Overproduction and characterization of two distinct aldehyde-oxidizing enzymes from Gluconobacter oxydans 621H

Identification of a Novel Dehydrogenase from Gluconobacter oxydans for Degradation of Inhibitors Derived from Lignocellulosic Biomass

Inhibitors from lignocellulosic biomass have become the bottleneck of biorefinery development. Gluconobacter oxydans DSM2003 showed a high performance of inhibitors degradation, which had a short lag time in non-detoxified corn stover hydrolysate and could convert 90% of aldehyde inhibitors to weaker toxic acids. In this study, an aldehyde dehydrogenase gene W826-RS0111485, which plays an important function in the conversion of aldehyde inhibitors in Gluconobacter oxydans DSM2003, was identified. W826-RS0111485 was found by protein profiling, then a series of enzymatic properties were determined and were heterologously expressed in E. coli. The results indicated that NADP is the most suitable cofactor of the enzyme when aldehyde inhibitor is the substrate, and it had the highest oxidation activity to furfural among several aldehyde inhibitors. Under the optimal reaction conditions (50 °C, pH 7.5), the Km and Vmax of the enzyme under furfural stress were 2.45 and 80.97, respectively, and the Kcat was 232.22 min−1. The biodetoxification performance experiments showed that the recombinant E. coli containing the target gene completely converted 1 g/L furfural to furoic acid within 8 h, while the control E. coli only converted 18% furfural within 8 h. It was further demonstrated that W826-RS0111485 played an important role in the detoxification of furfural. The mining of this inhibitor degradation gene could provide a theoretical basis for rational modification of industrial strains to enhance its capacity of inhibitor degradation in the future.

pH regulatory divergent point for the selective bio-oxidation of primary diols during resting cell catalysis

Background

Hydroxyl acid is an important platform chemical that covers many industrial applications due to its dual functional modules. At present, the traditional technology for hydroxyl acid production mainly adopts the petroleum route with benzene, cyclohexane, butadiene and other non-renewable resources as raw materials which violates the development law of green chemistry. Conversely, it is well-known that biotechnology and bioengineering techniques possess several advantages over chemical methods, such as moderate reaction conditions, high chemical selectivity, and environmental-friendly. However, compared with chemical engineering, there are still some major obstacles in the industrial application of biotechnology. The critical issue of the competitiveness between bioengineering and chemical engineering is products titer and volume productivity. Therefore, based on the importance of hydroxyl acids in many fields, exploring a clean, practical and environmental-friendly preparation process of the hydroxyl acids is the core purpose of this study.

Results

To obtain high-purity hydroxyl acid, a microbiological regulation for its bioproduction by Gluconobacter oxydans was constructed. In the study, we found a critical point of chain length determine the end-products. Gluconobacter oxydans catalyzed diols with chain length ≤ 4, forming hydroxyl acids, and converting 1,5-pentylene glycol and 1,6-hexylene glycol to diacids. Based on this principle, we successfully synthesized 75.3 g/L glycolic acid, 83.2 g/L 3-hydroxypropionic acid, and 94.3 g/L 4-hydroxybutyric acid within 48 h. Furthermore, we directionally controlled the products of C5/C6 diols by adjusting pH, resulting in 102.3 g/L 5‑hydroxyvaleric acid and 48.8 g/L 6-hydroxycaproic acid instead of diacids. Combining pH regulation and cell-recycling technology in sealed-oxygen supply bioreactor, we prepared 271.4 g 5‑hydroxyvaleric acid and 129.4 g 6-hydroxycaproic acid in 6 rounds.

Conclusions

In this study, a green scheme of employing G. oxydans as biocatalyst for superior-quality hydroxyl acids (C2–C6) production is raised up. The proposed strategy commendably demonstrated a novel technology with simple pH regulation for high-value production of hydroxyl acids via green bioprocess developments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02171-5.

Heterologous expression of membrane-bound alcohol dehydrogenase-encoding genes for glyceric acid production using Gluconobacter sp. CHM43 and its derivatives

In contrast to D-glyceric acid (D-GA) production with 99% enantiomeric excess (ee) by Acetobacter tropicalis NBRC 16470, Gluconobacter sp. CHM43 produced 19.6 g L^-1 of D-GA with 73.7% ee over 4 days of incubation in flask culture. To investigate the reason for this enantiomeric composition of GA, the genes encoding membrane-bound alcohol dehydrogenase (mADH) of A. tropicalis NBRC 16470, composed of three subunits (adhA, adhB, and adhS), were cloned using the broad-host-range vector pBBR1MCS-2 and heterologously expressed in Gluconobacter sp. CHM43 and its ΔadhAB ΔsldBA derivative TORI4. Reverse-transcription quantitative real-time polymerase chain reaction demonstrated that adhABS genes from A. tropicalis were expressed in TORI4 transformants, and their membrane fraction exhibited mADH activities of 0.13 and 0.31 U/mg with or without AdhS, respectively. Compared with the GA production of TORI4-harboring pBBR1MCS-2 (1.23 g L^-1), TORI4 transformants expressing adhABS and adhAB showed elevated GA production of 2.46 and 3.67 g L^-1, respectively, suggesting a negative effect of adhS gene expression on GA production as well as mADH activity in TORI4. Although TORI4 was found to produce primarily L-GA with 42.5% ee, TORI4 transformants expressing adhABS and adhAB produced D-GA with 27.6% and 49.0% ee, respectively, demonstrating that mADH of A. tropicalis causes a sharp increase in the enantiomeric composition of D-GA. These results suggest that one reason for D-GA production with 73.7% ee in Gluconobacter spp. might be a property of the host, which possibly produces L-GA intracellularly. KEY POINTS: • Membrane-bound ADH from Acetobacter tropicalis showed activity in Gluconobacter sp. • D-GA production from glycerol was performed using recombinant Gluconobacter sp. • Enantiomeric excess of D-GA was affected by both membrane and intracellular ADHs.

Application of condensed molasses fermentation solubles and lactic acid bacteria in corn silage production

BACKGROUND

The aim of this study is to investigate the application of two lactic acid bacteria and dry condensed molasses fermentation solubles (CMS) in the making and preservation of corn silage. Forage corn materials are divided into eight treatment groups as follows: (i) control, (ii) B2 (Lactobacillus plantarum B2, 1 × 10⁹ cfu kg^-1 ), (iii) LAS (Lactobacillus buchneri 40788, 3 × 10⁸ cfu kg^-1 ), (iv) B2 + LAS, (v) CMS (35 g kg^-1 , fresh weight), (vi) B2 + CMS, (vii) LAS + CMS and (viii) B2 + LAS + CMS. The silage composition and aerobic stability are determined after ensiling for 90 days. Furthermore, the digestibility of the silage product and gas production are evaluated using a trotro digestion procedure.

RESULTS

The assay results indicate that the CMS supplementation and B2 inoculation significantly increased lactic acid concentration (P < 0.01). However, they also reduced the content of acetic acid and silage pH at the initial fermentation stage. The CMS supplemented with B2 (B2 + CMS) showed an improvement in the quality of silage, but a significant decrease in aerobic stability (P < 0.01). The B2 + LAS + CMS treatment yielded an increase in acetic acid production during the late fermentation period and is able to extend the aerobic stability of corn silage. Furthermore, this study shows that CMS supplementation alone can significantly improve the digestibility of the in vitro dry matter (P < 0.01) and the microbial protein synthesis efficiency (P = 0.01). In addition, the CMS supplementation is beneficial for enhancing the aerobic stability of corn silage.

CONCLUSIONS

These results suggest that the combination of CMS supplementation and a suitable inoculation lactic acid bacterial strain can be highly promising for enhancing the higher quality and stability of corn silage. © 2020 Society of Chemical Industry.

Novel plasmid-free Gluconobacter oxydans strains for production of the natural sweetener 5-ketofructose

Background

5-Ketofructose (5-KF) has recently been identified as a promising non-nutritive natural sweetener. Gluconobacter oxydans strains have been developed that allow efficient production of 5-KF from fructose by plasmid-based expression of the fructose dehydrogenase genes fdhSCL of Gluconobacter japonicus. As plasmid-free strains are preferred for industrial production of food additives, we aimed at the construction of efficient 5-KF production strains with the fdhSCL genes chromosomally integrated.

Results

For plasmid-free 5-KF production, we selected four sites in the genome of G. oxydans IK003.1 and inserted the fdhSCL genes under control of the strong P264 promoter into each of these sites. All four recombinant strains expressed fdhSCL and oxidized fructose to 5-KF, but site-specific differences were observed suggesting that the genomic vicinity influenced gene expression. For further improvement, a second copy of the fdhSCL genes under control of P264 was inserted into the second-best insertion site to obtain strain IK003.1::fdhSCL². The 5-KF production rate and the 5-KF yield obtained with this double-integration strain were considerably higher than for the single integration strains and approached the values of IK003.1 with plasmid-based fdhSCL expression.

Conclusion

We identified four sites in the genome of G. oxydans suitable for expression of heterologous genes and constructed a strain with two genomic copies of the fdhSCL genes enabling efficient plasmid-free 5-KF production. This strain will serve as basis for further metabolic engineering strategies aiming at the use of alternative carbon sources for 5-KF production and for bioprocess optimization.

Identification of NAD-Dependent Xylitol Dehydrogenase from Gluconobacter oxydans WSH-003

Gluconobacter oxydans plays an important role in the conversion of d-sorbitol to l-sorbose, which is an essential intermediate for the industrial-scale production of vitamin C. In the fermentation process, some d-sorbitol could be converted to d-fructose and other byproducts by uncertain dehydrogenases. Genome sequencing has revealed the presence of diverse genes encoding dehydrogenases in G. oxydans. However, the characteristics of most of these dehydrogenases remain unclear. Therefore, the analyses of these unknown dehydrogenases could be useful for identifying those related to the production of d-fructose and other byproducts. Accordingly, dehydrogenases in G. oxydans WSH-003, an industrial strain used for vitamin C production, were examined. A nicotinamide adenine dinucleotide (NAD)-dependent dehydrogenase, which was annotated as xylitol dehydrogenase 2, was identified, codon-optimized, and expressed in Escherichia coli BL21 (DE3) cells. The enzyme exhibited a high preference for NAD⁺ as the cofactor, while no activity with nicotinamide adenine dinucleotide phosphate, flavin adenine dinucleotide, or pyrroloquinoline quinone was noted. Although this enzyme presented high similarity with NAD-dependent xylitol dehydrogenase, it showed high activity to catalyze d-sorbitol to d-fructose. Unlike the optimum temperature and pH for most of the known NAD-dependent xylitol dehydrogenases (30-40 °C and about 6-8, respectively), those for the identified enzyme were 57 °C and 12, respectively. The values of K _m and V _max of the identified dehydrogenase toward l-sorbitol were 4.92 μM and 196.08 μM/min, respectively. Thus, xylitol dehydrogenase 2 can be useful for the cofactor-reduced nicotinamide adenine dinucleotide regeneration under alkaline conditions, or its knockout can improve the conversion ratio of d-sorbitol to l-sorbose.

Characterization of membrane-bound dehydrogenases of Gluconobacter oxydans 621H using a new system for their functional expression

Acetic acid bacteria are used in biotechnology due to their ability to incompletely oxidize a great variety of carbohydrates, alcohols, and related compounds in a regio- and stereo-selective manner. These reactions are catalyzed by membrane-bound dehydrogenases (mDHs), often with a broad substrate spectrum. In this study, the promoters of six mDHs of Gluconobacter oxydans 621H were characterized. The constitutive promoter of the alcohol dehydrogenase and the glucose-repressed promoter of the inositol dehydrogenase were used to construct a shuttle vector system for the fully functional expression of mDHs in the multi-deletion strain G. oxydans BP.9 that lacks its mDHs. This system was used to express each mDH of G. oxydans 621H, in order to individually characterize the substrates, they oxidize. From 55 tested compounds, the alcohol dehydrogenase oxidized 30 substrates and the polyol dehydrogenase 25. The substrate spectrum of alcohol dehydrogenase overlapped largely with the aldehyde dehydrogenase and partially with polyol dehydrogenase. Thus, we were able to resolve the overlapping substrate spectra of the main mDHs of G. oxydans 621H. The described approach could also be used for the expression and detailed characterization of substrates used by mDHs from other acetic acid bacteria or a metagenome.

(13)C Tracers for Glucose Degrading Pathway Discrimination in Gluconobacter oxydans 621H

Gluconobacter oxydans 621H is used as an industrial production organism due to its exceptional ability to incompletely oxidize a great variety of carbohydrates in the periplasm. With glucose as the carbon source, up to 90% of the initial concentration is oxidized periplasmatically to gluconate and ketogluconates. Growth on glucose is biphasic and intracellular sugar catabolism proceeds via the Entner-Doudoroff pathway (EDP) and the pentose phosphate pathway (PPP). Here we studied the in vivo contributions of the two pathways to glucose catabolism on a microtiter scale. In our approach we applied specifically (13)C labeled glucose, whereby a labeling pattern in alanine was generated intracellularly. This method revealed a dynamic growth phase-dependent pathway activity with increased activity of EDP in the first and PPP in the second growth phase, respectively. Evidence for a growth phase-independent decarboxylation-carboxylation cycle around the pyruvate node was obtained from (13)C fragmentation patterns of alanine. For the first time, down-scaled microtiter plate cultivation together with (13)C-labeled substrate was applied for G. oxydans to elucidate pathway operation, exhibiting reasonable labeling costs and allowing for sufficient replicate experiments.

Residues that influence coenzyme preference in the aldehyde dehydrogenases

To find out the residues that influence the coenzyme preference of aldehyde dehydrogenases (ALDHs), we reviewed, analyzed and correlated data from their known crystal structures and amino-acid sequences with their published kinetic parameters for NAD(P)(+). We found that the conformation of the Rossmann-fold loops participating in binding the adenosine ribose is very conserved among ALDHs, so that coenzyme specificity is mainly determined by the nature of the residue at position 195 (human ALDH2 numbering). Enzymes with glutamate or proline at 195 prefer NAD(+) because the side-chains of these residues electrostatically and/or sterically repel the 2'-phosphate group of NADP(+). But contrary to the conformational rigidity of proline, the conformational flexibility of glutamate may allow NADP(+)-binding in some enzymes by moving the carboxyl group away from the 2'-phosphate group, which is possible if a small neutral residue is located at position 224, and favored if the residue at position 53 interacts with Glu195 in a NADP(+)-compatible conformation. Of the residues found at position 195, only glutamate interacts with the NAD(+)-adenosine ribose; glutamine and histidine cannot since their side-chain points are opposite to the ribose, probably because the absence of the electrostatic attraction by the conserved nearby Lys192, or its electrostatic repulsion, respectively. The shorter side-chains of other residues-aspartate, serine, threonine, alanine, valine, leucine, or isoleucine-are distant from the ribose but leave room for binding the 2'-phosphate group. Generally, enzymes having a residue different from Glu bind NAD(+) with less affinity, but they can also bind NADP(+) even sometimes with higher affinity than NAD(+), as do enzymes containing Thr/Ser/Gln195. Coenzyme preference is a variable feature within many ALDH families, consistent with being mainly dependent on a single residue that apparently has no other structural or functional roles, and therefore can easily be changed through evolution and selected in response to physiological needs.

Succinic semialdehyde reductase Gox1801 from Gluconobacter oxydans in comparison to other succinic semialdehyde-reducing enzymes

Gluconobacter oxydans is an industrially important bacterium that possesses many uncharacterized oxidoreductases, which might be exploited for novel biotechnological applications. In this study, gene gox1801 was homologously overexpressed in G. oxydans and it was found that the relative expression of gox1801 was 13-fold higher than that in the control strain. Gox1801 was predicted to belong to the 3-hydroxyisobutyrate dehydrogenase-type proteins. The purified enzyme had a native molecular mass of 134 kDa and forms a homotetramer. Analysis of the enzymatic activity revealed that Gox1801 is a succinic semialdehyde reductase that used NADH and NADPH as electron donors. Lower activities were observed with glyoxal, methylglyoxal, and phenylglyoxal. The enzyme was compared to the succinic semialdehyde reductase GsSSAR from Geobacter sulfurreducens and the γ-hydroxybutyrate dehydrogenase YihU from Escherichia coli K-12. The comparison revealed that Gox1801 is the first enzyme from an aerobic bacterium reducing succinic semialdehyde with high catalytic efficiency. As a novel succinic semialdehyde reductase, Gox1801 has the potential to be used in the biotechnological production of γ-hydroxybutyrate.

The consequence of an additional NADH dehydrogenase paralog on the growth of Gluconobacter oxydans DSM3504

Acetic acid bacteria such as Gluconobacter oxydans are used in several biotechnological processes due to their ability to perform rapid incomplete regio- and stereo-selective oxidations of a great variety of carbohydrates, alcohols, and related compounds by their membrane-bound dehydrogenases. In order to understand the growth physiology of industrial strains such as G. oxydans ATCC 621H that has high substrate oxidation rates but poor growth yields, we compared its genome sequence to the genome sequence of strain DSM 3504 that reaches an almost three times higher optical density. Although the genome sequences are very similar, DSM 3504 has additional copies of genes that are absent from ATCC 621H. Most importantly, strain DSM 3504 contains an additional type II NADH dehydrogenase (ndh) gene and an additional triosephosphate isomerase (tpi) gene. We deleted these additional paralogs from DSM 3504, overexpressed NADH dehydrogenase in ATCC 621H, and monitored biomass and the concentration of the representative cell components as well as O2 and CO2 transfer rates in growth experiments on mannitol. The data revealed a clear competition of membrane-bound dehydrogenases and NADH dehydrogenase for channeling electrons in the electron transport chain of Gluconobacter and an important role of the additional NADH dehydrogenase for increased growth yields. The less active the NADH dehydrogenase is, the more active is the membrane-bound polyol dehydrogenase. These results were confirmed by introducing additional ndh genes via plasmid pAJ78 in strain ATCC 621H, which leads to a marked increase of the growth rate.

Structural insights into substrate and coenzyme preference by SDR family protein Gox2253 from Gluconobater oxydans

Gox2253 from Gluconobacter oxydans belongs to the short-chain dehydrogenases/reductases family, and catalyzes the reduction of heptanal, octanal, nonanal, and decanal with NADPH. To develop a robust working platform to engineer novel G. oxydans oxidoreductases with designed coenzyme preference, we adopted a structure based rational design strategy using computational predictions that considers the number of hydrogen bonds formed between enzyme and docked coenzyme. We report the crystal structure of Gox2253 at 2.6 Å resolution, ternary models of Gox2253 mutants in complex with NADH/short-chain aldehydes, and propose a structural mechanism of substrate selection. Molecular dynamics simulation shows that hydrogen bonds could form between 2'-hydroxyl group in the adenosine moiety of NADH and the side chain of Gox2253 mutant after arginine at position 42 is replaced with tyrosine or lysine. Consistent with the molecular dynamics prediction, Gox2253-R42Y/K mutants can use both NADH and NADPH as a coenzyme. Hence, the strategies here could provide a practical platform to engineer coenzyme selectivity for any given oxidoreductase and could serve as an additional consideration to engineer substrate-binding pockets.

Effects of 8 chemical and bacterial additives on the quality of corn silage

This project aimed to evaluate the effects 8 additives on the fermentation, dry matter (DM) losses, nutritive value, and aerobic stability of corn silage. Corn forage harvested at 31% DM was chopped (10mm) and treated with (1) deionized water (control); (2) Buchneri 500 (BUC; 1×10(5) cfu/g of Pediococcus pentosaceus 12455 and 4×10(5) cfu/g of Lactobacillus buchneri 40788; Lallemand Animal Nutrition, Milwaukee, WI); (3) sodium benzoate (BEN; 0.1% of fresh forage); (4) Silage Savor acid mixture (SAV: 0.1% of fresh forage; Kemin Industries Inc., Des Moines, IA); (5) 1×10(6) cfu/g of Acetobacter pasteurianus-ATCC 9323; (6) 1×10(6) cfu/g of Gluconobacter oxydans-ATCC 621; (7) Ecosyl 200T (1×10(5) cfu/g of Lactobacillus plantarum MTD/1; Ecosyl Products Inc., Byron, IL); (8) Silo-King WS (1.5×10(5) cfu/g of L. plantarum, P. pentosaceus and Enterococcus faecium; Agri-King, Fulton, IL); and (9) Biomax 5 (BIO; 1×10(5) cfu/g of L. plantarum PA-28 and K-270; Chr. Hansen Animal Health and Nutrition, Milwaukee, WI). Treated forage was ensiled in quadruplicate in mini silos at a density of 172 kg of DM/m(3) for 3 and 120 d. After 3 d of ensiling, the pH of all silages was below 4 but ethanol concentrations were least in BEN silage (2.03 vs. 3.24% DM) and lactic acid was greatest in SAV silage (2.97 vs. 2.51% DM). Among 120-d silages, additives did not affect DM recovery (mean=89.8% ± 2.27) or in vitro DM digestibility (mean=71.5% ± 0.63). The SAV silage had greater ammonia-N (0.85 g/kg of DM) and butyric acid (0.22 vs. 0.0% DM) than other treatments. In contrast, BEN and Silo-King silages had the least ammonia-N concentration and had no butyric acid. The BEN and A. pasteurianus silages had the lowest pH (3.69) and BEN silage had the least ethanol (1.04% DM) and ammonia nitrogen (0.64 g/kg DM) concentrations, suggesting that fermentation was more extensive and protein degradation was less in BEN silages. The BUC and BIO silages had greater acetic acid concentrations than control silages (3.19 and 3.19 vs. 2.78% DM), but yeast counts did not differ. Aerobic stability was increased by 64% by BUC (44.30 h) and by 35% by BEN (36.49 h), but other silages had similar values (27.0±1.13 h).

Structural basis for cofactor and substrate selection by cyanobacterium succinic semialdehyde dehydrogenase

Aldehyde dehydrogenase (ALDH) catalyzes the oxidation of aldehydes to carboxylic acids. Cyanobacterium Synechococcus contains one ALDH enzyme (Sp2771), together with a novel 2-oxoglutarate decarboxylase, to complete a non-canonical tricarboxylic acid cycle. However, the molecular mechanisms for substrate selection and cofactor preference by Sp2771 are largely unknown. Here, we report crystal structures of wild type Sp2771, Sp2771 S419A mutant and ternary structure of Sp2771 C262A mutant in complex with NADP(+) and SSA, as well as binary structure of Gluconobacter oxydans aldehyde dehydrogenase (Gox0499) in complex with PEG. Structural comparison of Sp2771 with Gox0499, coupled with mutational analysis, demonstrates that Ser157 residue in Sp2771 and corresponding Pro159 residue in Gox0499 play critical structural roles in determining NADP(+) and NAD(+) preference for Sp2771 and Gox0499, respectively, whereas size and distribution of hydrophobic residues along the substrate binding funnel determine substrate selection. Hence, our work has provided insightful structural information into cofactor and substrate selection by ALDH.

Role of the pentose phosphate pathway and the Entner-Doudoroff pathway in glucose metabolism of Gluconobacter oxydans 621H

Glucose catabolism by the obligatory aerobic acetic acid bacterium Gluconobacter oxydans 621H proceeds in two phases comprising rapid periplasmic oxidation of glucose to gluconate (phase I) and oxidation of gluconate to 2-ketogluconate or 5-ketogluconate (phase II). Only a small amount of glucose and part of the gluconate is taken up into the cells. To determine the roles of the pentose phosphate pathway (PPP) and the Entner-Doudoroff pathway (EDP) for intracellular glucose and gluconate catabolism, mutants defective in either the PPP (Δgnd, Δgnd zwf*) or the EDP (Δedd-eda) were characterized under defined conditions of pH 6 and 15 % dissolved oxygen. In the presence of yeast extract, neither of the two pathways was essential for growth with glucose. However, the PPP mutants showed a reduced growth rate in phase I and completely lacked growth in phase II. In contrast, the EDP mutant showed the same growth behavior as the reference strain. These results demonstrate that the PPP is of major importance for cytoplasmic glucose and gluconate catabolism, whereas the EDP is dispensable. Reasons for this difference are discussed.

Properties of recombinant Strep-tagged and untagged hyperthermophilic D-arabitol dehydrogenase from Thermotoga maritima

The first hyperthermophilic D-arabitol dehydrogenase from Thermotoga maritima was heterologously purified from Escherichia coli. The protein was purified with and without a Strep-tag. The enzyme exclusively catalyzed the NAD(H)-dependent oxidoreduction of D-arabitol, D-xylitol, D-ribulose, or D-xylulose. A twofold increase of catalytic rates was observed upon addition of Mg(2+) or K(+). Interestingly, only the tag-less protein was thermostable, retaining 90% of its activity after 90 min at 85 °C. However, the tag-less form of D-arabitol dehydrogenase had similar kinetic parameters compared to the tagged enzyme, demonstrating that the Strep-tag was not deleterious to protein function but decreased protein stability. A single band at 27.6 kDa was observed on SDS-PAGE and native PAGE revealed that the protein formed a homohexamer and a homododecamer. The enzyme catalyzed oxidation of D-arabitol to D: -ribulose and therefore belongs to the class of D-arabitol 2-dehydrogenases, which are typically observed in yeast and not bacteria. The product D-ribulose is a rare ketopentose sugar that has numerous industrially applications. Given its thermostability and specificity, D-arabitol 2-dehydrogenase is a desirable biocatalyst for the production of rare sugar precursors.

Metabolic engineering of Gluconobacter oxydans for improved growth rate and growth yield on glucose by elimination of gluconate formation

Gluconobacter oxydans N44-1, an obligatory aerobic acetic acid bacterium, oxidizes glucose primarily in the periplasm to the end products 2-ketogluconate and 2,5-diketogluconate, with intermediate formation of gluconate. Only a minor part of the glucose (less than 10%) is metabolized in the cytoplasm after conversion to gluconate or after phosphorylation to glucose-6-phosphate via the only functional catabolic routes, the pentose phosphate pathway and the Entner-Doudoroff pathway. This unusual method of glucose metabolism results in a low growth yield. In order to improve it, we constructed mutants of strain N44-1 in which the gene encoding the membrane-bound glucose dehydrogenase was inactivated either alone or together with the gene encoding the cytoplasmic glucose dehydrogenase. The growth and product formation from glucose of the resulting strains, N44-1 mgdH::kan and N44-1 DeltamgdH sgdH::kan, were analyzed. Both mutant strains completely consumed the glucose but produced neither gluconate nor the secondary products 2-ketogluconate and 2,5-diketogluconate. Instead, carbon dioxide formation of the mutants increased by a factor of 4 (N44-1 mgdH::kan) or 5.5 (N44-1 DeltamgdH sgdH::kan), and significant amounts of acetate were produced, presumably by the activities of pyruvate decarboxylase and acetaldehyde dehydrogenase. Most importantly, the growth yields of the two mutants increased by 110% (N44-1 mgdH::kan) and 271% (N44-1 DeltamgdH sgdH::kan). In addition, the growth rates improved by 39% (N44-1 mgdH::kan) and 78% (N44-1 DeltamgdH sgdH::kan), respectively, compared to the parental strain. These results show that the conversion of glucose to gluconate and ketogluconates has a strong negative impact on the growth of G. oxydans.

Characterization of two aldo-keto reductases from Gluconobacter oxydans 621H capable of regio- and stereoselective alpha-ketocarbonyl reduction

Two cytosolic NADPH-dependent carbonyl reductases from Gluconobacter oxydans 621H, Gox0644 and Gox1615, were heterologously produced in Escherichia coli. The recombinant proteins were purified to homogeneity and characterized. Gox0644 and Gox1615 were dimers with native molecular masses of 66.1 and 74.5 kDa, respectively. The enzymes displayed broad substrate specificities and reduced alpha-ketocarbonyls at the keto moiety most proximal to the terminus of the alkyl chain to produce alpha-hydroxy carbonyls, as demonstrated by NMR. With respect to stereoselectivity, protein Gox0644 specifically reduced 2,3-pentanedione to 2R-hydroxy-pentane-3-one, whereas Gox1615 produced 2S-hydroxy-pentane-3-one. Both enzymes also reduced 1-phenyl-1,2-propanedione to 2-hydroxy-1-phenylpropane-1-one, which is a key intermediate in the production of numerous pharmaceuticals, such as antifungal azoles and antidepressants. Gox0644 displayed highest activities with 2,3-diones, alpha-ketoaldehydes, alpha-keto esters, and 2,5-diketogluconate. Gox1615 was less active with these substrates, but displayed a broader substrate spectrum reducing a variety of alpha-diketones and aldehydes.

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.

Vinyl ketone reduction by three distinct Gluconobacter oxydans 621H enzymes

Three cytosolic NADPH-dependent flavin-associated proteins (Gox2107, Gox0502, and Gox2684) from Gluconobacter oxydans 621H were overproduced in Escherichia coli, and the recombinant enzymes were purified and characterized. Apparent native molecular masses of 65.2, 78.2, and 78.4 kDa were observed for Gox2107, Gox0502, and Gox2684, corresponding to a trimeric structure for Gox2107 and dimers for Gox0502 and Gox2684. Analysis of flavin content revealed Gox2107 was flavin adenine dinucleotide dependent, whereas Gox0502 and Gox2684 contained flavin mononucleotide. The enzymes were able to reduce vinyl ketones and quinones, reducing the olefinic bond of vinyl ketones as shown by (1)H nuclear magnetic resonance. Additionally, Gox0502 and Gox2684 stereospecifically reduced 5S-(+)-carvone to 2R,5S-dihydrocarvone. All enzymes displayed highest activities with 3-butene-2-one and 1,4-naphthoquinone. Gox0502 and Gox2684 displayed a broader substrate spectrum also reducing short-chain alpha-diketones, whereas Gox2107 was most catalytically efficient.