NAD(H) recycling activity of an engineered bifunctional enzyme galactose dehydrogenase/lactate dehydrogenase

Synergistic Improvement of 5-Aminolevulinic Acid Production with Synthetic Scaffolds and System Pathway Engineering

Engineered synthetic scaffolds to organize metabolic pathway enzymes and system pathway engineering to fine-tune metabolic fluxes play essential roles in microbial production. Here, we first obtained the most favorable combination of key enzymes for 5-aminolevulinic acid (5-ALA) synthesis through the C5 pathway by screening enzymes from different sources and optimizing their combination in different pathways. Second, we successfully constructed a multienzyme complex assembly system with PduA*, which spatially recruits the above three key enzymes for 5-ALA synthesis in a designable manner. By further optimizing the ratio of these key enzymes in synthetic scaffolds, the efficiency of 5-ALA synthesis through the C5 pathway was significantly improved. Then, the competitive metabolism pathway was fine-tuned by rationally designing different antisense RNAs, further significantly increasing 5-ALA titers. Furthermore, for efficient 5-ALA synthesis, obstacles of NADH and NADPH imbalances and feedback inhibition of the synthesis pathway were also overcome through engineering the NADPH regeneration pathway and transport pathway, respectively. Finally, combining these strategies with further fermentation optimization, we achieved a final 5-ALA titer of 11.4 g/L. These results highlight the importance of synthetic scaffolds and system pathway engineering to improve the microbial cell factory production performance.

Using Unnatural Protein Fusions to Engineer a Coenzyme Self-Sufficiency System for D-Phenyllactic Acid Biosynthesis in Escherichia coli

The biosynthetic production of D-penyllactic acid (D-PLA) is often affected by insufficient supply and regeneration of cofactors, leading to high production cost, and difficulty in industrialization. In this study, a D-lactate dehydrogenase (D-LDH) and glycerol dehydrogenase (GlyDH) co-expression system was constructed to achieve coenzyme NADH self-sufficiency and sustainable production of D-PLA. Using glycerol and sodium phenylpyruvate (PPA) as co-substrate, the E. coli BL21 (DE3) harboring a plasmid to co-express LfD-LDH and BmGlyDH produced 3.95 g/L D-PLA with a yield of 0.78 g/g PPA, similar to previous studies. Then, flexible linkers were used to construct fusion proteins composing of D-LDH and GlyDH. Under the optimal conditions, 5.87 g/L D-PLA was produced by expressing LfD-LDH-l3-BmGlyDH with a yield of 0.97 g/g PPA, which was 59.3% increased compared to expression of LfD-LDH. In a scaled-up reaction, a productivity of 5.83 g/L/h was reached. In this study, improving the bio-catalytic efficiency by artificial redox self-equilibrium system with a bifunctional fusion protein could reduce the bio-production cost of D-PLA, making this bio-production of D-PLA a more promising industrial technology.

Engineering bi-functional enzyme complex of formate dehydrogenase and leucine dehydrogenase by peptide linker mediated fusion for accelerating cofactor regeneration

This study reports the application of peptide linker in the construction of bi-functional formate dehydrogenase (FDH) and leucine dehydrogenase (LeuDH) enzymatic complex for efficient cofactor regeneration and L-tert leucine (L-tle) biotransformation. Seven FDH-LeuDH fusion enzymes with different peptide linker were successfully developed and displayed both parental enzyme activities. The incorporation order of FDH and LeuDH was investigated by predicting three-dimensional structures of LeuDH-FDH and FDH-LeuDH models using the I-TASSER server. The enzymatic characterization showed that insertion of rigid peptide linker obtained better activity and thermal stability in comparison with flexible peptide linker. The production rate of fusion enzymatic complex with suitable flexible peptide linker was increased by 1.2 times compared with free enzyme mixture. Moreover, structural analysis of FDH and LeuDH suggested the secondary structure of the N-, C-terminal domain and their relative positions to functional domains was also greatly relevant to the catalytic properties of the fusion enzymatic complex. The results show that rigid peptide linker could ensure the independent folding of moieties and stabilized enzyme structure, while the flexible peptide linker was likely to bring enzyme moieties in close proximity for superior cofactor channeling.

Whole-cell biotransformation systems for reduction of prochiral carbonyl compounds to chiral alcohol in Escherichia coli

Lactobacillus brevis alcohol dehydrogenase (Lb-ADH) catalyzes reduction of prochiral carbonyl compounds to chiral alcohol and meanwhile consumes its cofactor NADH into NAD⁺, while the cofactor regeneration can be catalyzed by Candida boidinii formate dehydrogenase (Cb-FDH). This work presents three different Escherichia coli whole-cell biocatalyst systems expressing recombinant ADH/FDH, FDH-LIN1-ADH and FDH-LIN2-ADH, respectively, all of which display very high efficacies of prochiral carbonyl conversion with respect to conversion rates and enantiomeric excess values. ADH/FDH represents co-expression of Lb-ADH and Cb-FDH under different promoters in a single vector. Fusion of Lb-ADH and Cb-FDH by a linker peptide LIN1 (GGGGS)₂ or LIN2 (EAAAK)₂ generates the two bifunctional enzymes FDH-LIN1-ADH and FDH-LIN2-ADH, which enable efficient asymmetric reduction of prochiral ketones in whole-cell biotransformation.

Diversion of flux toward sesquiterpene production in Saccharomyces cerevisiae by fusion of host and heterologous enzymes

The ability to transfer metabolic pathways from the natural producer organisms to the well-characterized cell factory Saccharomyces cerevisiae is well documented. However, as many secondary metabolites are produced by collaborating enzymes assembled in complexes, metabolite production in yeast may be limited by the inability of the heterologous enzymes to collaborate with the native yeast enzymes. This may cause loss of intermediates by diffusion or degradation or due to conversion of the intermediate through competitive pathways. To bypass this problem, we have pursued a strategy in which key enzymes in the pathway are expressed as a physical fusion. As a model system, we have constructed several fusion protein variants in which farnesyl diphosphate synthase (FPPS) of yeast has been coupled to patchoulol synthase (PTS) of plant origin (Pogostemon cablin). Expression of the fusion proteins in S. cerevisiae increased the production of patchoulol, the main sesquiterpene produced by PTS, up to 2-fold. Moreover, we have demonstrated that the fusion strategy can be used in combination with traditional metabolic engineering to further increase the production of patchoulol. This simple test case of synthetic biology demonstrates that engineering the spatial organization of metabolic enzymes around a branch point has great potential for diverting flux toward a desired product.

Enantioselective reduction of prochiral ketones by engineered bifunctional fusion proteins

NADPH-dependent oxidoreductases are useful catalysts for the production of chiral synthons. However, preparative applications of oxidoreductases require efficient methods for in situ regeneration of the expensive nicotinamide cofactors. An advantageous method for cofactor regeneration is the construction of bifunctional fusion proteins composed of two enzymes, one catalysing the reduction reaction and the other one mediating the recycling of cofactors. Herein, we describe the in-frame fusion between an NADP+-accepting mutant of FDH (formate dehydrogenase) from Mycobacterium vaccae N10 and KR [3-ketoacyl-(acyl-carrier-protein) reductase] from Synechococcus sp. strain PCC 7942. The generation of linker insertion mutants led to a fusion protein exhibiting 100 and 80% of the enzymatic activities of native KR and FDH respectively. Escherichia coli cells expressing the fusion protein showed an approx. 2-fold higher initial reaction rate in the production of chiral alcohols than cells expressing the enzymes separately. The application of the engineered fusion protein in whole-cell bioreduction of pentafluoroacetophenone resulted in a substrate conversion of 99.97% with an excellent enantiomeric excess of 99.9% (S)-1-(pentafluorophenyl)ethanol.