Reconstruction of amino acid biosynthetic pathways increases the productivity of 2-keto- l -gulonic acid in Ketogulonicigenium vulgare - Bacillus endophyticus consortium via genes screening

Microbial Interactions in a Vitamin C Industrial Fermentation System: Novel Insights and Perspectives

In industrial production, the precursor of l-ascorbic acid (L-AA, also referred to as vitamin C), 2-keto-l-gulonic acid (2-KLG), is mainly produced using a classic two-step fermentation process performed by Gluconobacter oxydans, Bacillus megaterium, and Ketogulonicigenium vulgare. In the second step of the two-step fermentation process, the microbial consortium of K. vulgare and B. megaterium is used to achieve 2-KLG production. K. vulgare can transform l-sorbose to 2-KLG, but the yield of 2-KLG is much lower in the monoculture than in the coculture fermentation system. The relationship between the two strains is too diverse to analyze and has been a hot topic in the field of vitamin C fermentation. With the development of omics technology, the relationships between the two strains are well explained; nevertheless, the cell-cell communication is unclear. In this review, based on current omics results, the interactions between the two strains are summarized, and the potential cell-cell communications between the two strains are discussed, which will shed a light on the further understanding of synthetic consortia.

Construction of synthetic microbial consortia for 2-keto-L-gulonic acid biosynthesis

Currently, the establishment of synthetic microbial consortia with rational strategies has gained extensive attention, becoming one of the important frontiers of synthetic biology. Systems biology can offer insights into the design and construction of synthetic microbial consortia. Taking the high-efficiency production of 2-keto-l-gulonic acid (2-KLG) as an example, we constructed a synthetic microbial consortium “Saccharomyces cerevisiae-Ketogulonigenium vulgare” based on systems biology analysis. In the consortium, K. vulgare was the 2-KLG producing strain, and S. cerevisiae acted as the helper strain. Comparative transcriptomic analysis was performed on an engineered S. cerevisiae (VTC2) and a wild-type S. cerevisiae BY4741. The results showed that the up-regulated genes in VTC2, compared with BY4741, were mainly involved in glycolysis, TCA cycle, purine metabolism, and biosynthesis of amino acids, B vitamins, and antioxidant proteases, all of which play important roles in promoting the growth of K. vulgare. Furthermore, Vitamin C produced by VTC2 could further relieve the oxidative stress in the environment to increase the production of 2-KLG. Therefore, VTC2 would be of great advantage in working with K. vulgare. Thus, the synthetic microbial consortium "VTC2-K. vulgare" was constructed based on transcriptomics analyses, and the accumulation of 2-KLG was increased by 1.49-fold compared with that of mono-cultured K. vulgare, reaching 13.2 ± 0.52 g/L. In addition, the increased production of 2-KLG was accompanied by the up-regulated activities of superoxide dismutase and catalase in the medium and the up-regulated oxidative stress-related genes (sod, cat and gpd) in K. vulgare. The results indicated that the oxidative stress in the synthetic microbial consortium was efficiently reduced. Thus, systems analysis confirmed a favorable symbiotic relationship between microorganisms, providing guidance for further engineering synthetic consortia.

One-Step Biosynthesis of Vitamin C in Saccharomyces cerevisiae

Vitamin C (VC) is comprehensively applied in foods, cosmetics, pharmaceuticals, and especially clinical medicine. Nowadays, the industrial production of VC mainly relies on the classic two-step fermentation route, and researchers have explored the way for one-step fermentation of VC in recent years. In this study, a VC biosynthesis pathway that directly produced VC from glucose was reconstructed in Saccharomyces cerevisiae, and the protein engineering and metabolic engineering strategies were adopted to improve it. First, five exogenous modules from Arabidopsis were introduced into the chassis cells by synthetic biology approaches to obtain the strain YLAA harboring VC biosynthesis. In addition, L-galactose dehydrogenase (L-GalDH) and L-galactono-1,4-lactone dehydrogenase (L-GLDH) were fused and expressed in S. cerevisiae cells for the first time, which increased the intracellular VC accumulation by 2.78-fold, reaching 9.97 ± 0.09 mg/L. Through copy number engineering, it was further confirmed that the last step catalyzed by L-GLDH is the rate-limiting step. GDP-L-galactose phosphorylase (GPP) encoded by vtc2 is another rate-limiting enzyme confirmed by GAL1p overexpression results. Finally, by balancing gene expression and cell growth, the highest production strain with overexpressing vtc2 by multicopy plasmids was constructed. The VC accumulation reached 24.94 ± 1.16 mg/L, which was currently the highest production from glucose in S. cerevisiae. The production of the recombinant strain reached nearly 44 mg/L with the exogenous addition of L-galactose or glutathione. The results further emphasized the importance of the step catalyzed by GPP. The investigation provided experience for the efficient biosynthesis of VC and the determination of rate-limiting steps.

Production of 2-keto-L-gulonic acid by metabolically engineered Escherichia coli

The 2-keto-L-gulonic acid (2-KLG) is the direct precursor for industrial vitamin C production. The main biosynthetic method for 2-KLG production is the classical two-step fermentation route. However, disadvantages of this method are emerging, including high consumption of energy, difficulties in strain screening, complex operation, and poor stability. In this study, five recombinant Escherichia coli strains overexpressing different sorbose/sorbosone dehydrogenases were constructed and used for 2-KLG production. By optimizing catalytic conditions and further expressing pyrroloquinoline quinone in the recombinant strain, the titer of 2-KLG reached 72.4 g/L, with a conversion ratio from L-sorbose of 71.2% in a 5-L bioreactor. To achieve direct biosynthesis of 2-KLG from D-sorbitol, a co-culture system consisting of Gluconobacter oxydans and recombinant E. coli was designed. With this co-culture system, 16.8 g/L of 2-KLG was harvested, with a conversion ratio from D-sorbitol of 33.6%. The approaches developed here provide alternative routes for the efficient biosynthesis of 2-KLG.

Efficient Optimization of Gluconobacter oxydans Based on Protein Scaffold-Trimeric CutA to Enhance the Chemical Structure Stability of Enzymes for the Direct Production of 2-Keto-L-gulonic Acid

2-Keto-L-gulonic acid (2-KLG), the direct precursor of vitamin C, is produced by a two-step fermentation route from D-sorbitol in industry. However, this route is a complicated mix-culture system which involves three bacteria. Thus, replacement of the conventional two-step fermentation process with a one-step process could be revolutionary in vitamin C industry. The one-step fermentation of 2-keto-L-gulonic acid (2-KLG) has been achieved in our previous study; 32.4 g/L of 2-KLG production was obtained by the one-step strain G. oxydans/pGUC-tufB-sdh-GGGGS-sndh after 168 h. In this study, L-sorbose dehydrogenase (SDH) and L-sorbosone dehydrogenase (SNDH) were expressed in G. oxydans after the codon optimization. Furthermore, the trimeric protein CutA was used to improve the chemical structure stability of SDH and SNDH. The recombinant strain G. oxydans/pGUC-tufB-SH3-sdh-GGGGS-sndh-tufB-SH3lig-(GGGGS)2-cutA produced 40.3 g/L of 2-KLG after 168 h. In addition, the expression levels of the cofactor PQQ were enhanced to further improve 2-KLG production. With the stepwise metabolic engineering of G. oxydans, the final 2-KLG production was improved to 42.6 g/L. The efficient one-step production of 2-KLG was achieved, and the final one-step industrial-scale production of 2-KLG is drawing near.

High-Throughput Screening of a 2-Keto-L-Gulonic Acid-Producing Gluconobacter oxydans Strain Based on Related Dehydrogenases

High-throughput screening is a powerful tool for discovering strains in the natural environment that may be suitable for target production. Herein, a novel enzyme-based high-throughput screening method was developed for rapid screening of strains overproducing 2-keto-L-gulonic acid (2-KLG). The screening method detects changes in the fluorescence of reduced nicotinamide adenine dinucleotide (NADH) at 340 nm using a microplate reader when 2-KLG is degraded by 2-KLG reductase. In this research, three different 2-KLG reductases were expressed, purified, and studied. The 2-KLG reductase from Aspergillus niger were selected as the best appropriate reductase to establishment the method for its high activity below pH 7. Using the established method, and coupled with fluorescence-activated cell sorting, we achieved a high 2-KLG-producing strain of Gluconobacter oxydans WSH-004 from soil. When cultured with D-sorbitol as the substrate, the 2-KLG yield was 2.5 g/L from 50 g/L D-sorbitol without any side products. Compared with other reported screening methods, our enzyme-based method is more efficient and accurate for obtaining high-producing 2-KLG strains, and it is also convenient and cost-effective. The method is broadly applicable for screening keto acids and other products that can be oxidized via nicotinamide adenine dinucleotide (NAD⁺) or nicotinamide adenine dinucleotide phosphate (NADP⁺).

Integrated proteomic and metabolomic analysis of a reconstructed three-species microbial consortium for one-step fermentation of 2-keto-l-gulonic acid, the precursor of vitamin C

Microbial consortia, with the merits of strong stability, robustness, and multi-function, played critical roles in human health, bioenergy, and food manufacture, etc. On the basis of ‘build a consortium to understand it’, a novel microbial consortium consisted of Gluconobacter oxydans, Ketogulonicigenium vulgare and Bacillus endophyticus was reconstructed to produce 2-keto-l-gulonic acid (2-KGA), the precursor of vitamin C. With this synthetic consortium, 73.7 g/L 2-KGA was obtained within 30 h, which is comparable to the conventional industrial method. A combined time-series proteomic and metabolomic analysis of the fermentation process was conducted to further investigate the cell–cell interaction. The results suggested that the existence of B. endophyticus and G. oxydans together promoted the growth of K. vulgare by supplying additional nutrients, and promoted the 2-KGA production by supplying more substrate. Meanwhile, the growth of B. endophyticus and G. oxydans was compromised from the competition of the nutrients by K. vulgare, enabling the efficient production of 2-KGA. This study provides valuable guidance for further study of synthetic microbial consortia.

Antioxidant effect of glutathione on promoting 2-keto-l-gulonic acid production in vitamin C fermentation system

AIMS

Oxidative stress limited the growth of cells and 2-keto-l-gulonic acid (2-KGA) production in vitamin C (Vc) fermentation system. The study aims to investigate the antioxidant effect of glutathione on promoting 2-KGA in Vc fermentation system using Ketogulonicigenium vulgare 25B-1 and Bacillus endophyticus ST-1 as the co-culturing microbes.

METHODS AND RESULTS

The activities of antioxidant-related enzymes and qPCR were used to study the antioxidant effect of glutathione addition in Vc fermentation system. The addition of GSH and GSH/GSSG increased 2-KGA production and decreased fermentation time, and the highest 2-KGA production increased by 40·63% and the lowest fermentation time shortened to 60 h when the addition of optimal concentration ratio of GSH/GSSG was 50 : 1. Moreover, the increased production of 2-KGA was accompanied by up-regulated the activities of total antioxidant capacity (T-AOC), total superoxide dismutase (T-SOD), catalase (CAT) and over-expressed oxidative stress-related genes sod, gst, gr, zwf, gp, which resulted in scavenging reactive oxygen species to reduce oxidative stress in Vc fermentation system.

CONCLUSIONS

Glutathione showed a significant effect on increasing 2-KGA production and decreasing fermentation time in Vc fermentation system. GSH/GSSG could maintain a dynamic balance with two forms of glutathione and the optimal concentration ratio of GSH/GSSG was 50 : 1.

SIGNIFICANCE AND IMPACT OF THE STUDY

Glutathione is proved to be effective to relieve oxidative stress. The promotion effects of GSSG and GSH on 2-KGA production could help to further explore the optimization of co-culture fermentation process for Vc industrial production.

Establishing an innovative carbohydrate metabolic pathway for efficient production of 2-keto-L-gulonic acid in Ketogulonicigenium robustum initiated by intronic promoters

Background

2-Keto-l-gulonic acid (2-KGA), the precursor of vitamin C, is currently produced by two-step fermentation. In the second step, l-sorbose is transformed into 2-KGA by the symbiosis system composed of Ketogulonicigenium vulgare and Bacillus megaterium. Due to the different nutrient requirements and the uncertain ratio of the two strains, the symbiosis system significantly limits strain improvement and fermentation optimization.

Results

In this study, Ketogulonicigenium robustum SPU_B003 was reported for its capability to grow well independently and to produce more 2-KGA than that of K. vulgare in a mono-culture system. The complete genome of K. robustum SPU_B003 was sequenced, and the metabolic characteristics were analyzed. Compared to the four reported K. vulgare genomes, K. robustum SPU_B003 contained more tRNAs, rRNAs, NAD and NADP biosynthetic genes, as well as regulation- and cell signaling-related genes. Moreover, the amino acid biosynthesis pathways were more complete. Two species-specific internal promoters, P1 (orf_01408 promoter) and P2 (orf_02221 promoter), were predicted and validated by detecting their initiation activity. To efficiently produce 2-KGA with decreased CO₂ release, an innovative acetyl-CoA biosynthetic pathway (XFP-PTA pathway) was introduced into K. robustum SPU_B003 by expressing heterologous phosphoketolase (xfp) and phosphotransacetylase (pta) initiated by internal promoters. After gene optimization, the recombinant strain K. robustum/pBBR-P1_xfp2502-P2_pta2145 enhanced acetyl-CoA approximately 2.4-fold and increased 2-KGA production by 22.27% compared to the control strain K. robustum/pBBR1MCS-2. Accordingly, the transcriptional level of the 6-phosphogluconate dehydrogenase (pgd) and pyruvate dehydrogenase genes (pdh) decreased by 24.33 ± 6.67 and 8.67 ± 5.51%, respectively. The key genes responsible for 2-KGA biosynthesis, sorbose dehydrogenase gene (sdh) and sorbosone dehydrogenase gene (sndh), were up-regulated to different degrees in the recombinant strain.

Conclusions

The genome-based functional analysis of K. robustum SPU_B003 provided a new understanding of the specific metabolic characteristics. The new XFP-PTA pathway was an efficient route to enhance acetyl-CoA levels and to therefore promote 2-KGA production.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0932-9) contains supplementary material, which is available to authorized users.