Enhancing the 3-hydroxyvalerate component in bioplastic PHBV production by Cupriavidus necator

Microbial production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate), from lab to the shelf: A review

Polyhydroxyalkanoates (PHAs) are natural biopolyesters produced by microorganisms that represent one of the most promising candidates for the replacement of conventional plastics due to their complete biodegradability and advantageous material properties which can be modulated by varying their monomer composition. Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] has received particular research attention because it can be synthesized based on the same microbial platform developed for poly(3-hydroxybutyrate) [P(3HB)] without much modification, with as high productivity as P(3HB). It also offers more useful mechanical and thermal properties than P(3HB), which broaden its application as a biocompatible and biodegradable polyester. However, a significant commercial disadvantage of P(3HB-co-3HV) is its rather high production cost, thus many studies have investigated the economical synthesis of P(3HB-co-3HV) from structurally related and unrelated carbon sources in both wild-type and recombinant microbial strains. A large number of metabolic engineering strategies have also been proposed to tune the monomer composition of P(3HB-co-3HV) and thus its material properties. In this review, recent metabolic engineering strategies designed to for enhanced production of P(3HB-co-3HV) are discussed, along with their current status, limitations, and future perspectives.

Maximization of 3-hydroxyhexanoate fraction in poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) using lauric acid with engineered Cupriavidus necator H16

As polyhydroxybutyrate (P(3HB)) was struggling with mechanical properties, efforts have been directed towards increasing mole fraction of 3-hydroxyhexanoate (3HHx) in P(3HB-co-3HHx) to improve the properties of polyhydroxyalkanoates (PHAs). Although genetic modification had significant results, there were several issues related to cell growth and PHA production by deletion of PHA synthetic genes. To find out easier strategy for high 3HHx mole fraction without gene deletion, Cupriavidus necator H16 containing phaC2Ra-phaACn-phaJ1Pa was examined with various oils resulting that coconut oil gave the highest 3HHx mole fraction. When fatty acid composition analysis with GC-MS was applied, coconut oil was found to have very different composition from other vegetable oil containing very high lauric acid (C12) content. To find out specific fatty acid affecting 3HHx fraction, different fatty acids from caproic acid (C6) to stearic acid (C18) was evaluated and the 3HHx mole fraction was increased to 26.5 ± 1.6 % using lauric acid. Moreover, the 3HHx mole fraction could be controlled from 9 % to 31.1 % by mixing bean oil and lauric acid with different ratios. Produced P(3HB-co-3HHx) exhibited higher molecular than P(3HB-co-3HHx) from phaB-deletion mutant. This study proposes another strategy to increase 3HHx mole fraction with easier way by modifying substrate composition without applying deletion tools.

Influence of Rheological and Morphological Characteristics of Polyhydroxybutyrate on Its Meltblown Process Behavior

Polyhydroxybutyrate (PHB) is a promising biopolymer. However, processing PHB in pure form in thermoplastic processes is limited due to its rapid degradation, very low initial crystallization rate, strong post-crystallization, and its low final stretchability. In this article, we screened commercial PHBs for morphological characteristics, rheological properties, and "performance" in the meltblown process in order to reveal process-relevant properties and overcome the shortcoming of PHB in thermoplastic processes for fiber formation. An evaluation of degradation (extruded (meltblown) material vs. granules) was performed via rheological and SEC analysis. The study revealed large differences in the minimum melt temperature (175 up to 200 °C) and the grade-dependent limitation of accessible throughput on a 500 mm plant. The average fiber diameter could be lowered from around 10 μm to 2.4 μm in median, which are the finest reported values in the literature so far. It was found that the determination of the necessary process temperature can be predicted well from the complex shear viscosity. Different to expectations, it became apparent that a broader initial molar mass distribution (>8) is suitable to overcome the state-of-the art limitations of PHAs in order to stabilize fiber formation, increase the productivity, and obtain better resistance towards thermal degradation in process. Accordingly, longer polymer chain fractions could be more affected by degradation than medium and short polymer chains in the distribution. Further, a low initial narrow distributed molar mass resulted in too brittle fabrics.

Fermentative bioconversion of food waste into biopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) using Cupriavidus necator

Dependency on plastic commodities has led to a recurrent increase in their global production every year. Conventionally, plastic products are derived from fossil fuels, leading to severe environmental concerns. The recent coronavirus disease 2019 pandemic has triggered an increase in medical waste. Conversely, it has disrupted the supply chain of personal protective equipment (PPE). Valorisation of food waste was performed to cultivate C. necator for fermentative production of biopolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The increase in biomass, PHBV yield and molar 3-hydroxy valerate (3HV) content was estimated after feeding volatile fatty acids. The fed-batch fermentation strategy reported in this study produced 15.65 ± 0.14 g/L of biomass with 5.32 g/L of PHBV with 50% molar 3HV content. This is a crucial finding, as molar concentration of 3HV can be modulated to suit the specification of biopolymer (film or fabric). The strategy applied in this study addresses the issue of global food waste burden and subsequently generates biopolymer PHBV, turning waste to wealth.

Biomedical Applications of Polyhydroxyalkanoate in Tissue Engineering

Tissue engineering technology aids in the regeneration of new tissue to replace damaged or wounded tissue. Three-dimensional biodegradable and porous scaffolds are often utilized in this area to mimic the structure and function of the extracellular matrix. Scaffold material and design are significant areas of biomaterial research and the most favorable material for seeding of in vitro and in vivo cells. Polyhydroxyalkanoates (PHAs) are biopolyesters (thermoplastic) that are appropriate for this application due to their biodegradability, thermo-processability, enhanced biocompatibility, mechanical properties, non-toxicity, and environmental origin. Additionally, they offer enormous potential for modification through biological, chemical and physical alteration, including blending with various other materials. PHAs are produced by bacterial fermentation under nutrient-limiting circumstances and have been reported to offer new perspectives for devices in biological applications. The present review discusses PHAs in the applications of conventional medical devices, especially for soft tissue (sutures, wound dressings, cardiac patches and blood vessels) and hard tissue (bone and cartilage scaffolds) regeneration applications. The paper also addresses a recent advance highlighting the usage of PHAs in implantable devices, such as heart valves, stents, nerve guidance conduits and nanoparticles, including drug delivery. This review summarizes the in vivo and in vitro biodegradability of PHAs and conducts an overview of current scientific research and achievements in the development of PHAs in the biomedical sector. In the future, PHAs may replace synthetic plastics as the material of choice for medical researchers and practitioners.

A Critical Review on the Economically Feasible and Sustainable Poly(3-Hydroxybutyrate-co-3-hydroxyvalerate) Production from Alkyl Alcohols

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) is the most studied short-chain-length polyhydroxyalkanoates (PHA) with high application importance in various fields. The domination of high-cost propionate and valerate over other 3-hydroxyvalerate (3HV) precursors owing to their wide preference among PHA-producing bacteria has hindered the development of diverse production processes. As alkyl alcohols are mainly produced from inexpensive starting materials through oxo synthesis, they contribute a cost-effective advantage over propionate and valerate. Moreover, alkyl alcohols can be biosynthesized from natural substrates and organic wastes. Despite their great potential, their toxicity to most PHA-producing bacteria has been the major drawback for their wide implementation as 3HV precursors for decades. Although the standard PHA-producing bacteria Cupriavidus necator showed promising alcohol tolerance, the 3HV yield was discouraging. Continuous discovery of alkyl alcohols-utilizing PHA-producing bacteria has enabled broader choices in 3HV precursor selection for diverse P(3HB-co-3HV) production processes with higher economic feasibility. Besides continuous effort in searching for promising wild-type strains, genetic engineering to construct promising recombinant strains based on the understanding of the mechanisms involved in alkyl alcohols toxicity and tolerance is an alternative approach. However, more studies are required for techno-economic assessment to analyze the economic performance of alkyl alcohol-based production compared to that of organic acids.

Genetic incorporation of oil-utilizing ability in Cupriavidus malaysiensis USMAA2-4 for sustainable polyhydroxyalkanoates production from palm olein and 1-pentanol

The sustainability in polyhydroxyalkanoates (PHA) production is drawing increasing attention as the effort to increase the economic feasibility for commercialization pursues. Oleic acid is widely preferred by bacteria but its employment for PHA production makes sustainability rather dubious. This study showed promising poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] content of 68 wt % by lipase genes-harbouring Cupriavidus malaysiensis USMAA2-4 transformant from palm olein and 1-pentanol. High oleic acid content and low oil saturation caused palm olein to outperform crude palm oil, crude palm kernel oil and soybean oil due to its preference for oleic acid shown by previous screening. The transformant showed 8-fold and 40-fold higher lipase activity compared to C. necator H16 and its wild-type respectively. The transformant was unaffected by Co²⁺ but the growth of C. necator H16 was inversely proportional to Co²⁺ concentration and the employment of 1-pentanol also ceased its growth and PHA accumulation. Although the inhibitory effect of Fe²⁺, Cu²⁺ and Zn²⁺ at high molarity on LipA decreased PHA content of C. malaysiensis USMAA2-4 transformant by 23-24 wt %, the lipase activity was restorable with high molarity of Ca²⁺, thus resulted in higher PHA content. The transformant enabled the employment of low-cost 1-pentanol as the precursor for cost-effective PHA production and its preference for palm olein contributed to higher sustainability.

Anabolism of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by Cupriavidus necator DSM 545 from spent coffee grounds oil

Oil extracted from spent coffee grounds (SCG) [yield 16.8 % (w/w)] was discovered to be a highly suitable carbon substrate for the biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3 HV)] copolymers by Cupriavidus necator DSM 545 in the absence of any traditional 3 HV precursors. Cells cultivated in a 3 L bioreactor (batch) reached a total biomass concentration of 8.9 g L^-1 with a P(3HB-co-3 HV) (6.8 mol% 3 HV) content of 89.6 % (w/w). In contrast, cells grown on sunflower oil reached a total biomass concentration of 9.4 gL^-1 with a P(3HB-co-3 HV) (0.2 mol% 3 HV) content of 88.1 % (w/w). It is proposed that the organism could synthesize 3 HV monomers from succinyl CoA, an intermediate of the tricarboxylic acid (TCA) cycle, via the succinate-propionate metabolic pathway.

Microbial and enzymatic conversion of levulinic acid, an alternative building block to fermentable sugars from cellulosic biomass

Levulinic acid (LA) is an important chemical building block listed among the top 12 value-added chemicals by the United States Department of Energy, and can be obtained through the hydrolysis of lignocellulosic biomass. Using the same approach as in the catalytic production of LA from biomass, catalytic methods to upgrade LA to higher value chemicals have been investigated. Since the discovery of the catabolic genes and enzymes in the LA metabolic pathway, bioconversion of LA into useful chemicals has attracted attention, and can potentially broaden the range of biochemical products derived from cellulosic biomass. With a brief introduction to the LA catabolic pathway in Pseudomonas spp., this review summarizes the current studies on the microbial conversion of LA into bioproducts, including the recent developments to achieve higher yields through genetic engineering of Escherichia coli cells. Three different types of reactions during the enzymatic conversion of LA are also discussed. KEY POINTS: • Levulinic acid is an alternative building block to sugars from cellulosic biomass. • Introduction of levulinic acid bioconversion with natural and engineered microbes. • Initial enzymatic conversion of levulinic acid proceeds via three different pathways. • 4-Hydroxyvalerate is one of the target chemicals for levulinic acid bioconversion.

The effect of methane and odd-chain fatty acids on 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV) synthesis by a Methylosinus-dominated mixed culture

A methanotrophic community was enriched in a semi-continuous reactor under non-aseptic conditions with methane and ammonia as carbon and nitrogen source. After a year of operation, Methylosinus sp., accounted for 80% relative abundance of the total sequences identified from potential polyhydroxyalkanoates (PHAs) producers, dominated the methane-fed enrichment. Prior to induction of PHA accumulation, cells harvested from the parent reactor contained low level of PHA at 4.0 ± 0.3 wt%. The cells were later incubated in the absence of ammonia with various combinations of methane, propionic acid, and valeric acid to induce biosynthesis of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). Previous studies reported that methanotrophic utilization of odd-chain fatty acids for the production of PHAs requires reducing power from methane oxidation. However, our findings demonstrated that the PHB-containing methanotrophic enrichment does not require methane availability to generate 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate (3HV)—when odd-chain fatty acids are presented. The enrichment yielded up to 14 wt% PHA with various mole fractions of 3HV monomer depending on the availability of methane and odd-fatty acids. Overall, the addition of valeric acid resulted in a higher PHA content and a higher 3HV fraction. The highest 3HV fraction (up to 65 mol%) was obtained from the methane–valeric acid experiment, which is higher than those previously reported for PHA-producing methanotrophic mixed microbial cultures.

Recent Advances in the Use of Polyhydroyalkanoates in Biomedicine

Polyhydroxyalkanoates (PHAs), a family of natural biopolyesters, are widely used in many applications, especially in biomedicine. Since they are produced by a variety of microorganisms, they possess special properties that synthetic polyesters do not have. Their biocompatibility, biodegradability, and non-toxicity are the crucial properties that make these biologically produced thermoplastics and elastomers suitable for their applications as biomaterials. Bacterial or archaeal fermentation by the combination of different carbohydrates or by the addition of specific inductors allows the bioproduction of a great variety of members from the PHAs family with diverse material properties. Poly(3-hydroxybutyrate) (PHB) and its copolymers, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHVB) or poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (PHB4HB), are the most frequently used PHAs in the field of biomedicine. PHAs have been used in implantology as sutures and valves, in tissue engineering as bone graft substitutes, cartilage, stents for nerve repair, and cardiovascular patches. Due to their good biodegradability in the body and their breakdown products being unhazardous, they have also been remarkably applied as drug carriers for delivery systems. As lately there has been considerable and growing interest in the use of PHAs as biomaterials and their application in the field of medicine, this review provides an insight into the most recent scientific studies and advances in PHAs exploitation in biomedicine.

Cloud-point extraction of green-polymers from Cupriavidus necator lysate using thermoseparating-based aqueous two-phase extraction

Polyhydroxyalkanoates (PHAs), a class of renewable and biodegradable green polymers, have gained attraction as a potential substitute for the conventional plastics due to the increasing concern towards environmental pollution as well as the rapidly depleting petroleum reserve. Nevertheless, the high cost of downstream processing of PHA has been a bottleneck for the wide adoption of PHAs. Among the options of PHAs recovery techniques, aqueous two-phase extraction (ATPE) outshines the others by having the advantages of providing a mild environment for bioseparation, being green and non-toxic, the capability to handle a large operating volume and easily scaled-up. Utilizing unique properties of thermo-responsive polymer which has decreasing solubility in its aqueous solution as the temperature rises, cloud point extraction (CPE) is an ATPE technique that allows its phase-forming component to be recycled and reused. A thorough literature review has shown that this is the first time isolation and recovery of PHAs from Cupriavidus necator H16 via CPE was reported. The optimum condition for PHAs extraction (recovery yield of 94.8% and purification factor of 1.42 fold) was achieved under the conditions of 20 wt/wt % ethylene oxide-propylene oxide (EOPO) with molecular weight of 3900 g/mol and 10 mM of sodium chloride addition at thermoseparating temperature of 60°C with crude feedstock limit of 37.5 wt/wt %. Recycling and reutilization of EOPO 3900 can be done at least twice with satisfying yield and PF. CPE has been demonstrated as an effective technique for the extraction of PHAs from microbial crude culture.

Improvement of the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) production by dual feeding with levulinic acid and sodium propionate in Cupriavidus necator

In the context of increasing volatility of oil prices, replacement of petroleum based plastics by bioplastics is a topic of increasing interest. Poly(hydroxyalkanoate)s (PHAs) are among the most promising families in this field. Controlling composition of the polymer on the monomeric level remains a pivotal issue. This control is even more difficult to achieve when the polymer is not synthesized by chemists, but produced by nature, in this case, bacteria. In this study mechanism and role of two 3-hydroxyvalerate (3-HV) inducing substrates on the production of PHBV with high, 80%, 3-HV content were evaluated. It was found that levulinic acid contributes to biomass and bio-polymer content enhancement, whereas sodium propionate mainly contributes to 3-HV enhancement. Optimized proportions of feeding substrates at 1 g/L and 2.5 g/L, respectively for levulinic acid and sodium propionate allowed a 100% productivity enhancement, at 3.9 mg/L/hour, for the production of PHBV with 80% 3-HV.

From organic pollutants to bioplastics: insights into the bioremediation of aromatic compounds by Cupriavidus necator

Organic pollution by aromatic compounds is of increasing concern to our environment. Therefore, the transformation of aromatic pollutants into valuable aliphatic and biodegradable bioplastics was studied. Since benzoic acid was found to be the key compound for such bioremediation processes, its transformation, and metabolic pathways of digestion, by Cupriavidus necator were specifically analysed. It was found that the degradation of aromatic compounds follows the 2,3-dioxygenase pathway in this strain and that the batch transformations of benzoic acid with either fresh or adapted cells were limited to an initial concentration of 2.5 g/L of pollutant. The repeated fed-batch with partial withdrawal process, however, showed a 17.5-fold improvement, thus allowing the transformation of a total of 43.7 g/L in 12 weeks.

Metabolic engineering of Escherichia coli for biosynthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from glucose

The Escherichia coli XL1-blue strain was metabolically engineered to synthesize poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] through 2-ketobutyrate, which is generated via citramalate pathway, as a precursor for propionyl-CoA. Two different metabolic pathways were examined for the synthesis of propionyl-CoA from 2-ketobutyrate. The first pathway is composed of the Dickeya dadantii 3937 2-ketobutyrate oxidase or the E. coli pyruvate oxidase mutant (PoxB L253F V380A) for the conversion of 2-ketobutyrate into propionate and the Ralstonia eutropha propionyl-CoA synthetase (PrpE) or the E. coli acetyl-CoA:acetoacetyl-CoA transferase for further conversion of propionate into propionyl-CoA. The second pathway employs pyruvate formate lyase encoded by the E. coli tdcE gene or the Clostridium difficile pflB gene for the direct conversion of 2-ketobutyrate into propionyl-CoA. As the direct conversion of 2-ketobutyrate into propionyl-CoA could not support the efficient production of P(3HB-co-3HV) from glucose, the first metabolic pathway was further examined. When the recombinant E. coli XL1-blue strain equipped with citramalate pathway expressing the E. coli poxB L253F V380A gene and R. eutropha prpE gene together with the R. eutropha PHA biosynthesis genes was cultured in a chemically defined medium containing 20 g/L of glucose as a sole carbon source, P(3HB-co-2.3 mol% 3HV) was produced up to the polymer content of 61.7 wt.%. Moreover, the 3HV monomer fraction in P(3HB-co-3HV) could be increased up to 5.5 mol% by additional deletion of the prpC and scpC genes, which are responsible for the metabolism of propionyl-CoA in host strains.

Biosynthesis and thermal properties of PHBV produced from levulinic acid by Ralstonia eutropha

Levulinic acid (LA) can be cost-effectively produced from a vast array of renewable carbohydrate-containing biomaterials. LA could facilitate the commercialization of the polymer poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) and PHBV-based products as carbon substrates. Therefore, this paper focused on the production of PHBV by Ralstonia eutropha with LA for hydroxyvalerate (HV) production, which plays an important role in enhancing the thermal properties of PHBV. Accordingly, the HV content of PHBV varied from 0–40.9% at different concentrations of LA. Stimulation of cell growth and PHBV accumulation were observed when 2–6 g L⁻¹ LA was supplied to the culture. The optimal nitrogen sources were determined to be 0.5 g L⁻¹ ammonium chloride and 2 g L⁻¹ casein peptone. It was determined that the optimal pH for cell growth and PHBV accumulation was 7.0. When the cultivation was performed in large scale (2 L fermenter) with a low DO concentration of 30% and a pH of 7.0, a high maximum dry cell weight of 15.53 g L⁻¹ with a PHBV concentration of 12.61 g L⁻¹ (53.9% HV), up to 81.2% of the dry cell weight, was obtained. The melting point of PHBV found to be decreased as the fraction of HV present in the polymer increased, which resulted in an improvement in the ductility and flexibility of the polymer. The results of this study will improve the understanding of the PHBV accumulation and production by R. eutropha and will be valuable for the industrial production of biosynthesized polymers.

Metabolic engineering of Escherichia coli for the production of 1-propanol

An engineered Escherichia coli strain that produces 1-propanol under aerobic condition was developed based on an L-threonine-overproducing E. coli strain. First, a feedback resistant ilvA gene encoding threonine dehydratase was introduced and the competing metabolic pathway genes were deleted. Further engineering was performed by overexpressing the cimA gene encoding citramalate synthase and the ackA gene encoding acetate kinase A/propionate kinase II, introducing a modified adhE gene encoding an aerobically functional AdhE, and by deleting the rpoS gene encoding the stationary phase sigma factor. Fed-batch culture of the final engineered strain harboring pBRthrABC-tac-cimA-tac-ackA and pTacDA-tac-adhE(mut) allowed production of 10.8 g L(-1) of 1-propanol with the yield and productivity of 0.107 g g(-1) and 0.144 g L(-1) h(-1), respectively, from 100 g L(-1) of glucose, and 10.3 g L(-1) of 1-propanol with the yield and productivity of 0.259 g g(-1) and 0.083 g L(-1) h(-1), respectively, from 40 g L(-1) glycerol.

Novel approach for productivity enhancement of polyhydroxyalkanoates (PHA) production by Cupriavidus necator DSM 545

In the global context of increased concerns for our environment, the use of bioplastics as a replacement for existing petroleum-based polymers is an important challenge. Indeed, bioplastics hardly meet economical and technical constraints. One, of the most promising among currently studied bioplastics, is the polyhydroxyalkanoate (PHA). To circumvent the economical issue for this particular biopolymer one solution can be the enhancement of the overall productivity by the improvement of the nutritional medium of the microorganism producing the biopolymer. Thus, several nutrition media, supplemented or not with sodium glutamate, were tested for the growth and the PHA production by Cupriavidus necator DSM 545 strain. The most efficient for the biomass and the PHA production improvement were found to be the Luria broth (LB) and the Bonnarme's media, both supplemented with 10 g/L sodium glutamate. Hence the overall productivity was 33 times enhanced comparing to traditional cultivation methods. These results open a new route for the PHA production by C. necator which appears to be more suitable on a rich, or enriched, medium with no limiting factors.