Metabolic engineering for the synthesis of polyesters: A 100-year journey from polyhydroxyalkanoates to non-natural microbial polyesters.

As concerns increase regarding sustainable industries and environmental pollutions caused by the accumulation of non-degradable plastic wastes, bio-based polymers, particularly biodegradable plastics, have attracted considerable attention as potential candidates for solving these problems by substituting petroleum-based plastics. Among these candidates, polyhydroxyalkanoates (PHAs), natural polyesters that are synthesized and accumulated in a range of microorganisms, are considered as promising biopolymers since they have biocompatibility, biodegradability, and material properties similar to those of commodity plastics. Accordingly, substantial efforts have been made to gain a better understanding of mechanisms related to the biosynthesis and properties of PHAs and to develop natural and recombinant microorganisms that can efficiently produce PHAs comprising desired monomers with high titer and productivity for industrial applications. Recent advances in biotechnology, including those related to evolutionary engineering, synthetic biology, and systems biology, can provide efficient and effective tools and strategies that reduce time, labor, and costs to develop microbial platform strains that produce desired chemicals and materials. Adopting these technologies in a systematic manner has enabled microbial fermentative production of non-natural polyesters such as poly(lactate) [PLA], poly(lactate-co-glycolate) [PLGA], and even polyesters consisting of aromatic monomers from renewable biomass-derived carbohydrates, which can be widely used in current chemical industries. In this review, we present an overview of strain development for the production of various important natural PHAs, which will give the reader an insight into the recent advances and provide indicators for the future direction of engineering microorganisms as plastic cell factories. On the basis of our current understanding of PHA biosynthesis systems, we discuss recent advances in the approaches adopted for strain development in the production of non-natural polyesters, notably 2-hydroxycarboxylic acid-containing polymers, with particular reference to systems metabolic engineering strategies.

Solution blowing spinning of polylactate/polyvinyl alcohol/ZnO nanocomposite toward green and sustainable preparation of wound dressing nanofibrous films.

The outstanding biodegradability, biocompatibility, affordability, and renewability of polylactic acid have made it a prominent biomaterial. Herein, an innovative, easy, and eco-friendly technique is used to prepare sodium polylactate (SP)-based nanofibers. Solution blowing spinning (SBS) was used to create fibrous mats of SP and polyvinyl alcohol (PVA). SBS's SP nanfibers were crosslinked using an aqueous solution of calcium chloride to produce moisture-resistant calcium polylactate nanofibrous spun mats. Both of UV-visible absorption spectra and transmission electron microscopy were utilized to study the produced zinc oxide (ZnO) nanoparticles (NPs) to indicate a diameter of around 15-23 nm with a high intensity absorption intensity at 370 nm. New polylactate copolymer was synthesized and characterized by infrared and NMR spectroscopic techniques. In order to prepare SP/PVA/ZnO nanocomposite nanofibers, various ZnO ratios were used. The morphologies of the composite nanofibers were investigated by infrared spectroscopy (FTIR), energy-dispersive X-ray analyzer, and scanning electron microscopy. The cytotoxicity tests of the prepared mat were studied by conducting experiments with L-929 cells at various time intervals. The prepared composite SP/PVA/ZnO nanofibers were subjected to cytotoxicity tests to determine their cytocompatibility. Results showed that those with ZnO concentrations between 0.5% and 2% were found to be less harmful than those with higher concentrations. A variety of bacterial species, including Bacillus pumilus and Staphylococcus aureus, as well as Klebseilla pneumoniae and Escherichia coli, were used to test the antibacterial properties of SP/PVA/ZnO spun mats. The ZnO NPs integrated in the SP/PVA fibrous mats were responsible for their antibacterial properties. After finding the appropriate concentration of ZnO that is least harmful while yet giving a satisfactory antibacterial activity, this biomaterial might be perfect for wound dressing applications. HIGHLIGHTS: New eco-friendly biodegradable sodium polylactate (SP) copolymer was synthesized. Zinc oxide nanoparticles (ZnO NPs) with a diameter of 15-23 nm were prepared. High antibacterial SP/PVA/ZnO fibers were prepared by solution blowing spinning. SP/PVA/ZnO nanofibers (180-220 nm) with various ratios of ZnO were presented. Cytotoxicity results showed that the cell viability decreases with increasing ZnO.

Biocatalytic synthesis of polylactate and its copolymers by engineered microorganisms.

Poly(lactate), also called poly(lactic acid) or poly(lactide) [PLA], has been one of the most attractive bio-based polymers since it possesses desirable material properties for its use in general performance plastics in addition to biodegradability and biocompatibility. PLA has been produced by biological and chemical hybrid process comprising microbial fermentation for lactate (LA) production followed by purification and chemical polymerization process of LA. Recently, the direct one-step fermentative processes for production of PLA and several LA-containing polyesters have been developed by employing metabolically engineered microorganisms. Since natural microorganisms cannot produce the LA-containing polymers, several engineering strategies have been employed together based on the polyhydroxyalkanoate (PHA) biosynthesis system. In this chapter, we summarize strategies and procedures on developing the engineered microorganisms producing PLA and its copolymers, cultivating the cells, and extracting the polymers from the cells. Focuses were given on construction of enzymatic polymerization process of LA: design of metabolic pathway for PLA by mimicking PHA biosynthetic pathway, examination of possible enzymes, and engineering of the enzymes for better performances. This synthetic pathway has been established in a microorganism producing LA that enabled one-step fermentative production of LA-containing polyesters from carbohydrates derived from renewable biomass. Polymer production has been further enhanced by implementing strain engineering to concentrate the metabolic fluxes toward PLA formation. In addition, various monomers such as glycolate, 2-hydroxybutyrate, and phenyllactate have been copolymerized with LA by the microbial system. These fermentative production systems developed by using the engineered microorganisms can be versatile and sustainable platforms for the production of LA-containing polyesters and other non-natural polymers.