Directed Evolution of Sequence-Regulating Polyhydroxyalkanoate Synthase to Synthesize a Medium-Chain-Length-Short-Chain-Length (MCL-SCL) Block Copolymer

A Review of Current Achievements and Recent Challenges in Bacterial Medium-Chain-Length Polyhydroxyalkanoates: Production and Potential Applications

Using petroleum-derived plastics has contributed significantly to environmental issues, such as greenhouse gas emissions and the accumulation of plastic waste in ecosystems. Researchers have focused on developing ecofriendly polymers as alternatives to traditional plastics to address these concerns. This review provides a comprehensive overview of medium-chain-length polyhydroxyalkanoates (mcl-PHAs), biodegradable biopolymers produced by microorganisms that show promise in replacing conventional plastics. The review discusses the classification, properties, and potential substrates of less studied mcl-PHAs, highlighting their greater ductility and flexibility compared to poly(3-hydroxybutyrate), a well-known but brittle PHA. The authors summarize existing research to emphasize the potential applications of mcl-PHAs in biomedicine, packaging, biocomposites, water treatment, and energy. Future research should focus on improving production techniques, ensuring economic viability, and addressing challenges associated with industrial implementation. Investigating the biodegradability, stability, mechanical properties, durability, and cost-effectiveness of mcl-PHA-based products compared to petroleum-based counterparts is crucial. The future of mcl-PHAs looks promising, with continued research expected to optimize production techniques, enhance material properties, and expand applications. Interdisciplinary collaborations among microbiologists, engineers, chemists, and materials scientists will drive progress in this field. In conclusion, this review serves as a valuable resource to understand mcl-PHAs as sustainable alternatives to conventional plastics. However, further research is needed to optimize production methods, evaluate long-term ecological impacts, and assess the feasibility and viability in various industries.

Engineering of the Long-Main-Chain Monomer-Incorporating Polyhydroxyalkanoate Synthase PhaCAR for the Biosynthesis of Poly[(R)-3-hydroxybutyrate-co-6-hydroxyhexanoate]

Polyhydroxyalkanoate (PHA) synthases (PhaCs) are useful and versatile tools for the production of aliphatic polyesters. Here, the chimeric PHA synthase PhaCAR was engineered to increase its capacity to incorporate unusual 6-hydroxyhexanoate (6HHx) units. Mutations at positions 149 and 314 in PhaCAR were previously found to increase the incorporation of an analogous natural monomer, 3-hydroxyhexanoate (3HHx). We attempted to repurpose the mutations to produce 6HHx-containing polymers. Site-directed saturation mutants at these positions were applied for P(3HB-co-6HHx) synthesis in Escherichia coli. As a result, the N149D and F314Y mutants effectively increased the 6HHx fraction. Moreover, the pairwise NDFY mutation further increased the 6HHx fraction, which reached 22 mol %. This increase was presumably caused by altered enzyme activity rather than altered expression levels, as assessed based on immunoblot analysis. The glass transition temperature and crystallinity of P(3HB-co-6HHx) decreased as the 6HHx fraction increased.

Engineering Pseudomonas chlororaphis HT66 for the Biosynthesis of Copolymers Containing 3-Hydroxybutyrate and Medium-Chain-Length 3-Hydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are promising alternatives to petroleum-based plastics, owing to their biodegradability and superior material properties. Here, the controllable biosynthesis of scl-co-mcl PHA containing 3-hydroxybutyrate (3HB) and mcl 3-hydroxyalkanoates was achieved in Pseudomonas chlororaphis HT66. First, key genes involved in fatty acid β-oxidation, the de novo fatty acid biosynthesis pathway, and the phaC1-phaZ-phaC2 operon were deleted to develop a chassis strain. Subsequently, an acetoacetyl-CoA reductase gene phaB and a PHA synthase gene phaC with broad substrate specificity were heterologously expressed for producing and polymerizing the 3HB monomer with mcl 3-hydroxyalkanoates under the assistance of native β-ketothiolase gene phaA. Furthermore, the monomer composition of scl-co-mcl PHA was regulated by adjusting the amount of glucose and dodecanoic acid supplemented. Notably, the cell dry weight and scl-co-mcl PHA content reached 14.2 g/L and 60.1 wt %, respectively, when the engineered strain HT11Δ::phaCB was cultured in King's B medium containing 5 g/L glucose and 5 g/L dodecanoic acid. These results demonstrated that P. chlororaphis can be a platform for producing scl-co-mcl PHA and has the potential for industrial application.

Synthesis and physical properties of polyhydroxyalkanoate (PHA)-based block copolymers: A review

Polyhydroxyalkanoates (PHAs) are a group of natural polyesters that are synthesised by microorganisms. In general, their thermoplasticity and (in some forms) their elasticity makes them attractive alternatives to petrochemical-derived polymers. However, the high crystallinity of some PHAs - such as poly(3-hydroxybutyrate) (P3HB) - results in brittleness and a narrow processing window for applications such as packaging. The production of copolymeric PHA materials is one approach to improving the mechanical and thermal properties of PHAs. Another solution is the manufacture of PHA-based block copolymers. The incorporation of different polymer and copolymer blocks coupled to PHA, and the resulting tailorable microstructure of these block copolymers, can result in a step-change improvement in PHA-based material properties. A range of production strategies for PHA-based block copolymers has been reported in the literature, including biological production and chemical synthesis. Biological production is typically less controllable, with products of a broad molecular weight and compositional distribution, unless finely controlled using genetically modified organisms. By contrast, chemical synthesis delivers relatively controllable block structures and narrowly defined compositions. This paper reviews current knowledge in the areas of the production and properties of PHA-based block copolymers, and highlights knowledge gaps and future potential areas of research.

Production of poly(3-hydroxybutyrate)/poly(lactic acid) from industrial wastewater by wild-type Cupriavidus necator H16

The massive production of urban and industrial wastes has created a clear need for alternative waste management processes. One of the more promising strategies is to use waste as raw material for the production of biopolymers such as polyhydroxyalkanoates (PHAs). In this work, a lactate-enriched stream obtained by anaerobic digestion (AD) of wastewater (WW) from a candy production plant was used as a feedstock for PHA production in wild-type Cupriavidus necator H16. Unexpectedly, we observed the accumulation of poly(3-hydroxybutyrate)/poly(lactic acid) (P(3HB)/PLA), suggesting that the non-engineered strain already possesses the metabolic potential to produce these polymers of interest. The systematic study of factors, such as incubation time, nitrogen and lactate concentration, influencing the synthesis of P(3HB)/PLA allowed the production of a panel of polymers in a resting cell system with tailored lactic acid (LA) content according to the GC-MS of the biomass. Further biomass extraction suggested the presence of methanol soluble low molecular weight molecules containing LA, while 1 % LA could be detected in the purified polymer fraction. These results suggested that the cells are producing a blend of polymers. A proteomic analysis of C. necator resting cells under P(3HB)/PLA production conditions provides new insights into the latent pathways involved in this process. This study is a proof of concept demonstrating that LA can polymerize in a non-modified organism and paves the way for new metabolic engineering approaches for lactic acid polymer production in the model bacterium C. necator H16.

Toward the production of block copolymers in microbial cells: achievements and perspectives

The microbial production of polyhydroxyalkanoate (PHA) block copolymers has attracted research interests because they can be expected to exhibit excellent physical properties. Although post-polymerization conjugation and/or extension have been used for PHA block copolymer synthesis, the discovery of the first sequence-regulating PHA synthase, PhaCAR, enabled the direct synthesis of PHA-PHA type block copolymers in microbial cells. PhaCAR spontaneously synthesizes block copolymers from a mixture of substrates. To date, Escherichia coli and Ralstonia eutropha have been used as host strains, and therefore, sequence regulation is not a host-specific phenomenon. The monomer sequence greatly influences the physical properties of the polymer. For example, a random copolymer of 3-hydroxybutyrate and 2-hydroxybutyrate deforms plastically, while a block copolymer of approximately the same composition exhibits elastic deformation. The structure of the PHA block copolymer can be expanded by in vitro evolution of the sequence-regulating PHA synthase. An engineered variant of PhaCAR can synthesize poly(D-lactate) as a block copolymer component, which allows for greater flexibility in the molecular design of block copolymers. Therefore, creating sequence-regulating PHA synthases with a further broadened substrate range will expand the variety of properties of PHA materials. This review summarizes and discusses the sequence-regulating PHA synthase, analytical methods for verifying block sequence, properties of block copolymers, and mechanisms of sequence regulation. KEY POINTS: • Spontaneous monomer sequence regulation generates block copolymers • Poly(D-lactate) segment can be synthesized using a block copolymerization system • Block copolymers exhibit characteristic properties.

Current advances and emerging trends in sustainable polyhydroxyalkanoate modification from organic waste streams for material applications

The current processes for producing polyhydroxyalkanoates (PHAs) are costly, owing to the high cost of cultivation feedstocks, and the need to sterilise the growth medium, which is energy-intensive. PHA has been identified as a promising biomaterial with a wide range of potential applications and its functionalization from waste streams has made significant advances recently, which can help foster the growth of a circular economy and waste reduction. Recent developments and novel approaches in the functionalization of PHAs derived from various waste streams offer opportunities for addressing these issues. This study focuses on the development of sustainable, efficient, and cutting-edge methods, such as advanced bioprocess engineering, novel catalysts, and advances in materials science. Chemical techniques, such as epoxidation, oxidation, and esterification, have been employed for PHA functionalization, while enzymatic and microbial methods have indicated promise. PHB/polylactic acid blends with cellulose fibers showed improved tensile strength by 24.45-32.08 % and decreased water vapor and oxygen transmission rates while PHB/Polycaprolactone blends with a 1:1 ratio demonstrated an elongation at break four to six times higher than pure PHB, without altering tensile strength or elastic modulus. Moreover, PHB films blended with both polyethylene glycol and esterified sodium alginate showed improvements in crystallinity and decreased hydrophobicity.

Pseudomonas umsongensis GO16 as a platform for the in vivo synthesis of short and medium chain length polyhydroxyalkanoate blends

Polyhydroxyalkanoates (PHAs) are biological polyesters, viewed as a replacement for petrochemical plastic. However, they suffer from suboptimal physical and mechanical properties. Here, it was shown that a metabolically versatile Pseudomonas umsongensis GO16 can synthesise a blend of short chain length (scl) and medium chain length (mcl)-PHA. A defined mix of butyric (BA) and octanoic acid (OA) in different ratios was used. The PHA monomer composition varied depending on the feeding strategy. When OA and BA were fed at 80:20 ratio it showed 14, 8, 77 and 1 mol% of (R)-3-hydroxybutyrate, (R)-3-hydroxyhexanoate, (R)-3-hydroxyoctanoate and (R)-3-hydroxydecanoate respectively. The polymer characterisation clearly shows that polyhydroxybutyrate (PHB) and mcl-PHA are produced individually. The two polymers are blended on the PHA granule level, as demonstrated by fluorescence microscopy and yeast two-hybrid assay. The resulting blend has a specific viscoelasticity compared to PHB and PHO. Mcl-PHA acts as a plasticiser and reduces PHB brittleness.

Exploring the Feasibility of Cell-Free Synthesis as a Platform for Polyhydroxyalkanoate (PHA) Production: Opportunities and Challenges

The extensive utilization of traditional petroleum-based plastics has resulted in significant damage to the natural environment and ecological systems, highlighting the urgent need for sustainable alternatives. Polyhydroxyalkanoates (PHAs) have emerged as promising bioplastics that can compete with petroleum-based plastics. However, their production technology currently faces several challenges, primarily focused on high costs. Cell-free biotechnologies have shown significant potential for PHA production; however, despite recent progress, several challenges still need to be overcome. In this review, we focus on the status of cell-free PHA synthesis and compare it with microbial cell-based PHA synthesis in terms of advantages and drawbacks. Finally, we present prospects for the development of cell-free PHA synthesis.

Real-time NMR analysis of polyhydroxyalkanoate synthase reaction that synthesizes block copolymer comprising glycolate and 3-hydroxybutyrate

The sequence-regulating polyhydroxyalkanoate (PHA) synthase PhaC_AR spontaneously synthesizes the homo-random block copolymer, poly[3-hydroxybutyrate (3HB)]-b-poly[glycolate (GL)-ran-3HB]. In this study, a real-time in vitro chasing system was established using a high-resolution 800 MHz nuclear magnetic resonance (NMR) and ¹³C-labeled monomers to monitor the polymerization of GL-CoA and 3HB-CoA into this atypical copolymer. Consequently, PhaC_AR initially consumed only 3HB-CoA and subsequently consumed both substrates. The structure of the nascent polymer was analyzed by extracting it with deuterated hexafluoro-isopropanol. In the primary reaction product, a 3HB-3HB dyad was detected, and GL-3HB linkages were subsequently formed. According to these results, the P(3HB) homopolymer segment is synthesized prior to the random copolymer segment. This is the first report of its kind which proposes the application of real-time NMR to a PHA synthase assay, paving the way for elucidating the mechanisms of PHA block copolymerization.

Versatile aliphatic polyester biosynthesis system for producing random and block copolymers composed of 2-, 3-, 4-, 5-, and 6-hydroxyalkanoates using the sequence-regulating polyhydroxyalkanoate synthase PhaCAR

BACKGROUND

Polyhydroxyalkanoates (PHAs) are microbial polyesters synthesized by PHA synthases. Naturally occurring PHA copolymers possess a random monomer sequence. The development of PhaCAR, a unique sequence-regulating PHA synthase, has enabled the spontaneous biosynthesis of PHA block copolymers. PhaCAR synthesizes both a block copolymer poly(2-hydroxybutyrate)-b-poly(3-hydroxybutyrate) [P(2HB)-b-P(3HB)], and a random copolymer, poly(3HB-co-3-hydroxyhexanoate), indicating that the combination of monomers determines the monomer sequence. Therefore, in this study, we explored the substrate scope of PhaCAR and the monomer sequences of the resulting copolymers to identify the determinants of the monomer sequence. PhaCAR is a class I PHA synthase that is thought to incorporate long-main-chain hydroxyalkanoates (LMC HAs, > C3 in the main [backbone] chain). Thus, the LMC monomers, 4-hydroxy-2-methylbutyrate (4H2MB), 5-hydroxyvalerate (5HV), and 6-hydroxyhexanoate (6HHx), as well as 2HB, 3HB, and 3-hydroxypropionate (3HP) were tested.

RESULTS

Recombinant Escherichia coli harboring PhaCAR, CoA transferase and CoA ligase genes was used for PHA production. The medium contained the monomer precursors, 2HB, 3HB, 3HP, 4H2MB, 5HV, and 6HHx, either individually or in combination. As a result, homopolymers were obtained only for 3HB and 3HP. Moreover, 3HB and 3HP were randomly copolymerized by PhaCAR. 3HB-based binary copolymers P(3HB-co-LMC HA)s containing up to 2.9 mol% 4H2MB, 4.8 mol% 5HV, or 1.8 mol% 6HHx were produced. Differential scanning calorimetry analysis of the copolymers indicated that P(3HB-co-LMC HA)s had a random sequence. In contrast, combining 3HP and 2HB induced the synthesis of P(3HP)-b-P(2HB). Similarly, P(2HB) segment-containing block copolymers P(3HB-co-LMC HA)-b-P(2HB)s were synthesized. Binary copolymers of LMC HAs and 2HB were not obtained, indicating that the 3HB or 3HP unit is essential to the polymer synthesis.

CONCLUSION

PhaCAR possesses a wide substrate scope towards 2-, 3-, 4-, 5-, and 6-hydroxyalkanoates. 3HB or 3HP units are essential for polymer synthesis using PhaCAR. The presence of a 2HB monomer is key to synthesizing block copolymers, such as P(3HP)-b-P(2HB) and P(3HB-co-LMC HA)-b-P(2HB)s. The copolymers that did not contain 2HB units had a random sequence. This study's results provide insights into the mechanism of sequence regulation by PhaCAR and pave the way for designing PHA block copolymers.