Combined evolutionary and metabolic engineering improve 2-keto-L-gulonic acid production in Gluconobacter oxydans WSH-004

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.

Enhancing the biosynthesis of 2-keto-L-gulonic acid through multi-strategy metabolic engineering in Pseudomonas putida KT2440

2-KGA, a precursor for the synthesis of Vitamin C, is currently produced in China utilizing the "two-step fermentation" technique. Nevertheless, this method exhibits many inherent constraints. This study presents a comprehensive metabolic engineering strategy to establish and optimize a one-step 2-KGA fermentation process from D-sorbitol in Pseudomonas putida KT2440. In general, the endogenous promoters were screened to identify promoter P1 for subsequent heterologous gene expression in KT2440. Following the screening and confirmation of suitable heterologous gene elements such as sldh, sdh, cytc551, pqqAB, and irrE, genetic recombination was performed in KT2440. In comparison to the initial achievement of expressing only sldh and sdh in KT2440, a yield of merely 0.42 g/L was obtained. However, by implementing four metabolic engineering strategies, the recombinant strain KT20 exhibited a significant enhancement in its ability to produce 2-KGA with a remarkable yield of up to 6.5 g/L - representing an impressive 15.48-fold improvement.

Efficient production of 2-keto-L-gulonic acid from D-glucose in Gluconobacter oxydans ATCC9937 by mining key enzyme and transporter

Direct production of 2-keto-L-gulonic acid (2-KLG, the precursor of vitamin C) from D-glucose through 2,5-diketo-D-gluconic acid (2,5-DKG) is a promising alternative route. To explore the pathway of producing 2-KLG from D-glucose, Gluconobacter oxydans ATCC9937 was selected as a chassis strain. It was found that the chassis strain naturally has the ability to synthesize 2-KLG from D-glucose, and a new 2,5-DKG reductase (DKGR) was found on its genome. Several major issues limiting production were identified, including the insufficient catalytic capacity of DKGR, poor transmembrane movement of 2,5-DKG and imbalanced D-glucose consumption flux inside and outside of the host strain cells. By identifying novel DKGR and 2,5-DKG transporter, the whole 2-KLG biosynthesis pathway was systematically enhanced by balancing intracellular and extracellular D-glucose metabolic flux. The engineered strain produced 30.5 g/L 2-KLG with a conversion ratio of 39.0%. The results pave the way for a more economical large-scale fermentation process for vitamin C.

Temporal sorting of microdroplets can identify productivity differences of itaconic acid from libraries of Yarrowia lipolytica

Microdroplet screening of microorganisms can improve the rate of strain selection and characterization within the canonical design-build-test paradigm. However, a full analysis of the microdroplet environment and how well these conditions translate to culturing conditions and techniques is lacking in the field. Quantification of three different biosensor/analyte combinations at 12 hour timepoints reveals the potential for extended dose-response ranges as compared to traditional in vitro conditions. Using these dynamics, we present an application and analysis of microfluidic droplet screening utilizing whole-cell biosensors, ultimately identifying an altered productivity profile of itaconic acid in a Yarrowia lipolytica-based piggyBac transposon library. Specifically, we demonstrate that the timepoint for microdroplet selection can influence the outcome of the selection and thus shift the identified strain productivity and final titer. In this case, strains selected at earlier timepoints showed increased early productivity in flask scale, with the converse true as well. Differences in response indicate microdroplet assays require tailored development to more accurately sort for phenotypes that are scalable to larger incubation volumes. Likewise, these results further highlight that screening conditions are critical parameters for success in high-throughput applications.

New perspectives into Gluconobacter-catalysed biotransformations

Different from other aerobic microorganisms that oxidise carbon sources to water and carbon dioxide, Gluconobacter catalyses the incomplete oxidation of various substrates with regio- and stereoselectivity. This ability, as well as its capacity to release the resulting products into the reaction media, place Gluconobacter as a privileged member of a non-model microorganism class that may boost industrial biotechnology. Knowledge of new technologies applied to Gluconobacter has been piling up in recent years. Advancements in its genetic modification, application of immobilisation tools and careful designs of the transformations, have improved productivities and stabilities of Gluconobacter strains or enabled new bioconversions for the production of valuable marketable chemicals. In this work, the latest advancements applied to Gluconobacter-catalysed biotransformations are summarised with a special focus on recent available tools to improve them. From genetic and metabolic engineering to bioreactor design, the most recent works on the topic are analysed in depth to provide a comprehensive resource not only for scientists and technologists working on/with Gluconobacter, but for the general biotechnologist.

Combined engineering of l-sorbose dehydrogenase and fermentation optimization to increase 2-keto-l-gulonic acid production in Escherichia coli

One-step fermentation to produce 2-keto-l-gulonic acid (2-KLG), the precursor of vitamin C, is a long-term goal. Improvement of the enzyme's activity through engineering could benefit 2-KLG production. This study aimed to conduct a semi-rational design of l-sorbose dehydrogenase (SDH) through structure-directed, to screen mutants that could enhance the 2-KLG titer. First, the predicted structure of SDH was obtained using AlphaFold2. The key mutation sites in the substrate pocket were identified by Ala scanning. Subsequently, the mutant V336I/V368A was obtained by iterative saturation mutagenesis, which increased the yield of 2-KLG 1.9-fold. Finally, 5.03 g/L of 2-KLG was obtained by a two-stage temperature control fermentation method, and the conversion rate was 50%. Furthermore, experiments showed that knockdown of the l-sorbose-associated phosphotransferase system delays 2-KLG production. The results show that the production of 2-KLG was effectively increased through a combination of SDH engineering and fermentation optimization.

Efficient 1,3-dihydroxyacetone biosynthesis in Gluconobacter oxydans using metabolic engineering and a fed-batch strategy

1,3-Dihydroxyacetone (DHA) is a commercially important chemical and widely used in cosmetics, pharmaceuticals, and food industries as it prevents excessive water evaporation, and provides anti-ultraviolet radiation protection and antioxidant activity. Currently, the industrial production of DHA is based on a biotechnological synthetic route using Gluconobacter oxydans. However, achieving higher production requires more improvements in the synthetic process. In this study, we compared DHA synthesis levels in five industrial wild-type Gluconobacter strains, after which the G. oxydans WSH-003 strain was selected. Then, 16 dehydrogenase genes, unrelated to DHA synthesis, were individually knocked out, with one strain significantly enhancing DHA production, reaching 89.49 g L-1 and 42.27% higher than the wild-type strain. By optimizing the culture media, including seed culture and fermentation media, DHA production was further enhanced. Finally, using an established fed-batch fermentation system, DHA production reached 198.81 g L-1 in a 5 L bioreactor, with a glycerol conversion rate of 82.84%.