Evaluating the Ability of Polyhydroxyalkanoate Synthase Mutants to Produce P(3HB-co- 3HA) from Soybean Oil

Recent Development of Polyhydroxyalkanoates (PHA)-Based Materials for Antibacterial Applications: A Review

The diseases caused by microorganisms are innumerable existing on this planet. Nevertheless, increasing antimicrobial resistance has become an urgent global challenge. Thus, in recent decades, bactericidal materials have been considered promising candidates to combat bacterial pathogens. Recently, polyhydroxyalkanoates (PHAs) have been used as green and biodegradable materials in various promising alternative applications, especially in healthcare for antiviral or antiviral purposes. However, it lacks a systematic review of the recent application of this emerging material for antibacterial applications. Therefore, the ultimate goal of this review is to provide a critical review of the state of the art recent development of PHA biopolymers in terms of cutting-edge production technologies as well as promising application fields. In addition, special attention was given to collecting scientific information on antibacterial agents that can potentially be incorporated into PHA materials for biological and durable antimicrobial protection. Furthermore, the current research gaps are declared, and future research perspectives are proposed to better understand the properties of these biopolymers as well as their possible applications.

Engineering of Aeromonas caviae Polyhydroxyalkanoate Synthase Through Site-Directed Mutagenesis for Enhanced Polymerization of the 3-Hydroxyhexanoate Unit

Polyhydroxyalkanoate (PHA) synthase is an enzyme that polymerizes the acyl group of hydroxyacyl-coenzyme A (CoA) substrates. Aeromonas caviae PHA synthase (PhaC_Ac) is an important biocatalyst for the synthesis of a useful PHA copolymer, poly[(R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate] [P(3HB-co-3HHx)]. Previously, a PhaC_Ac mutant with double mutations in asparagine 149 (replaced by serine [N149S]) and aspartate 171 (replaced by glycine [D171G]) was generated to synthesize a 3HHx-rich P(3HB-co-3HHx) and was named PhaC_Ac NSDG. In this study, to further increase the 3HHx fraction in biosynthesized PHA, PhaC_Ac was engineered based on the three-dimensional structural information of PHA synthases. First, a homology model of PhaC_Ac was built to target the residues for site-directed mutagenesis. Three residues, namely tyrosine 318 (Y318), serine 389 (S389), and leucine 436 (L436), were predicted to be involved in substrate recognition by PhaC_Ac. These PhaC_Ac NSDG residues were replaced with other amino acids, and the resulting triple mutants were expressed in the engineered strain of Ralstonia eutropha for application in PHA biosynthesis from palm kernel oil. The S389T mutation allowed the synthesis of P(3HB-co-3HHx) with an increased 3HHx fraction without a significant reduction in PHA yield. Thus, a new workhorse enzyme was successfully engineered for the biosynthesis of a higher 3HHx-fraction polymer.

Polyhydroxyalkanoates (PHAs): Biopolymers for Biofuel and Biorefineries

Fossil fuels are energy recourses that fulfill most of the world's energy requirements. However, their production and use cause severe health and environmental problems including global warming and pollution. Consequently, plant and animal-based fuels (also termed as biofuels), such as biogas, biodiesel, and many others, have been introduced as alternatives to fossil fuels. Despite the advantages of biofuels, such as being renewable, environmentally friendly, easy to source, and reducing the dependency on foreign oil, there are several drawbacks of using biofuels including high cost, and other factors discussed in the fuel vs. food debate. Therefore, it is imperative to produce novel biofuels while also developing suitable manufacturing processes that ease the aforementioned problems. Polyhydroxyalkanoates (PHAs) are structurally diverse microbial polyesters synthesized by numerous bacteria. Moreover, this structural diversity allows PHAs to readily undergo methyl esterification and to be used as biofuels, which further extends the application value of PHAs. PHA-based biofuels are similar to biodiesel except for having a high oxygen content and no nitrogen or sulfur. In this article, we review the microbial production of PHAs, biofuel production from PHAs, parameters affecting the production of fuel from PHAs, and PHAs biorefineries. In addition, future work on the production of biofuels from PHAs is also discussed.

Degradation of plant arabinogalactan proteins by intestinal bacteria: characteristics and functions of the enzymes involved

Arabinogalactan proteins (AGPs) are complex plant proteoglycans that function as dietary fiber utilized by human intestinal bacteria such as Bifidobacterium and Bacteroides species. However, the degradative mechanism is unknown because of the complexity of sugar chains of AGPs as well as variation among plant species and organs. Recently, AGP degradative enzymes have been characterized in Bifidobacterium and Bacteroides species. In this review, we summarize the characteristics and functions of AGP degradative enzymes in human intestinal bacteria.

Bacterial polyhydroxyalkanoates: Still fabulous?

Bacterial polyhydroxyalkanoates (PHA) are polyesters accumulated as carbon and energy storage materials under limited growth conditions in the presence of excess carbon sources. They have been developed as biomaterials with unique properties for the past many years being considered as a potential substitute for conventional non-degradable plastics. Due to the increasing concern towards global climate change, depleting petroleum resource and problems with an utilization of a growing number of synthetic plastics, PHAs have gained much more attention from industry and research. These environmentally friendly microbial polymers have great potential in biomedical, agricultural, and industrial applications. However, their production on a large scale is still limited. This paper describes the backgrounds of PHAs and discussed the current state of knowledge on the polyhydroxyalkanoates. Ability of bacteria to convert different carbon sources to PHAs, the opportunities and challenges of their introduction to global market as valuable renewable products have been also discussed.

Microbial production of polyhydroxyalkanoates (PHAs) and its copolymers: A review of recent advancements

Traditional mineral oil based plastics are important commodity to enhance the comfort and quality of life but the accumulation of these plastics in the environment has become a major universal problem due to their low biodegradation. Solution to the plastic waste management includes incineration, recycling and landfill disposal methods. These processes are very time consuming and expensive. Biopolymers are important alternatives to the petroleum-based plastics due to environment friendly manufacturing processes, biodegradability and biocompatibility. Therefore use of novel biopolymers, such as polylactide, polysaccharides, aliphatic polyesters and polyhydroxyalkanoates is of interest. PHAs are biodegradable polyesters of hydroxyalkanoates (HA) produced from renewable resources by using microorganisms as intracellular carbon and energy storage compounds. Even though PHAs are promising candidate for biodegradable polymers, however, the production cost limit their application on an industrial scale. This article provides an overview of various substrates, microorganisms for the economical production of PHAs and its copolymers. Recent advances in PHAs to reduce the cost and to improve the performance of PHAs have also been discussed.

Challenges and Opportunities for Customizing Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) as an alternative to synthetic plastics have been gaining increasing attention. Being natural in their origin, PHAs are completely biodegradable and eco-friendly. However, consistent efforts to exploit this biopolymer over the last few decades have not been able to pull PHAs out of their nascent stage, inspite of being the favorite of the commercial world. The major limitations are: (1) the high production cost, which is due to the high cost of the feed and (2) poor thermal and mechanical properties of polyhydroxybutyrate (PHB), the most commonly produced PHAs. PHAs have the physicochemical properties which are quite comparable to petroleum based plastics, but PHB being homopolymers are quite brittle, less elastic and have thermal properties which are not suitable for processing them into sturdy products. These properties, including melting point (Tm), glass transition temperature (Tg), elastic modulus, tensile strength, elongation etc. can be improved by varying the monomeric composition and molecular weight. These enhanced characteristics can be achieved by modifications in the types of substrates, feeding strategies, culture conditions and/or genetic manipulations.

Biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyalkanoates) by recombinant Escherichia coli from glucose

The polyhydroxyalkanoate (PHA) copolymers consisting of short-chain-length (scl) and medium-chain-length (mcl) monomers have various properties ranging from stiff to flexible depending on the molar fraction of the monomer compositions. It has been reported that PhaG, which is first known as a (R)-3-hydroxyacyl-acyl carrier protein (ACP)-CoA transferase, actually functions as a 3-hydroxyacyl-ACP thioesterase, and the product of PP0763 gene from Pseudomonas putida KT2440 has a (R)-3-hydroxyacyl (3HA)-CoA ligase activity (Wang et al., Appl. Environ. Microbiol., 78, 519-527, 2012). In this study, we found a new (R)-3HA-CoA ligase (the product of PA3924 gene) from Pseudomonas aeruginosa PAO. The PA3924 gene was coexpressed with PHA synthase 1 gene (phaC1Ps) and phaGPs gene from Pseudomonas sp. 61-3, and β-ketothiolase gene (phbARe) and acetoacetyl-CoA reductase gene (phbBRe) from Ralstonia eutropha in Escherichia coli LS5218 at 25°C. As a result, the copolymer containing 94.6 mol% 3-hydroxybutyrate (3HB) and 5.4 mol% mcl-3-hydroxyalkanoates (3HA) consisting of C8, C10, C12 and C14 was synthesized by recombinant E. coli LS5218 from glucose as the sole carbon source. The concentration of P(3HB-co-3HA) (scl-co-mcl-PHA) synthesized by the recombinant E. coli LS5218 harboring phaC1Ps, phaGPs, phbABRe and the PA3924 genes was approximately 7-fold higher than that of the recombinant LS5218 harboring phaC1Ps, phaGPs, phbABRe and the PP0763 genes. The number-average molecular weight of the P(3HB-co-5.4% 3HA) copolymer was 233 × 10(3), which was relatively high molecular weight. In addition, the physical and the mechanical properties of the copolymer were demonstrated to improve the brittleness of P(3HB) homopolymer.

Biosynthesis of polyhydroxyalkanaotes by a novel facultatively anaerobic Vibrio sp. under marine conditions

Marine bacteria have recently attracted attention as potentially useful candidates for the production of practical materials from marine ecosystems, including the oceanic carbon dioxide cycle. The advantages of using marine bacteria for the biosynthesis of poly(hydroxyalkanoate) (PHA), one of the eco-friendly bioplastics, include avoiding contamination with bacteria that lack salt-water resistance, ability to use filtered seawater as a culture medium, and the potential for extracellular production of PHA, all of which would contribute to large-scale industrial production of PHA. A novel marine bacterium, Vibrio sp. strain KN01, was isolated and characterized in PHA productivity using various carbon sources under aerobic and aerobic-anaerobic marine conditions. The PHA contents of all the samples under the aerobic-anaerobic condition, especially when using soybean oil as the sole carbon source, were enhanced by limiting the amount of dissolved oxygen. The PHA accumulated using soybean oil as a sole carbon source under the aerobic-anaerobic condition contained 14% 3-hydroxypropionate (3HP) and 3% 5-hydroxyvalerate (5HV) units in addition to (R)-3-hydroxybutyrate (3HB) units and had a molecular weight of 42 × 10³ g/mol. The present result indicates that the activity of the beta-oxidation pathway under the aerobic-anaerobic condition is reduced due to a reduction in the amount of dissolved oxygen. These findings have potential for use in controlling the biosynthesis of long main-chain PHA by regulating the activity of the beta-oxidation pathway, which also could be regulated by varying the dissolved oxygen concentration.

Characterization and functional analyses of R-specific enoyl coenzyme A hydratases in polyhydroxyalkanoate-producing Ralstonia eutropha

A genome survey of polyhydroxyalkanoate (PHA)-producing Ralstonia eutropha H16 detected the presence of 16 orthologs of R-specific enoyl coenzyme A (enoyl-CoA) hydratase, among which three proteins shared high homologies with the enzyme specific to enoyl-CoAs of medium chain length encoded by phaJ4 from Pseudomonas aeruginosa (phaJ4(Pa)). The recombinant forms of the three proteins, termed PhaJ4a(Re) to PhaJ4c(Re), actually showed enoyl-CoA hydratase activity with R specificity, and the catalytic efficiencies were elevated as the substrate chain length increased from C(4) to C(8). PhaJ4a(Re) and PhaJ4b(Re) showed >10-fold-higher catalytic efficiency than PhaJ4c(Re). The functions of the new PhaJ4 proteins were investigated using previously engineered R. eutropha strains as host strains; these strains are capable of synthesizing poly((R)-3-hydroxybutyrate-co-(R)-3-hydroxyhexanoate) [P(3HB-co-3HHx)] from soybean oil. Deletion of phaJ4a(Re) from the chromosome resulted in significant decrease of 3HHx composition in the accumulated copolyester, whereas no change was observed with deletion of phaJ4b(Re) or phaJ4c(Re), indicating that only PhaJ4a(Re) was one of the major enzymes supplying the (R)-3HHx-CoA monomer through β-oxidation. Introduction of phaJ4a(Re) or phaJ4b(Re) into the R. eutropha strains using a broad-host-range vector enhanced the 3HHx composition of the copolyesters, but the introduction of phaJ4c(Re) did not. The two genes were then inserted into the pha operon on chromosome 1 of the engineered R. eutropha by homologous recombination. These modifications enabled the biosynthesis of P(3HB-co-3HHx) composed of a larger 3HHx fraction without a negative impact on cell growth and PHA production on soybean oil, especially when phaJ4a(Re) or phaJ4b(Re) was tandemly introduced with phaJ(Ac) from Aeromonas caviae.