol, and d-sorbitol disappeared. Unexpectedly, the strain defective in both gndL and sldA (double mutant) still showed activity with GA. Moreover, 2KGA production was still observed in gndL and double mutant strains. 5KGA production was not observed at all in sldA and double mutant strains. Thus, it seems that "G. dioxyacetonicus" IFO 3271 has another membrane-bound enzyme that reacts with GA, producing 2KGA.

Fructose/dioxygen biofuel cell based on direct electron transfer-type bioelectrocatalysis

One-compartment biofuel cells without separators have been constructed, in which d-fructose dehydrogenase (FDH) from Gluconobacter sp. and laccase from Trametes sp. (TsLAC) work as catalysts of direct electron transfer (DET)-type bioelectrocatalysis in the two-electron oxidation of d-fructose and four-electron reduction of dioxygen as fuels, respectively. FDH adsorbs strongly and stably on Ketjen black (KB) particles that have been modified on carbon papers (CP) and produces the catalytic current with the maximum density of about 4 mA cm(-2) without mediators at pH 5. The catalytic wave of the d-fructose oxidation is controlled by the enzyme kinetics. The location and the shape of the catalytic waves suggest strongly that the electron is directly transferred to the KB particles from the heme c site in FDH, of which the formal potential has been determined to be 39 mV vs. Ag|AgCl|sat. KCl. Electrochemistry of three kinds of multi-copper oxidases has also been investigated and TsLAC has been selected as the best one of the DET-type bioelectrocatalyst for the four-electron reduction of dioxygen in view of the thermodynamics and kinetics at pH 5. In the DET-type bioelectrocatalysis, the electron from electrodes seems to be transferred to the type I copper site of multi-copper oxidases. TsLAC adsorbed on carbon aerogel (CG) particles with an average pore size of 22 nm, that have been modified on CP electrodes, produces the catalytic reduction current of dioxygen with a density of about 4 mA cm(-2), which is governed by the mass transfer of the dissolved dioxygen. The FDH-adsorbed KB-modified CP electrodes and the TsLAC-adsorbed CG-modified CP electrodes have been combined to construct one-compartment biofuel cells without separators. The open-circuit voltage was 790 mV. The maximum current density of 2.8 mA cm(-2) and the maximum power density of 850 microW cm(-2) have been achieved at 410 mV of the cell voltage under stirring.

Gluconobacter in biosensors: applications of whole cells and enzymes isolated from gluconobacter and acetobacter to biosensor construction

Bacteria belonging to the genus Acetobacter and Gluconobacter, and enzymes isolated from them, have been extensively used for biosensor construction in the last decade. Bacteria used as a biocatalyst are easy to prepare and use in amperometric biosensors. They contain multiple enzyme activities otherwise not available commercially. The range of compounds analyzable by Gluconobacter biosensors includes: mono- and poly-alcohols, multiple aldoses and ketoses, several disaccharides, triacylglycerols, and complex parameters like utilizable saccharides or biological O2 demand. Here, the recent trends in Gluconobacter biosensors and current practical applications are summarized.

Structure-guided engineering of xylitol dehydrogenase cosubstrate specificity

Xylitol dehydrogenase (XDH) is one of several enzymes responsible for assimilating xylose into eukaryotic metabolism and is useful for fermentation of xylose contained in agricultural byproducts to produce ethanol. For efficient xylose utilization at high flux rates, cosubstrates should be recycled between the NAD+-specific XDH and the NADPH-preferring xylose reductase, another enzyme in the pathway. To understand and alter the cosubstrate specificity of XDH, we determined the crystal structure of the Gluconobacter oxydans holoenzyme to 1.9 angstroms resolution. The structure reveals that NAD+ specificity is largely conferred by Asp38, which interacts with the hydroxyls of the adenosine ribose. Met39 stacked under the purine ring and was also located near the 2' hydroxyl. Based on the location of these residues and on sequence alignments with related enzymes of various cosubstrate specificities, we constructed a double mutant (D38S/M39R) that was able to exclusively use NADP+, with no loss of activity.

Molecular properties of membrane-bound FAD-containing D-sorbitol dehydrogenase from thermotolerant Gluconobacter frateurii isolated from Thailand

There are two types of membrane-bound D-sorbitol dehydrogenase (SLDH) reported: PQQ-SLDH, having pyrroloquinoline quinone (PQQ), and FAD-SLDH, containing FAD and heme c as the prosthetic groups. FAD-SLDH was purified and characterized from the PQQ-SLDH mutant strain of a thermotolerant Gluconobacter frateurii, having molecular mass of 61.5 kDa, 52 kDa, and 22 kDa. The enzyme properties were quite similar to those of the enzyme from mesophilic G. oxydans IFO 3254. This enzyme was shown to be inducible by D-sorbitol, but not PQQ-SLDH. The oxidation product of FAD-SLDH from D-sorbitol was identified as L-sorbose. The cloned gene of FAD-SLDH had three open reading frames (sldSLC) corresponding to the small, the large, and cytochrome c subunits of FAD-SLDH respectively. The deduced amino acid sequences showed high identity to those from G. oxydans IFO 3254: SldL showed to other FAD-enzymes, and SldC having three heme c binding motives to cytochrome c subunits of other membrane-bound dehydrogenases.

Biofuel cell based on direct bioelectrocatalysis

A biofuel cell, consisting of two 3mm diameter carbon rod electrodes and operating at ambient temperature in aqueous solution, pH 6, is described. Biofuel cell based on enzymes able to exchange directly electrons with carbon electrodes was constructed and characterized. Anode of the biofuel cell was based on immobilized Quino-hemoprotein alcohol dehydrogenase from Gluconobacter sp. 33 (QH-ADH), cathode on co-immobilized glucose oxidase from Aspergilus niger (GO(x)) and microperoxidase 8 from the horse heart (MP-8) acting in the consecutive mode. Two enzymes GO(x) and MP-8 applied in the design of biofuel cell cathode were acting in consecutive mode and by hydrogen peroxide oxidized MP-8 was directly accepting electrons from carbon rod electrode. If ethanol was applied as an energy source the maximal open circuit potential of the biofuel cell was -125 mV. If glucose was applied as energy source the open circuit potential of the cell was +145 mV. The maximal open circuit potential (270 mV) was achieved in the presence of extent concentration (over 2 mM) of both substrates (ethanol and glucose). Operational half-life period (tau(1/2)) of the biofuel cell was found to be 2.5 days.

Batch and fed-batch production of coenzyme Q10 in recombinant Escherichia coli containing the decaprenyl diphosphate synthase gene from Gluconobacter suboxydans

Coenzyme Q(10) (CoQ(10)) is a quinine consisting of ten units of the isoprenoid side-chain. Because it limits the oxidative attack of free radicals to DNA and lipids, CoQ(10) has been used as an antioxidant for foods, cosmetics and pharmaceuticals. Decaprenyl diphosphate synthase (DPS) is the key enzyme for synthesis of the decaprenyl tail in CoQ(10) with isopentenyl diphosphate. The ddsA gene coding for DPS from Gluconobacter suboxydans was expressed under the control of an Escherichia coli constitutive promoter. Analysis of the cell extract in recombinant E. coli BL21/pACDdsA by high performance liquid chromatography and mass spectrometry showed that CoQ(10) rather than endogenous CoQ(8) was biologically synthesized as the major coenzyme Q. Expression of the ddsA gene with low copy number led to the accumulation of CoQ(10) to 0.97 mg l(-1) in batch fermentation. A high cell density (103 g l(-1)) in fed-batch fermentation of E. coli BL21/pACDdsA increased the CoQ(10) concentration to 25.5 mg l (-1) and its productivity to 0.67 mg l(-1) h(-1), which were 26.0 and 6.9 times higher than the corresponding values for batch fermentation.

Purification and characterization of membrane-bound quinoprotein quinate dehydrogenase

Several bacterial strains carrying quinoprotein quinate dehydrogenase (QDH) were screened through acetic acid bacteria and other bacteria. Strong enzyme activity was found in the membrane fraction of Gluconobacter melanogenus IFO 3294, G. oxydans IFO 3292, G. oxydans IFO 3244, and some strains of Acinetobacter calcoaceticus. Interestingly, in the membrane fraction of A. calcoaceticus AC3, which is unable to produce pyrroloquinoline quinone (PQQ), fairly large amounts of apo-QDH were formed, and were converted to holo-QDH only by the addition of PQQ. It was difficult to detach PQQ from the holo-QDH by EDTA treatment, and EDTA treatment with apo-QDH prior to PQQ addition gave no significant holo-QDH. For QDH purification, Gluconobacter strains were not suitable due to the presence of huge amounts of quinohemoprotein alcohol dehydrogenase (ADH) in the same membrane, which was co-solubilized with QDH and disturbed purification of QDH. Purification of holo-QDH was done with Acinetobacter sp. SA1 instead, which contained no ADH. Apo-QDH was purified from A. aclcoaceticus AC3. This is the first report dealing with QDH purification, and two different criteria of QDH purification were given. A combination of two steps using butyl-Toyopearl and hydroxyapatite columns gave a highly purified holo-QDH which was monodispersed and showed enough purity, though the specific activity did not increase as much as expected. When QDH purification was done with A. calcoaceticus AC3 in the absence of PQQ, purified apo-QDH appeared to be a dimer, which was converted to the monomer on addition of PQQ. Since QDH was highly hydrophobic, one-step chromatography on a DEAE-Sepharose column was tried. Purified holo-QDH of higher specific activity was obtained with a higher yield. The molecular mass of QDH was estimated to be 88 kDa. There was no characteristic absorption spectrum with the purified QDH except for a small bump around 420 nm. QDH oxidized only quinate and shikimate so far examined. The optimal QDH activity was found at pH 6-7 when assayed with artificial electron acceptors. QDH was formed in the presence or absence of quinate in the culture medium, although stronger induction was usually observed in the presence of quinate.

Cloning of a gluconate/polyol dehydrogenase gene from Gluconobacter suboxydans IFO 12528, characterisation of the enzyme and its use for the production of 5-ketogluconate in a recombinant Escherichia coli strain

A 5-ketogluconate (5-KGA)-forming membrane quinoprotein, gluconate dehydrogenase, was isolated from Gluconobacter suboxydans strain IFO 12528 and partially sequenced. Partial sequences of five internal tryptic peptides were elucidated by mass spectrometry and used to isolate the two adjacent genes encoding the enzyme (EBI accession no. AJ577472). These genes share close homology with sorbitol dehydrogenase from another strain of G. suboxydans (IFO 3255). Substrate specificity of gluconate 5-dehydrogenase (GA 5-DH) turned out to be quite broad, covering many polyols, amino derivatives of carbohydrates, and simple secondary alcohols. There is a broad correlation between the substrate specificity of GA 5-DH and the empirical Bertrand-Hudson rule that predicts the specificity of oxidation of polyols by acetic acid bacteria. Escherichia coli transformed with the genes encoding gluconate dehydrogenase were able to convert gluconic acid into 5-KGA at 75% yield. Furthermore, it was found that 5-KGA can be converted into tartaric acid semialdehyde by a transketolase. These results provide a basis for designing a direct fermentation-based process for conversion of glucose into tartaric acid.

Membrane-bound D-sorbitol dehydrogenase of Gluconobacter suboxydans IFO 3255--enzymatic and genetic characterization

Gluconobacter strains effectively produce L-sorbose from D-sorbitol because of strong activity of the D-sorbitol dehydrogenase (SLDH). L-sorbose is one of the important intermediates in the industrial vitamin C production process. Two kinds of membrane-bound SLDHs, which consist of three subunits, were reportedly found in Gluconobacter strains [Agric. Biol. Chem. 46 (1982) 135,FEMS Microbiol. Lett. 125 (1995) 45]. We purified a one-subunit-type SLDH (80 kDa) from the membrane fraction of Gluconobacter suboxydans IFO 3255 solubilized with Triton X-100 in the presence of D-sorbitol, but the cofactor could not be identified from the purified enzyme. The SLDH was active on mannitol, glycerol and other sugar alcohols as well as on D-sorbitol to produce respective keto-aldoses. Then, the SLDH gene (sldA) was cloned and sequenced. It encodes the polypeptide of 740 residues, which contains a signal sequence of 24 residues. SLDH had 35-37% identity to those of membrane-bound quinoprotein glucose dehydrogenases (GDHs) from Escherichia coli, Gluconobacter oxydans and Acinetobacter calcoaceticus except the N-terminal hydrophobic region of GDH. Additionally, the sldB gene located just upstream of sldA was found to encode the polypeptide consisting of 126 very hydrophobic residues that is similar to the one-sixth N-terminal region of the GDH. Development of the SLDH activity in E. coli required co-expression of the sldA and sldB genes and the presence of PQQ. The sldA gene disruptant showed undetectable oxidation activities on D-sorbitol in growing culture, and resting-cell reaction (pH 4.5 and 7); in addition, they showed undetectable activities on D-mannitol and glycerol. The disruption of the sldB gene by a gene cassette with a downward promoter to express the sldA gene resulted in formation of a larger size of the SLDH protein and in undetectable oxidation of the polyols. In conclusion, the SLDH of the strain 3255 functions as the main polyol dehydrogenase in vivo. The sldB polypeptide possibly has a chaperone-like function to process the SLDH polypeptide into a mature and active form.

5-keto-D-gluconate production is catalyzed by a quinoprotein glycerol dehydrogenase, major polyol dehydrogenase, in gluconobacter species

Acetic acid bacteria, especially Gluconobacter species, have been known to catalyze the extensive oxidation of sugar alcohols (polyols) such as D-mannitol, glycerol, D-sorbitol, and so on. Gluconobacter species also oxidize sugars and sugar acids and uniquely accumulate two different keto-D-gluconates, 2-keto-D-gluconate and 5-keto-D-gluconate, in the culture medium by the oxidation of D-gluconate. However, there are still many controversies regarding their enzyme systems, especially on D-sorbitol and also D-gluconate oxidations. Recently, pyrroloquinoline quinone-dependent quinoprotein D-arabitol dehydrogenase and D-sorbitol dehydrogenase have been purified from G. suboxydans, both of which have similar and broad substrate specificity towards several different polyols. In this study, both quinoproteins were shown to be identical based on their immuno-cross-reactivity and also on gene disruption and were suggested to be the same as the previously isolated glycerol dehydrogenase (EC 1.1.99.22). Thus, glycerol dehydrogenase is the major polyol dehydrogenase involved in the oxidation of almost all sugar alcohols in Gluconobacter sp. In addition, the so-called quinoprotein glycerol dehydrogenase was also uniquely shown to oxidize D-gluconate, which was completely different from flavoprotein D-gluconate dehydrogenase (EC 1.1.99.3), which is involved in the production of 2-keto-D-gluconate. The gene disruption experiment and the reconstitution system of the purified enzyme in this study clearly showed that the production of 5-keto-D-gluconate in G. suboxydans is solely dependent on the quinoprotein glycerol dehydrogenase.

Main polyol dehydrogenase of Gluconobacter suboxydans IFO 3255, membrane-bound D-sorbitol dehydrogenase, that needs product of upstream gene, sldB, for activity

The D-sorbitol dehydrogenase gene, sldA, and an upstream gene, sldB, encoding a hydrophobic polypeptide, SldB, of Gluconobacter suboxydans IFO 3255 were disrupted in a check of their biological functions. The bacterial cells with the sldA gene disrupted did not produce L-sorbose by oxidation of D-sorbitol in resting-cell reactions at pHs 4.5 and 7.0, indicating that the dehydrogenase was the main D-sorbitol-oxidizing enzyme in this bacterium. The cells did not produce D-fructose from D-mannitol or dihydroxyacetone from glycerol. The disruption of the sldB gene resulted in undetectable oxidation of D-sorbitol, D-mannitol, or glycerol, although the cells produced the dehydrogenase. The cells with the sldB gene disrupted produced more of what might be signal-unprocessed SldA than the wild-type cells did. SldB may be a chaperone-like component that assists signal processing and folding of the SldA polypeptide to form active D-sorbitol dehydrogenase.

NADPH-dependent L-sorbose reductase is responsible for L-sorbose assimilation in Gluconobacter suboxydans IFO 3291

The NADPH-dependent L-sorbose reductase (SR) of L-sorbose-producing Gluconobacter suboxydans IFO 3291 contributes to intracellular L-sorbose assimilation. The gene disruptant showed no SR activity and did not assimilate the once-produced L-sorbose, indicating that the SR functions mainly as an L-sorbose-reducing enzyme in vivo and not as a D-sorbitol-oxidizing enzyme.

Molecular cloning and functional expression of D-sorbitol dehydrogenase from Gluconobacter suboxydans IF03255, which requires pyrroloquinoline quinone and hydrophobic protein SldB for activity development in E. coli

The sldA gene that encodes the D-sorbitol dehydrogenase (SLDH) from Gluconobacter suboxydans IFO 3255 was cloned and sequenced. It encodes a polypeptide of 740 residues, which contains a signal sequence of 24 residues. SLDH had 35-37% identity to the membrane-bound quinoprotein glucose dehydrogenases (GDHs) from E. coli, Gluconobacter oxydans, and Acinetobacter calcoaceticus except the N-terminal hydrophobic region of GDH. Additionally, the sldB gene located just upstream of sldA was found to encode a polypeptide consisting of 126 very hydrophobic residues that is similar in sequence to the one-sixth N-terminal region of the GDH. For the development of the SLDH activity in E. coli, co-expression of the sldA and sldB genes and the presence of pyrrloquinolone quinone as a co-factor were required.

Purification and properties of membrane-bound D-sorbitol dehydrogenase from Gluconobacter suboxydans IFO 3255

D-Sorbitol dehydrogenase was solubilized from the membrane fraction of Gluconobacter suboxydans IFO 3255 with Triton X-100 in the presence of D-sorbitol. Purification of the enzyme was done by fractionation with column chromatographies of DEAE-Cellulose, DEAE-Sepharose, hydroxylapatite, and Sephacryl HR300 in the presence of Triton X-100. The molecular mass of the enzyme was 800 kDa, consisting of homologous subunits of 80 kDa. The optimum pH of the enzyme activity was 6.0, and the optimum temperature was 30 degrees C. Western blot analysis suggested the occurrence of the enzyme in all the Gluconobacter strains tested.

Membrane-bound quinoprotein D-arabitol dehydrogenase of Gluconobacter suboxydans IFO 3257: a versatile enzyme for the oxidative fermentation of various ketoses

Solubilization of membrane-bound quinoprotein D-arabitol dehydrogenase (ARDH) was done successfully with the membrane fraction of Gluconobacter suboxydans IFO 3257. In enzyme solubilization and subsequent enzyme purification steps, special care was taken to purify ARDH as active as it was in the native membrane, after many disappointing trials. Selection of the best detergent, keeping ARDH as the holoenzyme by the addition of PQQ and Ca2+, and of a buffer system involving acetate buffer supplemented with Ca2+, were essential to treat the highly hydrophobic and thus labile enzyme. Purification of the enzyme was done by two steps of column chromatography on DEAE-Toyopearl and CM-Toyopearl in the presence of detergent and Ca2+. ARDH was homogenous and showed a single sedimentation peak in analytical ultracentrifugation. ARDH was dissociated into two different subunits upon SDS-PAGE with molecular masses of 82 kDa (subunit I) and 14 kDa (subunit II), forming a heterodimeric structure. ARDH was proven to be a quinoprotein by detecting a liberated PQQ from SDS-treated ARDH in HPLC chromatography. More preliminarily, an EDTA-treated membrane fraction lost the enzyme activity and ARDH activity was restored to the original level by the addition of PQQ and Ca2+. The most predominant unique character of ARDH, the substrate specificity, was highly versatile and many kinds of substrates were oxidized irreversibly by ARDH, not only pentitols but also other polyhydroxy alcohols including D-sorbitol, D-mannitol, glycerol, meso-erythritol, and 2,3-butanediol. ARDH may have its primary function in the oxidative fermentation of ketose production by acetic acid bacteria. ARDH contained no heme component, unlike the type II or type III quinoprotein alcohol dehydrogenase (ADH) and did not react with primary alcohols.

Purification and characterization of membrane-bound quinoprotein cyclic alcohol dehydrogenase from Gluconobacter frateurii CHM 9

A quinoprotein catalyzing oxidation of cyclic alcohols was found in the membrane fraction for the first time, after extensive screening among aerobic bacteria. Gluconobacter frateurii CHM 9 was finally selected in this study. The enzyme tentatively named membrane-bound cyclic alcohol dehydrogenase (MCAD) was found to occur specifically in the membrane fraction, and pyrroloquinoline quinone (PQQ) was functional as the primary coenzyme in the enzyme activity. MCAD catalyzed only oxidation reaction of cyclic alcohols irreversibly to corresponding ketones. Unlike already known cytosolic NAD(P)H-dependent alcohol-aldehyde or alcohol-ketone oxidoreductases, MCAD was unable to catalyze the reverse reaction of cyclic ketones or aldehydes to cyclic alcohols. MCAD was solubilized and purified from the membrane fraction of the organism to homogeneity. Differential solubilization to eliminate the predominant quinoprotein alcohol dehydrogenase (ADH), and the subsequent two steps of column chromatographies, brought MCAD to homogeneity. Purified MCAD had a molecular mass of 83 kDa by SDS-PAGE. Substrate specificity showed that MCAD was an enzyme oxidizing a wide variety of cyclic alcohols. Some minor enzyme activity was found with aliphatic secondary alcohols and sugar alcohols, but not primary alcohols, differentiating MCAD from quinoprotein ADH. NAD-dependent cytosolic cyclic alcohol dehydrogenase (CCAD) in the same organism was crystallized and its catalytic and physicochemical properties were characterized. Judging from the catalytic properties of CCAD, it was apparent that CCAD was distinct from MCAD in many respects and seemed to make no contributions to cyclic alcohol oxidation.

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.

Membrane-bound sugar alcohol dehydrogenase in acetic acid bacteria catalyzes L-ribulose formation and NAD-dependent ribitol dehydrogenase is independent of the oxidative fermentation

To identify the enzyme responsible for pentitol oxidation by acetic acid bacteria, two different ribitol oxidizing enzymes, one in the cytosolic fraction of NAD(P)-dependent and the other in the membrane fraction of NAD(P)-independent enzymes, were examined with respect to oxidative fermentation. The cytoplasmic NAD-dependent ribitol dehydrogenase (EC 1.1.1.56) was crystallized from Gluconobacter suboxydans IFO 12528 and found to be an enzyme having 100 kDa of molecular mass and 5 s as the sedimentation constant, composed of four identical subunits of 25 kDa. The enzyme catalyzed a shuttle reversible oxidoreduction between ribitol and D-ribulose in the presence of NAD and NADH, respectively. Xylitol and L-arabitol were well oxidized by the enzyme with reaction rates comparable to ribitol oxidation. D-Ribulose, L-ribulose, and L-xylulose were well reduced by the enzyme in the presence of NADH as cosubstrates. The optimum pH of pentitol oxidation was found at alkaline pH such as 9.5-10.5 and ketopentose reduction was found at pH 6.0. NAD-Dependent ribitol dehydrogenase seemed to be specific to oxidoreduction between pentitols and ketopentoses and D-sorbitol and D-mannitol were not oxidized by this enzyme. However, no D-ribulose accumulation was observed outside the cells during the growth of the organism on ribitol. L-Ribulose was accumulated in the culture medium instead, as the direct oxidation product catalyzed by a membrane-bound NAD(P)-independent ribitol dehydrogenase. Thus, the physiological role of NAD-dependent ribitol dehydrogenase was accounted to catalyze ribitol oxidation to D-ribulose in cytoplasm, taking D-ribulose to the pentose phosphate pathway after being phosphorylated. L-Ribulose outside the cells would be incorporated into the cytoplasm in several ways when need for carbon and energy sources made it necessary to use L-ribulose for their survival. From a series of simple experiments, membrane-bound sugar alcohol dehydrogenase was concluded to be the enzyme responsible for L-ribulose production in oxidative fermentation by acetic acid bacteria.