Construction of an Electron Transfer Mediator Pathway for Bioelectrosynthesis by Escherichia coli.

Microbial electrosynthesis (MES) or electro-fermentation (EF) is a promising microbial electrochemical technology for the synthesis of valuable chemicals or high-value fuels with aid of microbial cells as catalysts. By introducing electrical energy (current), fermentation environments can be altered or controlled in which the microbial cells are affected. The key role for electrical energy is to supply electrons to microbial metabolism. To realize electricity utility, a process termed inward extracellular electron transfer (EET) is necessary, and its efficiency is crucial to bioelectrochemical systems. The use of electron mediators was one of the main ways to realize electron transfer and improve EET efficiency. To break through some limitation of exogenous electron mediators, we introduced the phenazine-1-carboxylic acid (PCA) pathway from Pseudomonas aeruginosa PAO1 into Escherichia coli. The engineered E. coli facilitated reduction of fumarate by using PCA as endogenous electron mediator driven by electricity. Furthermore, the heterologously expressed PCA pathway in E. coli led to better EET efficiency and a strong metabolic shift to greater production of reduced metabolites, but lower biomass in the system. Then, we found that synthesis of adenosine triphosphate (ATP), as the "energy currency" in metabolism, was also affected. The reduction of menaquinon was demonstrated as one of the key reactions in self-excreted PCA-mediated succinate electrosynthesis. This study demonstrates the feasibility of electron transfer between the electrode and E. coli cells using heterologous self-excreted PCA as an electron transfer mediator in a bioelectrochemical system and lays a foundation for subsequent optimization.

Properties of Polyvinyl Alcohol Films Composited With Hemicellulose and Nanocellulose Extracted From Artemisia selengensis Straw.

Artemisia selengensis straw is an agricultural residue with great potential as a renewable resource because it is rich in lignocellulose. In this study, A. selengensis straw was used as a source of hemicelluloses (ASH) and cellulose nanocrystals (ASCNC) to produce biodegradable films. Different content levels of ASCNC were used as additives to improve composite films with ASH and polyvinyl alcohol (PVA). Various mechanical and hydrophobic properties of the films were analyzed. The composite films enhanced by ASCNC exhibited greater strength and were more effective as a barrier to water vapor when compared to that of the control ASH/PVA film. The tensile strength of the composite film was increased 80.1% to 36.21 MPa with ASCNC loading exceeding 9%, and the water vapor transmission rate decreased 15.45% when 12% ASCNC was added. Furthermore, the ASCNC-enhanced ASH/PVA composite film reduced a greater amount of light transmission than the control film.

Hierarchical porous and hydrophilic metal-organic frameworks with enhanced enzyme activity.

Metal-organic frameworks (MOFs) for enzyme encapsulation-induced biomimetic mineralization under mild reaction conditions are commonly microporous and hydrophobic, which result in a rather high mass transfer resistance of the reactants and restrain the enzyme catalytic activity. Herein, we prepared a type of hierarchical porous and hydrophilic MOF through the biomimetic mineralization of enzymes, zinc ions, 2-methylimidazole, and lithocholic acid. The hierarchical porous structure accelerated the diffusion process of the reactants and the increased hydrophilicity conferred interfacial activity and increased the enzyme catalytic activity. The immobilized enzyme retained higher catalytic activity than the free enzyme and exhibited enhanced resistance to alkaline, organic, and high-temperature conditions. The nanobiocatalyst was reusable and showed long-term storage stability.

Direct electron uptake from a cathode using the inward Mtr pathway in Escherichia coli.

Research on the biocathode-based bioelectrochemical system (BES) has attracted extensive attention because of its ability to increase the electricity-driven production of high-value fuels or chemicals by relying on microbial cells as catalysts. An extracellular electron transfer (EET) that makes electrical connections to microorganisms plays a key role in the BES. Compared with the better understanding of the EET-to-anode connection, the understanding of the mechanism and elements involved in inward EET from cathodes to microbes remains limited. Additionally, the low capability of the EET limits its applications in BESs for producing chemicals. Here, we introduced the Mtr pathway into Escherichia coli cells by expressing ccmABCDEFGH from E. coli and mtrABC from Shewanella oneidensis. Through selection by electrochemical pressure, the evolved E. coli could use electricity to increase the production of succinate in direct BES and enhance the electroactivity. In addition, in studying the mechanism of inward EET, menaquinone was found to be one of the key components of inward EET, and it is essential for the fumarate reduction reaction. Lastly, the intracellular NADH and ATP levels showed that there were differences in the energy conservation coupling between the electron transfer routes of the inward Mtr pathway and the electron mediator.

Artificial Nanometalloenzymes for Cooperative Tandem Catalysis.

Artificial metalloenzymes that combine the advantages of natural enzymes and metal catalysts have been getting more attention in research. As a proof of concept, an artificial nanometalloenzyme (CALB-Shvo@MiMBN) was prepared by co-encapsulation of metallo-organic catalyst and enzyme in a soft nanocomposite consisting of 2-methylimidazole, metal ions, and biosurfactant in mild reaction conditions using a one-pot self-assembly method. The artificial nanometalloenzyme with lipase acted as the core, and the metallo-organic catalyst embedded in micropore exhibited a spherical structure of 30-50 nm in diameter. The artificial nanometalloenzyme showed high catalytic efficiency in the dynamic kinetic resolution of racemic primary amines or secondary alcohols compared to the one-pot catalytic reaction of immobilized lipase and free metallo-organic catalyst. This artificial nanometalloenzyme holds great promise for integrated enzymatic and heterogeneous catalysis.