Comparative analysis of L-sorbose dehydrogenase by docking strategy for 2-keto-L-gulonic acid production in Ketogulonicigenium vulgare and Bacillus endophyticus consortium

Strain Development, Substrate Utilization, and Downstream Purification of Vitamin C

Vitamin C, C6H8O6, is a water-soluble vitamin that is widespread in nature. It is an essential nutrient involved in many biological processes in the living organisms: it enhances collagen biosynthesis, ensures the optimal functioning of enzymes and the immune system, has a major role in lipid and iron metabolism, and it enhances the biosynthesis of l-carnitine. Due to its antioxidant activity, vitamin C can neutralize the tissue-damaging effects of free radicals. Vitamin C is being related to the prevention of cancer and cardiovascular diseases. This review includes current information on the biosynthesis of ascorbic acid, as new methods are now challenging the traditional Reichstein process for vitamin C’s industrial-scale production. Different strains were analyzed in correlation with their ability to synthesize ascorbic acid, and several separation techniques were investigated for a more effective production of vitamin C.

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.

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.

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

Defect in the amino acid biosynthetic pathways of Ketogulonicigenium vulgare, the producing strain for 2-keto-l-gulonic acid (2-KGA), is the key reason for its poor growth and low productivity. In this study, five different strains were firstly reconstructed by expressing absent genes in threonine, proline and histidine biosynthetic pathways for better 2-KGA productivity. When mono-cultured in the shake flasks, the strain SyBE_Kv02080002 expressing hsk from Gluconobacter oxydans in threonine biosynthetic pathway achieved the highest biomass and the titer increased by 25.13%. When co-cultured with Bacillus endophyticus, the fermentation cycle decreased by 28.57% than that of the original consortium in 5-L fermenter. Furthermore, reconstruction of threonine biosynthetic pathway resulted in up-regulation of genes encoding sorbosone dehydrogenase and idonate-dehydrogenase, which increased the 2-KGA productivity in SyBE_Kv02080002. This study shows that reconstruction of absent biosynthetic pathways in bacteria is an effective way to enhance the productivity of target products.

Comparative genomics and metabolomics analyses of the adaptation mechanism in Ketogulonicigenium vulgare-Bacillus thuringiensis consortium

Adaptive evolution by serial subcultivation of co-cultured Bacillus thuringiensis and Ketogulonicigenium vulgare significantly enhanced the productivity of 2-keto-L-gulonic acid in two-step vitamin C production. The adaptation mechanism in K. vulgare-B. thuringiensis consortium was investigated in this study based on comparative genomics and metabolomics studies. It was found that the growth, anti-oxidation, transcription and regulation were significantly enhanced in the adapted consortium. The mutation of the genes, which encode amidohydrolase in adapted K. vulgare (K150) and amino acid permease in adapted B. thuringiensis (B150), resulted in the increase of some amino acids levels in each species, and further enhanced the metabolic exchange and growth ability of the two species. Besides, the mutation of the gene encoding spore germination protein enhanced the metabolic levels of tricarboxylic acid cycle, and decreased the sporulation in B150, which induced its growth. The mutation of the genes, which encode NADPH nitroreductase in K150 and NADPH-dependent FMN reductase in B150, may enhance the ability of anti-oxidation. Overall, the long-term adaptation of K. vulgare and B. thuringiensis influenced the global regulation and made them more inseparable in metabolite exchange. Our work will provide ideas for the molecular design and optimization in microbial consortium.