Plasmid Display for Stabilization of Enzymes Inside the Cell to Improve Whole-Cell Biotransformation Efficiency

Recent advances of 3-fucosyllactose in health effects and production

Human milk oligosaccharides (HMOs) have been recognized as gold standard for infant development. 3-Fucosyllactose (3-FL), being one of the Generally Recognized as Safe HMOs, represents a core trisaccharide within the realm of HMOs; however, it has received comparatively less attention in contrast to extensively studied 2'-fucosyllactose. The objective of this review is to comprehensively summarize the health effects of 3-FL, including its impact on gut microbiota proliferation, antimicrobial effects, immune regulation, antiviral protection, and brain maturation. Additionally, the discussion also covers the commercial application and regulatory approval status of 3-FL. Lastly, an organized presentation of large-scale production methods for 3-FL aims to provide a comprehensive guide that highlights current strategies and challenges in optimization.

Directed Evolution of Soluble α-1,2-Fucosyltransferase Using Kanamycin Resistance Protein as a Phenotypic Reporter for Efficient Production of 2'-Fucosyllactose

2'-Fucosyllactose (2'-FL), the most abundant fucosylated oligosaccharide in human milk, has multiple beneficial effects on human health. However, its biosynthesis by metabolically engineered Escherichia coli is often hampered owing to the insolubility and instability of α-1,2-fucosyltransferase (the rate-limiting enzyme). In this study, we aimed to enhance 2'-FL production by increasing the expression of soluble α-1,2-fucosyltransferase from Helicobacter pylori (FucT2). Because structural information regarding FucT2 has not been unveiled, we decided to improve the expression of soluble FucT2 in E. coli via directed evolution using a protein solubility biosensor that links protein solubility to antimicrobial resistance. For such a system to be viable, the activity of kanamycin resistance protein (Kan^R) should be dependent on FucT2 solubility. Kan^R was fused to the C-terminus of mutant libraries of FucT2, which were generated using a combination of error-prone PCR and DNA shuffling. Notably, one round of the directed evolution process, which consisted of mutant library generation and selection based on kanamycin resistance, resulted in a significant increase in the expression level of soluble FucT2. As a result, a batch fermentation with the ΔL M15 pBCGW strain, expressing the FucT2 mutant (F#1-5) isolated from the first round of the directed evolution process, resulted in the production of 0.31 g/l 2'-FL with a yield of 0.22 g 2'-FL/g lactose, showing 1.72- and 1.51-fold increase in the titer and yield, respectively, compared to those of the control strain. The simple and powerful method developed in this study could be applied to enhance the solubility of other unstable enzymes.

Optimization of protein trans-splicing in an inducible plasmid display system for high-throughput screening and selection of soluble proteins

Directed evolution is widely used to optimize protein folding and solubility in cells. Although the screening and selection of desired mutants is an essential step in directed evolution, it generally requires laborious optimization and/or specialized equipment. With a view toward designing a more practical procedure, we previously developed an inducible plasmid display system, in which the intein (auto-processing) and Oct-1 DNA-binding (DBD) domains were used as the protein trans-splicing domain and DNA-binding module, respectively. Specifically, the N-terminal (CfaN) and C-terminal (CfaC) domains of intein were fused to the C-terminal end of the His-tag and the N-terminal end of Oct-1 DBD to generate His₆-CfaN and CfaC-Oct-1, respectively. For such a system to be viable, the efficiency of protein trans-splicing without the protein of interest (POI) should be maximized, such that the probability of occurrence is solely dependent on the solubility of the POI. To this end, we initially prevented the degradation of l-arabinose (the inducer of the P_BAD promoter) by employing an Escherichia coli host strain deficient in the metabolism of l-arabinose. Given that a low expression of His₆-CfaN, compared with that of CfaC-Oct-1, was found to be conducive to the generation to a soluble product of the protein trans-splicing event, we designed the expression of His₆-CfaN and CfaC-Oct-1 to be inducible from the P_BAD and P_T7 promoters, respectively. The optimized system thus obtained enabled in vitro selection of the plasmid-protein complex with high yield. We believe that the inducible plasmid display system developed in this study would be applicable to high-throughput screening and/or selection of protein variants with enhanced solubility.

Enhancing acid tolerance of Escherichia coli via viroporin-mediated export of protons and its application for efficient whole-cell biotransformation

Escherichia coli-based whole-cell biocatalysts are widely used for the sustainable production of value-added chemicals. However, weak acids present as substrates and/or products obstruct the growth and fermentation capability of E. coli. Here, we show that a viroporin consisting of the influenza A matrix-2 (M2) protein, is activated by low pH and has proton channel activity in E. coli. The heterologous expression of the M2 protein in E. coli resulted in a significant increase in the intracellular pH and cell viability in the presence of various weak acids with different lengths of carbon chains. In addition, the feasibility of developing a robust and efficient E. coli-based whole-cell biocatalyst via introduction of the proton-selective viroporin was explored by employing (Z)-11-(heptanolyoxy)undec-9-enoic acid (ester) and 2-fucosyllactose (2'-FL) as model products, whose production is hampered by cytosolic acidification. The engineered E. coli strains containing the proton-selective viroporin exhibited approximately 80% and 230% higher concentrations of the ester and 2'-FL, respectively, than the control strains without the M2 protein. The simple and powerful strategy developed in this study can be applied to produce other valuable chemicals whose production involves substrates and/or products that cause cytosolic acidification.

Challenges Associated With the Formation of Recombinant Protein Inclusion Bodies in Escherichia coli and Strategies to Address Them for Industrial Applications

Recombinant proteins are becoming increasingly important for industrial applications, where Escherichia coli is the most widely used bacterial host for their production. However, the formation of inclusion bodies is a frequently encountered challenge for producing soluble and functional recombinant proteins. To overcome this hurdle, different strategies have been developed through adjusting growth conditions, engineering host strains of E. coli, altering expression vectors, and modifying the proteins of interest. These approaches will be comprehensively highlighted with some of the new developments in this review. Additionally, the unique features of protein inclusion bodies, the mechanism and influencing factors of their formation, and their potential advantages will also be discussed.

Inducible plasmid display system for high-throughput selection of proteins with improved solubility

Soluble expression of enzymes inside the cell is a prerequisite for the successful biotransformation of valuable products. Some key enzymes involved in biotransformation processes, however, are hardly expressed in their soluble forms. Here, we propose an inducible plasmid display, which is a molecular evolution strategy coupled with a high-throughput screening and/or selection method, as a simple and powerful tool for improving the solubility of target enzymes. Specifically, the Oct-1 DNA-binding domain and intein (i.e., auto-processing domain) were employed as anchoring and protein trans-splicing motifs to develop the system, in which the probability of protein trans-splicing is dependent on the soluble property of target proteins. The applicability of inducible plasmid display was investigated using an α-1,2-fucosyltransferase (FucT2) from Helicobacter pylori, a highly insoluble and unstable enzyme in the cytoplasmic space of Escherichia coli, as a model protein. One round of the overall inducible plasmid display process, which consists of in vivo production of FucT2 mutants and in vitro screening, enabled soluble expression of FucT2 and selection of plasmids containing the corresponding genetic information. The inducible plasmid display developed in this study will contribute to the rapid and efficient screening and/or selection of soluble proteins.

Development of fluorescent Escherichia coli for a whole-cell sensor of 2'-fucosyllactose

2′-Fucosyllactose (2′-FL), a major component of fucosylated human milk oligosaccharides, is beneficial to human health in various ways like prebiotic effect, protection from pathogens, anti-inflammatory activity and reduction of the risk of neurodegeneration. Here, a whole-cell fluorescence biosensor for 2′-FL was developed. Escherichia coli (E. coli) was engineered to catalyse the cleavage of 2′-FL into l-fucose and lactose by constitutively expressing α-l-fucosidase. Escherichia coli ∆L YA, in which lacZ is deleted and lacY is retained, was employed to disable lactose consumption. E. coli ∆L YA constitutively co-expressing α-l-fucosidase and a red fluorescence protein (RFP) exhibited increased fluorescence intensity in media containing 2′-FL. However, the presence of 50 g/L lactose reduced the RFP intensity due to lactose-induced cytotoxicity. Preadaptation of bacterial strains to fucose alleviated growth hindrance by lactose and partially recovered the fluorescence intensity. The fluorescence intensity of the cell was linearly proportional to 1–5 g/L 2′-FL. The whole-cell sensor will be versatile in developing a 2′-FL detection system.