Studies on the Microbial Synthesis and Characterization of Polyhydroxyalkanoates Containing 4-Hydroxyvalerate Using γ-Valerolactone

Controlled production of a polyhydroxyalkanoate (PHA) tetramer containing different mole fraction of 3-hydroxybutyrate (3HB), 3-hydroxyvalerate (3 HV), 4 HV and 5 HV units by engineered Cupriavidus necator

Polyhydroxyalkanoates (PHAs) are promising alternatives to existing petrochemical-based plastics because of their bio-degradable properties. However, the limited structural diversity of PHAs has hindered their application. In this study, high mole-fractions of Poly (39 mol% 3HB-co-17 mol% 3 HV-co-44 mol% 4 HV) and Poly (25 mol% 3HB-co-75 mol% 5 HV) were produced from 4- hydroxyvaleric acid and 5-hydroxyvaleric acid, using Cupriavidus necator PHB-4 harboring the gene phaCBP-M-CPF4 with modified sequences. In addition, the complex toxicity of precursor mixtures was tested, and it was confirmed that the engineered C. necator was capable of synthesizing Poly (32 mol% 3HB-co-11 mol% 3 HV-co-25 mol% 4 HV-co-32 mol% 5 HV) at low mixture concentrations. Correlation analyses of the precursor ratio and the monomeric mole fractions indicated that each mole fractions could be precisely controlled using the precursor proportion. Physical property analysis confirmed that Poly (3HB-co-3 HV-co-4 HV) is a rubber-like amorphous polymer and Poly (3HB-co-5 HV) has a high tensile strength and elongation at break. Poly (3HB-co-3 HV-co-4 HV-co-5 HV) had a much lower glass transition temperature than the co-, terpolymers containing 3 HV, 4 HV and 5 HV. This study expands the range of possible physical properties of PHAs and contributes to the realization of custom PHA production by suggesting a method for producing PHAs with various physical properties through mole-fraction control of 3 HV, 4 HV and 5 HV.

Biodegradable Polyhydroxyalkanoates with a Different Set of Valerate Monomers: Chemical Structure and Physicochemical Properties

The properties, features of thermal behavior and crystallization of copolymers containing various types of valerate monomers were studied depending on the set and ratio of monomers. We synthesized and studied the properties of three-component copolymers containing unusual monomers 4-hydroxyvalerate (4HV) and 3-hydroxy-4-methylvalerate (3H4MV), in addition to the usual 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) monomers. The results showed that P(3HB-co-3HV-co-4HV) and P(3HB-co-3HV-co-3H4MV) terpolymers tended to increase thermal stability, especially for methylated samples, including an increase in the gap between melting point (Tmelt) and thermal degradation temperature (Tdegr), an increase in the melting point and glass transition temperature, as well as a lower degree of crystallinity (40-46%) compared with P(3HB-co-3HV) (58-66%). The copolymer crystallization kinetics depended on the set and ratio of monomers. For terpolymers during exothermic crystallization, higher rates of spherulite formation (Gmax) were registered, reaching, depending on the ratio of monomers, 1.6-2.0 µm/min, which was several times higher than the Gmax index (0.52 µm/min) for the P(3HB-co-3HV) copolymer. The revealed differences in the thermal properties and crystallization kinetics of terpolymers indicate that they are promising polymers for processing into high quality products from melts.

Biosynthesis of versatile PHA copolymers by thermophilic members of the genus Aneurinibacillus

Thermophilic members of the genus Aneurinibacillus constitute a remarkable group of microorganisms that exhibit extraordinary flexibility in terms of polyhydroxyalkanoates (PHA) synthesis. In this study, we demonstrate that these Gram-positive bacteria are capable of the utilization of selected lactones, namely, γ-valerolactone (GVL), γ-hexalactone (GHL), and δ-valerolactone (DVL) as the structural precursors of related PHA monomers. In the presence of GVL, a PHA copolymer consisting of 3-hydroxybutyrate, 3-hydroxyvalerate, and also 4-hydroxyvalerate was synthesized, with a 4 HV fraction as high as 53.1 mol%. Similarly, the application of GHL resulted in the synthesis of PHA copolymer containing 4-hydroxyhexanaote (4HHx) (4HHx fraction reached up to 11.5 mol%) and DVL was incorporated into PHA in form of 5-hydroxyvalerate (5 HV) (maximal 5 HV content was 44.2 mol%). The produced materials were characterized by thermoanalytical and spectroscopic methods; the results confirmed extremely appealing material properties of produced copolymers. Further, due to their unique metabolic features and capability of incorporating various PHA monomers into the PHA chain, thermophilic Aneurinibacillus spp. can be considered not only promising chassis for PHA production but also potential donors of PHA-relevant genes to improve PHA production in other thermophiles by using approaches of synthetic biology.

Production of polyhydroxyalkanoates containing monomers conferring amorphous and elastomeric properties from renewable resources: Current status and future perspectives

Petrochemical-based plastics cause environmental pollution and threaten humans and ecosystems. Polyhydroxyalkanoate (PHA) is considered a promising alternative to nondegradable plastics since it is eco-friendly and biodegradable polymer having similar properties to conventional plastics. PHA's material properties are generally determined by composition and type of monomers in PHA. PHA can be designed in tailor-made manner for their suitable application areas. Among many monomers in PHAs, ω-hydroxalkanoates such as 3-hydroxypropionate (3HP), 4-hydroxybutyrate (4HB), 5-hydroxyvalerate (5HV), and 6-hydroxyhexanoate (6HHx) and medium-chain-length 3-hydroxyalkanoate such as 3-hydroxyhexanoate (3HHx) and 4-hydroxyvalerate (4HV), have been examined as potential monomers able to confer amorphous and elastomer properties when these are incorporated as comonomer in poly(3-hydroxybutyrate) copolymer that has 3HB as main monomer along with comonomers in different monomer fraction. Herein, recent advances in production of PHAs designed to have amorphous and elastomeric properties from renewable sources such as lignocellulose, levulinic acid, crude glycerol, and waste oil are discussed.

Biosynthesis and Properties of a P(3HB-co-3HV-co-4HV) Produced by Cupriavidus necator B-10646

Synthesis of P(3HB-co-3HV-co-4HV) copolymers by the wild-type strain Cupriavidus necator B-10646 on fructose or sodium butyrate as the main C-substrate with the addition of γ-valerolactone as a precursor of 3HV and 4HV monomers was studied. Bacterial cells were cultivated in the modes that enabled production of a series of copolymers with molar fractions of 3HV (from 7.3 to 23.4 mol.%) and 4HV (from 1.9 to 4.7 mol.%) with bacterial biomass concentration (8.2 ± 0.2 g/L) and PHA content (80 ± 2%). Using HPLC, DTA, DSC, X-Ray, SEM, and AFM, the physicochemical properties of copolymers and films prepared from them have been investigated as dependent on proportions of monomers. Copolymers are characterized by a reduced degree of crystallinity (C_x 38-49%) molecular weight characteristics M_n (45-87 kDa), and M_w (201-248 kDa) compared with P(3HB). The properties of the films surface of various composition including the porosity and surface roughness were studied. Most of the samples showed a decrease in the average pore area and an increase in their number with a total increase in 3HV and 4HV monomers. The results allow scaling up the productive synthesis of P(3HB-co-3HV-co-4HV) copolymers using Cupriavidus necator B-10646.

Substrate-inducible and antibiotic-free high-level 4-hydroxyvaleric acid production in engineered Escherichia coli

In this study, we developed a levulinic acid (LA)-inducible and antibiotic-free plasmid system mediated by HpdR/PhpdH and infA-complementation to produce 4-hydroxyvaleric acid (4-HV) from LA in an engineered Escherichia coli strain. The system was efficiently induced by the addition of the LA substrate and resulted in tight dose-dependent control and fine-tuning of gene expression. By engineering the 5′ untranslated region (UTR) of hpdR mRNA, the gene expression of green fluorescent protein (GFP) increased by at least two-fold under the hpdH promoter. Furthermore, by evaluating the robustness and plasmid stability of the proposed system, the engineered strain, IRV750f, expressing the engineered 3-hydroxybutyrate dehydrogenase (3HBDH∗) and formate dehydrogenase (CbFDH), produced 82 g/L of 4-HV from LA, with a productivity of 3.4 g/L/h and molar conversion of 92% in the fed-batch cultivation (5 L fermenter) without the addition of antibiotics or external inducers. Overall, the reported system was highly beneficial for the large-scale and cost-effective microbial production of value-added products and bulk chemicals from the renewable substrate, LA.

Metabolic Engineering of Escherichia coli for Production of Polyhydroxyalkanoates with Hydroxyvaleric Acid Derived from Levulinic Acid

Polyhydroxyalkanoates (PHAs) are emerging as alternatives to plastics by replacing fossil fuels with renewable raw substrates. Herein, we present the construction of engineered Escherichia coli strains to produce short-chain-length PHAs (scl-PHAs), including the monomers 4-hydroxyvalerate (4HV) and 3-hydroxyvalerate (3HV) produced from levulinic acid (LA). First, an E. coli strain expressing genes (lvaEDABC) from the LA metabolic pathway of Pseudomonas putida KT2440 was constructed to generate 4HV-CoA and 3HV-CoA. Second, both PhaAB enzymes from Cupriavidus necator H16 were expressed to supply 3-hydroxybutyrate (3HB)-CoA from acetyl-CoA. Finally, PHA synthase (PhaC_Cv) from Chromobacterium violaceum was introduced for the subsequent polymerization of these three monomers. The resulting E. coli strains produced four PHAs (w/w% of dry cell weight): 9.1 wt% P(4HV), 1.7 wt% P(3HV-co-4HV), 24.2 wt% P(3HB-co-4HV), and 35.6 wt% P(3HB-co-3HV-co-4HV).

Metabolic engineering of Pseudomonas putida for the production of various types of short-chain-length polyhydroxyalkanoates from levulinic acid

Poly(3-hydroxybutyrate), a short-chain-length polyhydroxyalkanoate (scl-PHA), is considered as a good alternative to conventional synthetic plastics. However, various biopolymers with diverse characteristics are still in demand. In this study, four different types of scl-PHA were successfully produced by engineering levulinic acid (LA) utilization metabolic pathway and expressing heterologous PHA synthase (PhaEC), acetyl-CoA acetyltransferase (PhaA), and acetyl-CoA reductase (PhaB) in Pseudomonas putida EM42. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)], poly(3-hydroxyvalerate-co-4-hydroxyvalerate) [P(3HV-co-4HV)] and poly(3-hydroxybutyrate-co-3-hydroxyvalerate-co-4-hydroxyvalerate) [P(3HB-co-3HV-co-4HV)] were produced by the natural LA pathway, poly(4-hydroxyvalerate) by lvaAB-deleted LA pathway, and P(3HV-co-4HV) and P(3HB-co-3HV-co-4HV) with relatively high 3HV by fadB-deleted LA pathway. PHA with different monomer fractions could be produced using different PHA synthases. Scl-PHA contents reached approximately 40% of cell dry mass under non-optimized flask culture. This demonstrates that the LA catabolic pathway may be a good alternative route to provide monomers for the production of various types of PHA.

Engineering the lva operon and Optimization of Culture Conditions for Enhanced Production of 4-Hydroxyvalerate from Levulinic Acid in Pseudomonas putida KT2440

Monomeric 4-hydroxyvalerate is a versatile chemical used to produce various commodities and fine chemicals. In the present study, the lvaAB gene was deleted from the lva operon in Pseudomonas putida KT2440 and tesB, obtained from Escherichia coli, was overexpressed under the control of the lva operon system, which is induced by the substrate levulinic acid and the product 4-hydroxyvalerate to produce 4-hydroxyvalerate from levulinic acid. The lvaAB-deleted strain showed almost complete conversion of levulinic acid to 4-hydroxyvalerate, compared with 24% conversion in the wild-type strain. In addition, under optimized culture conditions, the final engineered strain produced a maximum of 50 g/L 4-hydroxyvalerate with 97% conversion from levulinic acid. The system presented here could be applied to produce high titers of 4-hydroxyvalerate in a cost-effective manner at a large scale from renewable cellulosic biomass.

Ring-opening copolymerization thermodynamics and kinetics of γ-valerolactone/ϵ-caprolactone

The general misconception that γ-lactones are not thermodynamically polymerizable has limited the development of all γ-lactone-based copolymers. A few studies have reported copolymerization of these five-membered cyclic esters with more reactive monomers, yet a systematic investigation of kinetics and thermodynamics is still lacking. To explore the feasibility of the reaction, we combined equilibrium and non-isothermal syntheses for the copolymerization of γ-valerolactone with ϵ-caprolactone, initiated with methoxy polyethyleneglycol and catalyzed by Tin(II) 2-ethylhexanoate. Here, we present the polymerization kinetic and thermodynamic parameters for different monomer ratios in the reaction feed. We observed the dependency of enthalpy and entropy of polymerization upon monomer ratio changes, and estimated a linear increase in the activation energy by increasing the γ-valerolactone fraction in the starting monomer mixture. Our data demonstrate that γ-valerolactone can copolymerize with ϵ-caprolactone, but only under specific conditions. The reaction parameters determined in this study will enable preparation of additional γ-valerolactone-based copolymers and development of a family of degradable materials with improved properties in respect to commonly used polyesters.

Production and Characterization of Polyhydroxyalkanoate from Lignin Derivatives by Pandoraea sp. ISTKB

The present study investigates polyhydroxyalkanoate (PHA) production from lignin and its derivatives by a previously reported lignin-degrading bacterial strain Pandoraea sp. ISTKB. PHA production was screened by fluorescence microscopy and flow cytometry using a Nile red stain. PHA and biomass accumulation, while screening, was found to be maximum on 4-hydroxybenzoic acid followed by p-coumaric acid, vanillic acid, 2,6-dimethoxyphenol, and kraft lignin after 96 h. Monomer composition was analyzed by gas chromatography-mass spectrometry (GC-MS) and was followed by Fourier transform infrared and ¹H NMR analysis, indicating PHA to be a copolymer of P(hydroxybutyrate-co-hydroxyvalerate). Genomic analysis of Pandoraea sp. ISTKB also complemented the results of GC-MS and NMR, and the relevant genes responsible for the synthesis of small chain length PHA were discovered in the genome. Process parameters were optimized by response surface methodology for enhanced production of PHA and biomass on 4-hydroxybenzoate. Optimization results showed 30 and 66% increase in the biomass and PHA production, respectively. The results obtained were promising and indicated that if lignin is depolymerized into low-molecular-weight intermediates, then it can easily be utilized and converted into value-added products like PHA by microbes.

Microbial degradation of linseed oil-based elastomer and subsequent accumulation of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer

The microbial synthesis of environment-friendly poly(3-hydroxybutyrate--co-3-hydroxyvalerate), PHBV, has been performed by using an alkaliphilic microorganism, Alkaliphilus oremlandii OhILAs strain (GenBank Accession number NR_043674.1), at pH 8 and at a temperature of 30-32 °C through the biodegradation of linseed oil-based elastomer. The yield of the copolymer on dry cell weight basis is 90 %. The elastomers used for the biodegradation have been synthesized by cationic polymerization technique. The yield of the PHBV copolymer also varies with the variation of linseed oil content (30-60 %) in the elastomer. Spectroscopic characterization ((1)H NMR and FTIR) of the accumulated product through biodegradation of linseed oil-based elastomers indicates that the accumulated product is a PHBV copolymer consisting of 13.85 mol% of 3-hydroxyvalerate unit. The differential scanning calorimetry (DSC) results indicate a decrease in the melting (T m) and glass transition temperature (T g) of PHBV copolymer with an increase in the content of linseed oil in the elastomer, which is used for the biodegradation. The gel permeation chromatography (GPC) results indicate that the weight average molecular weight (M w) of PHBV copolymer decreases with an increasing concentration of linseed oil in the elastomer. The surface morphology of the elastomer before and after biodegradation is observed under scanning electron microscope (SEM) and atomic force microscope (AFM); these results indicate about porous morphology of the biodegraded elastomer.