α-Substituted 3-hydroxy acid production from glucose in Escherichia coli

Polyhydroxyalkanoates (PHAs) are renewably-derived, microbial polyesters composed of hydroxy acids (HAs). Demand for sustainable plastics alternatives, combined with the unfavorable thermal properties exhibited by some PHAs, motivates the discovery of novel PHA-based materials. Incorporation of α-substituted HAs yields thermostable PHAs; however, the reverse β-oxidation (rBOX) pathway, the canonical pathway for HA production, is unable to produce these monomers because it utilizes thiolases with narrow substrate specificity. Here, we present a thiolase-independent pathway to two α-substituted HAs, 3-hydroxyisobutyric acid (3HIB) and 3-hydroxy-2-methylbutyric acid (3H2MB). This pathway involves the conversion of glucose to various branched acyl-CoAs and ultimately to 3HIB or 3H2MB. As proof of concept, we engineered Escherichia coli for the specific production of 3HIB and 3H2MB from glucose at titers as high as 66 ± 5 mg/L and 290 ± 40 mg/L, respectively. Optimizing this pathway for 3H2MB production via a novel byproduct recycle increased titer by 60%. This work illustrates the utility of novel pathway design HA production leading to PHAs with industrially relevant properties.

Tacticity Characterization of Biosynthesized Polyhydroxyalkanoates Containing (S)- and (R)-3-Hydroxy-2-Methylpropionate Units

Polyhydroxyalkanoates (PHAs), aliphatic polyesters synthesized by microorganisms, have gained considerable attention as biodegradable plastics. Recently, α-carbon-methylated PHAs have been shown to exhibit several interesting properties that differ from those of conventional PHAs, such as their crystallization behavior and material properties. This study investigated α-carbon methylated (S)- and (R)-3-hydroxy-2-methylpropionate (3H2MP) as new repeating units. 3H2MP units were homopolymerized or copolymerized with (R)-3-hydroxybutyrate (3HB) by manipulating the culture conditions of recombinant Escherichia coli LSBJ. Consequently, PHAs with 3H2MP units ranging from 5 to 100 mol % were synthesized by external addition of (R)- and (S)-enantiomers or the racemic form of 3H2MPNa. The (S)-3H2MP precursor supplemented into the culture medium was almost directly polymerized into PHA while maintaining its chirality. Therefore, a highly isotactic P(3H2MP) (R:S = 1:99) was synthesized, which displayed a melting temperature of 114-119 °C and a relatively high enthalpy of fusion (68 J/g). In contrast, in cultures supplemented with (R)-3H2MP, the precursor was racemized and polymerized into PHA, resulting in the synthesis of the amorphous polymer atactic P(3H2MP) (R:S = 40:60). However, racemization was not observed at a low concentration of the (R)-3H2MP precursor, thereby synthesizing P(3HB-co-8 mol % 3H2MP) with 100% (R)-3H2MP units. The thermogravimetric analysis revealed that the thermal degradation temperatures at 5% weight loss of P(3H2MP)s occurred at approximately 313 °C, independent of tacticity, which is substantially higher than that of P(3HB) (257 °C). This study demonstrates a new concept for controlling the physical properties of biosynthesized PHA by manipulating the polymers' tacticity using 3H2MP units.

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.

Nature-inspired methylated polyhydroxybutyrates from C1 and C4 feedstocks

Polyolefin plastics are widely used due to their low cost and outstanding properties, but their environmental persistence presents a major societal challenge. Polyhydroxyalkanoates (PHA) are biodegradable substitutes for polyolefins, but their high cost and thermal instability are impediments to their widespread application. Here we report a series of methylated polyhydroxybutyrates, poly(3-hydroxy-2-methylbutyrate)s, which are structurally inspired by natural PHAs. The cis homopolymers exhibit tacticity-independent crystallinity, which allows for the discovery of high-melting, thermally stable and mechanically tough copolymers, and a full range of polyolefin-like properties can be further achieved by tailoring the cis/trans ratio of the repeating units. Moreover, these materials can be synthesized from inexpensive carbon monoxide and 2-butene feedstocks, and they can be chemically recycled or upcycled at their end of life. The versatile properties, abundant feedstocks and end-of-life utility of this family of polyesters will enable a powerful platform for the discovery of sustainable alternatives to polyolefin plastics.

Exploring Class I polyhydroxyalkanoate synthases with broad substrate specificity for polymerization of structurally diverse monomer units

Polyhydroxyalkanoate (PHA) synthases (PhaCs) are key enzymes in PHA polymerization. PhaCs with broad substrate specificity are attractive for synthesizing structurally diverse PHAs. In the PHA family, 3-hydroxybutyrate (3HB)-based copolymers are industrially produced using Class I PhaCs and can be used as practical biodegradable thermoplastics. However, Class I PhaCs with broad substrate specificities are scarce, prompting our search for novel PhaCs. In this study, four new PhaCs from the bacteria Ferrimonas marina, Plesiomonas shigelloides, Shewanella pealeana, and Vibrio metschnikovii were selected via a homology search against the GenBank database, using the amino acid sequence of Aeromonas caviae PHA synthase (PhaC_Ac), a Class I enzyme with a wide range of substrate specificities, as a template. The four PhaCs were characterized in terms of their polymerization ability and substrate specificity, using Escherichia coli as a host for PHA production. All the new PhaCs were able to synthesize P(3HB) in E. coli with a high molecular weight, surpassing PhaC_Ac. The substrate specificity of PhaCs was evaluated by synthesizing 3HB-based copolymers with 3-hydroxyhexanoate, 3-hydroxy-4-methylvalerate, 3-hydroxy-2-methylbutyrate, and 3-hydroxypivalate monomers. Interestingly, PhaC from P. shigelloides (PhaC_Ps) exhibited relatively broad substrate specificity. PhaC_Ps was further engineered through site-directed mutagenesis, and the variant resulted in an enzyme with improved polymerization ability and substrate specificity.

Poly(3-mercapto-2-methylpropionate), a Novel α-Methylated Bio-Polythioester with Rubber-like Elasticity, and Its Copolymer with 3-hydroxybutyrate: Biosynthesis and Characterization

A new polythioester (PTE), poly(3-mercapto-2-methylpropionate) [P(3M2MP)], and its copolymer with 3-hydroxybutyrate (3HB) were successfully biosynthesized from 3-mercapto-2-methylpropionic acid as a structurally-related precursor. This is the fourth PTE of biological origin and the first to be α-methylated. P(3M2MP) was biosynthesized using an engineered Escherichia coli LSBJ, which has a high molecular weight, amorphous structure, and elastomeric properties, reaching 2600% elongation at break. P(3HB-co-3M2MP) copolymers were synthesized by expressing 3HB-supplying enzymes. The copolymers were produced with high content in the cells and showed a high 3M2MP unit incorporation of up to 77.2 wt% and 54.8 mol%, respectively. As the 3M2MP fraction in the copolymer increased, the molecular weight decreased and the polymers became softer, more flexible, and less crystalline, with lower glass transition temperatures and higher elongations at break. The properties of this PTE were distinct from those of previously biosynthesized PTEs, indicating that the range of material properties can be further expanded by introducing α-methylated thioester monomers.

Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research

Transforming how plastics are made, unmade, and remade through innovative research and diverse partnerships that together foster environmental stewardship is critically important to a sustainable future. Designing, preparing, and implementing polymers derived from renewable resources for a wide range of advanced applications that promote future economic development, energy efficiency, and environmental sustainability are all central to these efforts. In this Chemical Reviews contribution, we take a comprehensive, integrated approach to summarize important and impactful contributions to this broad research arena. The Review highlights signature accomplishments across a broad research portfolio and is organized into four wide-ranging research themes that address the topic in a comprehensive manner: Feedstocks, Polymerization Processes and Techniques, Intended Use, and End of Use. We emphasize those successes that benefitted from collaborative engagements across disciplinary lines.

Microbial oversecretion of (R)-3-hydroxybutyrate oligomer with diethylene glycol terminal as a macromonomer for polyurethane synthesis

Poly((R)-3-hydroxybutyrate) (P(3HB)) is a polyester that is synthesized and accumulated in many prokaryotic cells. Recently, a new culture method for the secretion of the intracellularly synthesized (R)-3-hydroxybutyrate oligomer (3HBO) from recombinant Escherichia coli cells was developed. In this study, we attempted to produce microbial 3HBO capped with a diethylene glycol terminal (3HBO-DEG) as a macromonomer for polymeric materials. First, we prepared recombinant E. coli strains harboring genes encoding various polyhydroxyalkanoate (PHA) synthases (PhaC, PhaEC or PhaRC) that can incorporate chain transfer (CT) agents such as DEG into the polymer's terminal and generate CT end-capped oligomers. To this end, each strain was cultivated under DEG supplemental conditions, and the synthesis of 3HBO-DEG was confirmed. As a result, the highest secretory production of 3HBO-DEG was observed for the PHA synthase derived from Bacillus cereus YB-4 (PhaRC_YB4). To evaluate the usability of the secreted 3HBO-DEG as a macromonomer, 3HBO-DEG was purified from the culture medium and polymerized with 4,4'-diphenylmethane diisocyanate as a spacer compound. Characterization of the polymeric products revealed that 3HBO-based polyurethane was successfully obtained and was a flexible and transparent noncrystalline polymer, unlike P(3HB). These results suggested that microbial 3HBO-DEG is a promising platform building block for synthesizing polyurethane and various other polymers.

Kinetic understanding of nitrogen supply condition on biosynthesis of polyhydroxyalkanoate from benzoate by Pseudomonas putida KT2440

Nitrogen supply is critical to the synthesis of intracellular PHA in various bacteria. However, the specific role of the nitrogen in synthesizing PHA from benzoate, a lignin model compound use for the study of bacteria catabolism of aromatics, is still not clear. In this study, two culture conditions were maintained for Pseudomonas putida KT2440 to produce PHA using benzoate as a carbon source. Under nitrogen-limited and surplus conditions, the accumulation of PHA was to 37.3% and 0.25% of cell dry weight, respectively. A model fit to the kinetics of biomass growth and PHA accumulation showed good agreement with data. GC-MS and NMR showed that PHA contained six hydroxyl fatty acid monomers under nitrogen-limited conditions, while two monomers were identified under nitrogen surplus conditions. The average molecular weight of PHA increased after the nitrogen source was exhausted. These results provide a promising strategy for optimization of lignin to PHA yields.

Discovery and Engineering of Pathways for Production of α-Branched Organic Acids

Cell-based synthesis offers many opportunities for preparing small molecules from simple renewable carbon sources by telescoping multiple reactions into a single fermentation step. One challenge in this area is the development of enzymatic carbon-carbon bond forming cycles that enable a modular disconnection of a target structure into cellular building blocks. In this regard, synthetic pathways based on thiolase enzymes to catalyze the initial carbon-carbon bond forming step between acyl coenzyme A (CoA) substrates offer a versatile route for biological synthesis, but the substrate diversity of such pathways is currently limited. In this report, we describe the identification and biochemical characterization of a thiolase-ketoreductase pair involved in production of branched acids in the roundworm, Ascaris suum, that demonstrates selectivity for forming products with an α-methyl branch using a propionyl-CoA extender unit. Engineering synthetic pathways for production of α-methyl acids in Escherichia coli using these enzymes allows the construction of microbial strains that produce either chiral 2-methyl-3-hydroxy acids (1.1 ± 0.2 g L^-1) or branched enoic acids (1.12 ± 0.06 g L^-1) in the presence of a dehydratase at 44% and 87% yield of fed propionate, respectively. In vitro characterization along with in vivo analysis indicates that the ketoreductase is the key driver for selectivity, forming predominantly α-branched products even when paired with a thiolase that highly prefers unbranched linear products. Our results expand the utility of thiolase-based pathways and provide biosynthetic access to α-branched compounds as precursors for polymers and other chemicals.