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

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.

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.

Stepwise metabolic engineering of Corynebacterium glutamicum for the production of phenylalanine

Corynebacterium glutamicum was metabolically engineered to produce phenylalanine, a valuable aromatic amino acid that can be used as a raw material in the food and pharmaceutical industries. First, a starting phenylalanine-producer was constructed by overexpressing tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase and phenylalanine- and tyrosine-insensitive bifunctional enzyme chorismate mutase prephenate dehydratase from Escherichia coli, followed by the inactivation of enzymes responsible for the formation of dihydroxyacetone and the consumption of shikimate pathway-related compounds. Second, redirection of the carbon flow from tyrosine to phenylalanine was attempted by deleting of the tyrA gene encoding prephenate dehydrogenase, which catalyzes the committed step for tyrosine biosynthesis from prephenate. However, suppressor mutants were generated, and two mutants were isolated and examined for phenylalanine production and genome sequencing. The suppressor mutant harboring an amino acid exchange (L180R) on RNase J, which was experimentally proven to lead to a loss of function of the enzyme, showed significantly enhanced production of phenylalanine. Finally, modifications of phosphoenolpyruvate-pyruvate metabolism were investigated, revealing that the inactivation of either phosphoenolpyruvate carboxylase or pyruvate carboxylase, which are enzymes of the anaplerotic pathway, is an effective means for improving phenylalanine production. The resultant strain, harboring a phosphoenolpyruvate carboxylase deficiency, synthesized 50.7 mM phenylalanine from 444 mM glucose. These results not only provided new insights into the practical mutations in constructing a phenylalanine-producing C. glutamicum but also demonstrated the creation of a potential strain for the biosynthesis of phenylalanine-derived compounds represented by plant secondary metabolites.

Periplasmic dehydroshikimate dehydratase combined with quinate oxidation in Gluconobacter oxydans for protocatechuate production

Protocatechuate (3,4-dihydroxybenzoate) has antioxidant properties and is a raw material for the production of muconic acid, which is a key compound in the synthesis of polymers such as nylon and polyethylene terephthalate. Gluconobacter oxydans strain NBRC3244 has a periplasmic system for oxidation of quinate to produce 3-dehydroquinate. Previously, a periplasmic 3-dehydroshikimate production system was constructed by heterologously expressing Gluconacetobacter diazotrophicus dehydroquinate dehydratase in the periplasm of G. oxydans strain NBRC3244. 3-Dehydroshikimate is converted to protocatechuate by dehydration. In this study, we constructed a G. oxydans strain that expresses the Acinetobacter baylyi quiC gene, which encodes a dehydroshikimate dehydratase of which the subcellular localization is likely the periplasm. We attempted to produce protocatechuate by co-cultivation of two recombinant G. oxydans strains-one expressing the periplasmically targeted dehydroquinate dehydratase and the other expressing A. baylyi dehydroshikimate dehydratase. The co-cultivation system produced protocatechuate from quinate in a nearly quantitative manner.

Efficient Production of 2,5-Diketo-D-gluconic Acid by Reducing Browning Levels During Gluconobacter oxydans ATCC 9937 Fermentation

D-Glucose directly generates 2-keto-L-gulonic acid (2-KLG, precursor of vitamin C) through the 2,5-diketo-D-gluconic acid (2,5-DKG) pathway. 2,5-DKG is the main rate-limiting factor of the reaction, and there are few relevant studies on it. In this study, a more accurate quantitative method of 2,5-DKG was developed and used to screen G. oxydans ATCC9937 as the chassis strain for the production of 2,5-DKG. Combining the metabolite profile analysis and knockout and overexpression of production strain, the non-enzymatic browning of 2,5-DKG was identified as the main factor leading to low yield of the target compound. By optimizing the fermentation process, the fermentation time was reduced to 48 h, and 2,5-DKG production peaked at 50.9 g/L, which was 139.02% higher than in the control group. Effectively eliminating browning and reducing the degradation of 2,5-DKG will help increase the conversion of 2,5-DKG to 2-KLG, and finally, establish a one-step D-glucose to 2-KLG fermentation pathway.

The industrial versatility of Gluconobacter oxydans: current applications and future perspectives

Gluconobacter oxydans is a well-known acetic acid bacterium that has long been applied in the biotechnological industry. Its extraordinary capacity to oxidize a variety of sugars, polyols, and alcohols into acids, aldehydes, and ketones is advantageous for the production of valuable compounds. Relevant G. oxydans industrial applications are in the manufacture of L-ascorbic acid (vitamin C), miglitol, gluconic acid and its derivatives, and dihydroxyacetone. Increasing efforts on improving these processes have been made in the last few years, especially by applying metabolic engineering. Thereby, a series of genes have been targeted to construct powerful recombinant strains to be used in optimized fermentation. Furthermore, low-cost feedstocks, mostly agro-industrial wastes or byproducts, have been investigated, to reduce processing costs and improve the sustainability of G. oxydans bioprocess. Nonetheless, further research is required mainly to make these raw materials feasible at the industrial scale. The current shortage of suitable genetic tools for metabolic engineering modifications in G. oxydans is another challenge to be overcome. This paper aims to give an overview of the most relevant industrial G. oxydans processes and the current strategies developed for their improvement.

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.

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.

Relocation of dehydroquinate dehydratase to the periplasmic space improves dehydroshikimate production with Gluconobacter oxydans strain NBRC3244

3-Dehydroshikimate (3-DHS) is a key intermediate for the synthesis of various compounds, including the antiviral drug oseltamivir. The Gluconobacter oxydans strain NBRC3244 intrinsically oxidizes quinate to produce 3-dehydroquinate (3-DHQ) in the periplasmic space. Even though a considerable activity is detected in the recombinant G. oxydans homologously overexpressing type II dehydroquinate dehydratase (DHQase) encoded in the aroQ gene at a pH where it grows, an alkaline shift of the culture medium is required for 3-DHS production in the middle of cultivation. Here, we attempted to adopt type I DHQase encoded in the aroD gene of Gluconacetobacter diazotrophicus strain PAL5 because the type I DHQase works optimally at weak acid, which is preferable for growth conditions of G. oxydans. In addition, we anticipated that subcellular localization of DHQase is the cytoplasm, and therefore, transports of 3-DHQ and 3-DHS across the cytoplasmic membrane are rate-limiting steps in the biotransformation. The Sec- and TAT-dependent signal sequences for secretion were attached to the N terminus of AroD to change the subcellular localization. G. oxydans that expresses the TAT-AroD derivative achieved 3-DHS production at a tenfold higher rate than the reference strain that expresses wild-type AroD even devoid of alkaline shift. Enzyme activity with the intact cell suspension and signal sequence cleavage supported the relocation of AroD to the periplasmic space. The present study suggests that the relocation of DHQase improves 3-DHS production in G. oxydans and represents a proof of concept for the potential of enzyme relocation in metabolic engineering. KEY POINTS: • Type-I dehydroquinate dehydratase (DHQase) was expressed in Gluconobacter oxydans. • Cytoplasmic DHQase was relocated to the periplasmic space in G. oxydans. • Relocation of DHQase in G. oxydans improved 3-dehydroshikimate production.

On the way toward regulatable expression systems in acetic acid bacteria: target gene expression and use cases

Acetic acid bacteria (AAB) are valuable biocatalysts for which there is growing interest in understanding their basics including physiology and biochemistry. This is accompanied by growing demands for metabolic engineering of AAB to take advantage of their properties and to improve their biomanufacturing efficiencies. Controlled expression of target genes is key to fundamental and applied microbiological research. In order to get an overview of expression systems and their applications in AAB, we carried out a comprehensive literature search using the Web of Science Core Collection database. The Acetobacteraceae family currently comprises 49 genera. We found overall 6097 publications related to one or more AAB genera since 1973, when the first successful recombinant DNA experiments in Escherichia coli have been published. The use of plasmids in AAB began in 1985 and till today was reported for only nine out of the 49 AAB genera currently described. We found at least five major expression plasmid lineages and a multitude of further expression plasmids, almost all enabling only constitutive target gene expression. Only recently, two regulatable expression systems became available for AAB, an N-acyl homoserine lactone (AHL)-inducible system for Komagataeibacter rhaeticus and an L-arabinose-inducible system for Gluconobacter oxydans. Thus, after 35 years of constitutive target gene expression in AAB, we now have the first regulatable expression systems for AAB in hand and further regulatable expression systems for AAB can be expected. KEY POINTS: • Literature search revealed developments and usage of expression systems in AAB. • Only recently 2 regulatable plasmid systems became available for only 2 AAB genera. • Further regulatable expression systems for AAB are in sight.

Three ATP-dependent phosphorylating enzymes in the first committed step of dihydroxyacetone metabolism in Gluconobacter thailandicus NBRC3255

Dihydroxyacetone (DHA), a chemical suntan agent, is produced by the regiospecific oxidation of glycerol with Gluconobacter thailandicus NBRC3255. However, this microorganism consumes DHA produced in the culture medium. Here, we attempted to understand the pathway for DHA metabolism in NBRC3255 to minimize DHA degradation. The two gene products, NBRC3255_2003 (DhaK) and NBRC3255_3084 (DerK), have been annotated as DHA kinases in the NBRC 3255 draft genome. Because the double deletion derivative for dhaK and derK showed ATP-dependent DHA kinase activity similar to that of the wild type, we attempted to purify DHA kinase from ∆dhaK ∆derK cells to identify the gene for DHA kinase. The identified gene was NBRC3255_0651, of which the product was annotated as glycerol kinase (GlpK). Mutant strains with several combinations of deletions for the dhaK, derK, and glpK genes were constructed. The single deletion strain ∆glpK showed approximately 10% of wild-type activity and grew slower on glycerol than the wild type. The double deletion strain ∆derK ∆glpK and the triple deletion strain ∆dhaK ∆derK ∆glpK showed DHA kinase activity less than a detection limit and did not grow on glycerol. In addition, although ΔderK ΔglpK consumed a small amount of DHA in the late phase of growth, ∆dhaK ΔderK ΔglpK did not show DHA consumption on glucose-glycerol medium. The transformants of the ∆dhaK ΔderK ΔglpK strain that expresses one of the genes from plasmids showed DHA kinase activity. We concluded that all three DHA kinases, DhaK, DerK, and GlpK, are involved in DHA metabolism of G. thailandicus. KEY POINTS: • Dihydroxyacetone (DHA) is produced but degraded by Gluconobacter thailandicus. • Phosphorylation rather than reduction is the first committed step in DHA metabolism. • Three kinases are involved in DHA metabolism with the different properties.

The Auxiliary NADH Dehydrogenase Plays a Crucial Role in Redox Homeostasis of Nicotinamide Cofactors in the Absence of the Periplasmic Oxidation System in Gluconobacter oxydans NBRC3293

Gluconobacter oxydans has the unique property of a glucose oxidation system in the periplasmic space, where glucose is oxidized incompletely to ketogluconic acids in a nicotinamide cofactor-independent manner. Elimination of the gdhM gene for membrane-bound glucose dehydrogenase, the first enzyme for the periplasmic glucose oxidation system, induces a metabolic change whereby glucose is oxidized in the cytoplasm to acetic acid. G. oxydans strain NBRC3293 possesses two molecular species of type II NADH dehydrogenase (NDH), the primary and auxiliary NDHs that oxidize NAD(P)H by reducing ubiquinone in the cell membrane. The substrate specificities of the two NDHs are different from each other: primary NDH (p-NDH) oxidizes NADH specifically but auxiliary NDH (a-NDH) oxidizes both NADH and NADPH. We constructed G. oxydans NBRC3293 derivatives defective in the ndhA gene for a-NDH, in the gdhM gene, and in both. Our ΔgdhM derivative yielded higher cell biomass on glucose, as reported previously, but grew at a lower rate than the wild-type strain. The ΔndhA derivative showed growth behavior on glucose similar to that of the wild type. The ΔgdhM ΔndhA double mutant showed greatly delayed growth on glucose, but its cell biomass was similar to that of the ΔgdhM strain. The double mutant accumulated intracellular levels of NAD(P)H and thus shifted the redox balance to reduction. Accumulated NAD(P)H levels might repress growth on glucose by limiting oxidative metabolisms in the cytoplasm. We suggest that a-NDH plays a crucial role in redox homeostasis of nicotinamide cofactors in the absence of the periplasmic oxidation system in G. oxydans IMPORTANCE Nicotinamide cofactors NAD⁺ and NADP⁺ mediate redox reactions in metabolism. Gluconobacter oxydans, a member of the acetic acid bacteria, oxidizes glucose incompletely in the periplasmic space-outside the cell. This incomplete oxidation of glucose is independent of nicotinamide cofactors. However, if the periplasmic oxidation of glucose is abolished, the cells oxidize glucose in the cytoplasm by reducing nicotinamide cofactors. Reduced forms of nicotinamide cofactors are reoxidized by NADH dehydrogenase (NDH) on the cell membrane. We found that two kinds of NDH in G. oxydans have different substrate specificities: the primary enzyme is NADH specific, and the auxiliary one oxidizes both NADH and NADPH. Inactivation of the latter enzyme in G. oxydans cells in which we had induced cytoplasmic glucose oxidation resulted in elevated intracellular levels of NAD(P)H, limiting cell growth on glucose. We suggest that the auxiliary enzyme is important if G. oxydans grows independently of the periplasmic oxidation system.

Synthesis of the alternative sweetener 5-ketofructose from sucrose by fructose dehydrogenase and invertase producing Gluconobacter strains

A promising alternative to high-calorie sugars and artificial sweeteners is the microbially produced fructose derivative 5-ketofructose (5-KF). The key enzyme for biotransformation, fructose dehydrogenase (Fdh), was overproduced in Gluconobacter (G.) oxydans and G. japonicus LMG 26773. Furthermore, the fdh genes were integrated into the chromosome of G. oxydans (G. oxydans Δmgdh::fdh). All mutants showed high fructose oxidation rates forming 5-KF. G. japonicus LMG 26773 fdh was selected for 5-KF production from the cost-efficient and renewable feedstock sucrose because the organism possessed both, a highly active Fdh and an enzyme able to cleave sucrose. However, 5-KF yield was low because the strain formed levan and consumed 5-KF in the second growth phase. Several Gluconobacter strains were screened for sucrose-hydrolyzing enzymes. One of these proteins (Inv1417) was characterized and it was found that the enzyme showed the highest specific activity compared to all mesophilic invertases described so far (V_max = 2295 ± 243 U mg protein^-1). The corresponding gene was expressed in G. oxydans Δmgdh::fdh. The results clearly indicated that both heterologously produced enzymes Fdh and Inv1417 were active in this single-strain system for 5-KF synthesis. Overall 84 ± 2% of the available fructose units of sucrose were converted to 5-KF.

X-ray structure of the direct electron transfer-type FAD glucose dehydrogenase catalytic subunit complexed with a hitchhiker protein

The bacterial flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase complex derived from Burkholderia cepacia (BcGDH) is a representative molecule of direct electron transfer-type FAD-dependent dehydrogenase complexes. In this study, the X-ray structure of BcGDHγα, the catalytic subunit (α-subunit) of BcGDH complexed with a hitchhiker protein (γ-subunit), was determined. The most prominent feature of this enzyme is the presence of the 3Fe-4S cluster, which is located at the surface of the catalytic subunit and functions in intramolecular and intermolecular electron transfer from FAD to the electron-transfer subunit. The structure of the complex revealed that these two molecules are connected through disulfide bonds and hydrophobic interactions, and that the formation of disulfide bonds is required to stabilize the catalytic subunit. The structure of the complex revealed the putative position of the electron-transfer subunit. A comparison of the structures of BcGDHγα and membrane-bound fumarate reductases suggested that the whole BcGDH complex, which also includes the membrane-bound β-subunit containing three heme c moieties, may form a similar overall structure to fumarate reductases, thus accomplishing effective electron transfer.

Improved heterologous expression of the membrane-bound quinoprotein quinate dehydrogenase from Gluconobacter oxydans

Gluconobacter oxydans produces 3-dehydroquinate by oxidation of quinate through a reaction catalyzed by the quinate dehydrogenase (QDH), membrane-bound, pyrroloquinoline quinone (PQQ)-dependent dehydrogenase. We previously reported the nucleotide and deduced amino acid sequence of QDH and constructed a heterologous expression system of QDH in Pseudomonas sp. (A.S. Vangnai, W. Promden, W. De-Eknamkul, K. Matsushita, H. Toyama, Biochemistry (Moscow) 75:452-459, 2010). Through this study, we aim to update the sequences of QDH and improve the heterologous expression of QDH in Gluconobacter strains using a broad-host-range plasmid. Expression of QDH using a plasmid containing a long 5'-UTR was higher than that using a plasmid with a short 5'-UTR. In addition, the usage of the putative promoter region of the membrane-bound, alcohol dehydrogenase (ADH) of Gluconobacter resulted in higher expression levels compared to the usage of the lacZ promoter. Base substitution experiments allowed to identify the correct TTG initiation codon between two possibilities, and the result of these experiments were consistent with the N-terminal amino acid sequence of the expressed QDH. However, change of the TTG codon to ATG did not increase QDH expression. Therefore, the optimal plasmid for QDH expression included the structural gene with a long 5'-UTR and the ADH promoter. Cell membrane of the recombinant Gluconobacter strain presented approximately 10-times higher specific QDH activity than that observed in the wild-type strain.

Production of 5-ketofructose from fructose or sucrose using genetically modified Gluconobacter oxydans strains

The growing consumer demand for low-calorie, sugar-free foodstuff motivated us to search for alternative non-nutritive sweeteners. A promising sweet-tasting compound is 5-keto-D-fructose (5-KF), which is formed by membrane-bound fructose dehydrogenases (Fdh) in some Gluconobacter strains. The plasmid-based expression of the fdh genes in Gluconobacter (G.) oxydans resulted in a much higher Fdh activity in comparison to the native host G. japonicus. Growth experiments with G. oxydans fdh in fructose-containing media indicated that 5-KF was rapidly formed with a conversion efficiency of 90%. 5-KF production from fructose was also observed using resting cells with a yield of about 100%. In addition, a new approach was tested for the production of the sweetener 5-KF by using sucrose as a substrate. To this end, a two-strain system composed of the fdh-expressing strain and a G. oxydans strain that produced the sucrose hydrolyzing SacC was developed. The strains were co-cultured in sucrose medium and converted 92.5% of the available fructose units into 5-KF. The glucose moiety of sucrose was converted to 2-ketogluconate and acetate. With regard to the development of a sustainable and resource-saving process for the production of 5-KF, sugar beet extract was used as substrate for the two-strain system. Fructose as product from sucrose cleavage was mainly oxidized to 5-KF which was detected in a concentration of over 200 mM at the end of the fermentation process. In summary, the two-strain system was able to convert fructose units of sugar beet extract to 5-KF with an efficiency of 82 ± 5%.

Biotechnological production of alpha-keto acids: Current status and perspectives

Alpha-keto (α-keto) acids are used widely in feeds, food additives, pharmaceuticals, and in chemical synthesis processes. Although most α-keto acids are currently produced by chemical synthesis, their biotechnological production from renewable carbohydrates is a promising new approach. In this mini-review, we first present the different types of α-keto acids as well as their applications; next, we summarize the recent progresses in the biotechnological production of some important α-keto acids; namely, pyruvate, α-ketoglutarate, α-ketoisovalerate, α-ketoisocaproate, phenylpyruvate, α-keto-γ-methylthiobutyrate, and 2,5-diketo-d-gluconate. Finally, we discuss the future prospects as well as favorable directions for the biotechnological production of keto acids that ultimately would be more environment-friendly and simpler compared with the production by chemical synthesis.

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.

An Fe-S cluster in the conserved Cys-rich region in the catalytic subunit of FAD-dependent dehydrogenase complexes

Several bacterial flavin adenine dinucleotide (FAD)-harboring dehydrogenase complexes comprise three distinct subunits: a catalytic subunit with FAD, a cytochrome c subunit containing three hemes, and a small subunit. Owing to the cytochrome c subunit, these dehydrogenase complexes have the potential to transfer electrons directly to an electrode. Despite various electrochemical applications and engineering studies of FAD-dependent dehydrogenase complexes, the intra/inter-molecular electron transfer pathway has not yet been revealed. In this study, we focused on the conserved Cys-rich region in the catalytic subunits using the catalytic subunit of FAD dependent glucose dehydrogenase complex (FADGDH) as a model, and site-directed mutagenesis and electron paramagnetic resonance (EPR) were performed. By co-expressing a hitch-hiker protein (γ-subunit) and a catalytic subunit (α-subunit), FADGDH γα complexes were prepared, and the properties of the catalytic subunit of both wild type and mutant FADGDHs were investigated. Substitution of the conserved Cys residues with Ser resulted in the loss of dye-mediated glucose dehydrogenase activity. ICP-AEM and EPR analyses of the wild-type FADGDH catalytic subunit revealed the presence of a 3Fe-4S-type iron-sulfur cluster, whereas none of the Ser-substituted mutants showed the EPR spectrum characteristic for this cluster. The results suggested that three Cys residues in the Cys-rich region constitute an iron-sulfur cluster that may play an important role in the electron transfer from FAD (intra-molecular) to the multi-heme cytochrome c subunit (inter-molecular) electron transfer pathway. These features appear to be conserved in the other three-subunit dehydrogenases having an FAD cofactor.