Adjustable mutations in lactate (LA)-polymerizing enzyme for the microbial production of LA-based polyesters with tailor-made monomer composition

Complete sequence randomness of lactate-based copolymers (LAHBs) with varied lactate monomer fractions employing a series of propionyl-CoA transferases

Previously, we biosynthesized an evolved version of a bio-based polylactide (PLA) on microbial platforms using our engineered lactate-polymerizing enzyme (LPE). This lactate (LA)-based copolyester, LAHB, has advantages over PLA, including improved flexibility and biodegradability, and its properties can be regulated through the LA fraction. To expand the LA-incorporation capacity and improve polymer properties, in the state of in vivo LAHB production, propionyl-CoA transferases (PCTs) that exhibited enhanced production of LA-CoA than the conventional PCTs were selected. Here, the present study has demonstrated that the LA fraction of LAHB could be altered using various PCTs. Enhanced PCT performance was achieved by balancing polymer production and cell growth. Both events are governed by the use of acetyl-CoA, a commonly shared key metabolite. This could be attributed to the different reactivities of individual PCTs towards acetyl-CoA, which serves both as a CoA donor and a leading compound in the TCA cycle. Interestingly, we found complete sequence randomness in the LAHB copolymers, independent of the LA fraction. The mechanism of LA fraction-independent sequence randomness is discussed. This new PCT-based strategy synergistically combines with the evolution of LPE to advance the LAHB project, and enables us to perform advanced applications other than LAHB production utilizing CoA-linked substrates.

Modification of poly(lactate) via polymer blending with microbially produced poly[(R)-lactate-co-(R)-3-hydroxybutyrate] copolymers

This study investigated the effect of polymer blending of microbially produced poly[(R)-lactate-co-(R)-3-hydroxybutyrate] copolymers (LAHB) with poly(lactate) (PLA) on their mechanical, thermal, and biodegradable properties. Blending of high lactate (LA) content and high molecular weight LAHB significantly improved the tensile elongation of PLA up to more than 250 % at optimal LAHB composition of 20-30 wt%. Temperature-modulated differential scanning calorimetry and dynamic mechanical analysis revealed that PLA and LAHB were immiscible but interacted with each other, as indicated by the mutual plasticization effect. Detailed morphological characterization using scanning probe microscopy, small-angle X-ray scattering, and solid-state NMR confirmed that PLA and LAHB formed a two-phase structure with a characteristic length scale as small as 20 nm. Because of mixing in this order, the polymer blends were optically transparent. The biological oxygen demand test of the polymer blends in seawater indicated an enhancement of PLA biodegradation during biodegradation of the polymer blends.

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.

Cell-growth phase-dependent promoter replacement approach for improved poly(lactate-co-3-hydroxybutyrate) production in Escherichia coli

Escherichia coli is a useful platform for producing valuable materials through the implementation of synthetic gene(s) derived from other organisms. The production of lactate (LA)-based polyester poly[LA-co-3-hydroxybutyrate (3HB)] was carried out in E. coli using a set of five other species-derived genes: Pseudomonas sp. 61-3-derived phaC1STQK (for polymerization), Cupriavidus necator-derived phaAB (for 3HB-CoA generation), and Megasphaera elsdenii-derived pct (for LA-CoA generation) cloned into pTV118NpctphaC1p_s(ST/QK)AB. Here, we aimed to optimize the expression level and timing of these genes to improve the production of P(LA-co-3HB) and to manipulate the LA fraction by replacing the promoters with various promoters in E. coli. Evaluation of the effects of 21 promoter replacement plasmids revealed that the phaC1STQK-AB operon is critical for the stationary phase for P(LA-co-3HB) production. Interestingly, the effects of the promoters depended on the composition of the medium. In glucose-supplemented LB medium, the dps promoter replacement plasmid resulted in the greatest effect, increasing the accumulation to 8.8 g/L and an LA fraction of 14.1 mol% of P(LA-co-3HB), compared to 2.7 g/L and 8.1 mol% with the original plasmid. In xylose-supplemented LB medium, the yliH promoter replacement plasmid resulted in the greatest effect, with production of 5.6 g/L and an LA fraction of 40.2 mol% compared to 3.6 g/L and 22.6 mol% with the original plasmid. These results suggest that the selection of an appropriate promoter for expression of the phaC1STQK-AB operon could improve the production and LA fraction of P(LA-co-3HB). Here, we propose that the selection of cell-growth phase-dependent promoters is a versatile biotechnological strategy for effective intracellular production of polymeric materials such as P(LA-co-3HB), in combination with the selection of sugar-based carbon sources.

A review on microbial synthesis of lactate-containing polyesters

Degradable polylactic acids (PLA) have been widely used in agriculture, textile, medicine and degradable plastics industry, and can completely replace petroleum-based plastics in the future. At present, polylactic acid was chemically synthesized by ring-opening polymerisation or the direct polycondensation of lactic acid, which inevitably leads to chemical and heavy metal catalyst pollution. The current research focus has gradually shifted to the development of recombinant industrial strains for the efficiently production of lactate-containing polyesters from renewable resources. This review summarizes various explorations of metabolic pathway optimization and production cost control in the industrialization of lactate-containing polyesters bio-production. In particular, the effects of key enzymes, including CoA transferase, polyhydroxyalkanoate synthase, and their mutants, culture conditions, low-cost carbon sources, and recombinant strains on the yield and composition of lactate-containing polyesters are summarized and discussed. Future prospects and challenges for the industrialization of lactate-containing polyesters are also pointed out.

Control of D-lactic acid content in P(LA-3HB) copolymer in the yeast Saccharomyces cerevisiae using a synthetic gene expression system

The fully biobased polyhydroxyalkanoate (PHA) polymers provide interesting alternatives for petrochemical derived plastic materials. The mechanical properties of some PHAs, including the common poly(3-hydroxybutyrate) (PHB), are limited, but tunable by addition of other monomers into the polymer chain. In this study we present a precise synthetic biology method to adjust lactate monomer fraction of a polymer by controlling the monomer formation in vivo at gene expression level, independent of cultivation conditions. We used the modified doxycycline-based Tet-On approach to adjust the expression of the stereospecific D-lactate dehydrogenase gene (ldhA) from Leuconostoc mesenteroides to control D-lactic acid formation in yeast Saccharomyces cerevisiae. The synthetic Tet-On transcription factor with a VP16 activation domain was continuously expressed and its binding to a synthetic promoter with eight transcription factor specific binding sites upstream of the ldhA gene was controlled with the doxycycline concentration in the media. The increase in doxycycline concentration correlated positively with ldhA expression, D-lactic acid production, poly(D-lactic acid) (PDLA) accumulation in vivo, and D-lactic acid content in the poly(D-lactate-co-3-hydroxybutyrate) P(LA-3HB) copolymer. We demonstrated that the D-lactic acid content of the P(LA-3HB) copolymer can be adjusted linearly from 6 mol% to 93 mol% in vivo in S. cerevisiae. These results highlight the power of controlling gene expression and monomer formation in the tuning of the polymer composition. In addition, we obtained 5.6% PDLA and 19% P(LA-3HB) of the cell dry weight (CDW), which are over two- and five-fold higher accumulation levels, respectively, than reported in the previous studies with yeast. We also compared two engineered PHA synthases and discovered that in S. cerevisiae the PHA synthase PhaC1437_Ps6-19 produced P(LA-3HB) copolymers with lower D-lactic acid content, but with higher molecular weight, in comparison to the PHA synthase PhaC1Pre.

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P(LA-3HB) monomer structure was adjusted with controlled gene expression.

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Expression of D-lactate dehydrogenase (ldhA) was controlled with Tet-On approach.

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Lactic acid content in copolymer P(LA-3HB) was adjusted from 6 mol% up to 93 mol%.

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5.6% PDLA and 19% P(LA-3HB) of cell dry weight (CDW) were obtained in S. cerevisiae.

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PhaC1437_Ps6-19 P(LA-3HB) had lower D-lactic acid % than PhaC1Pre P(LA-3HB).

Finding of novel lactate utilizing Bacillus sp. YHY22 and its evaluation for polyhydroxybutyrate (PHB) production

Polyhydroxyalkanoates (PHAs) and their derivatives are biopolymers that have the potential of replacing petroleum-based plastics and can be produced and degraded via bacterial metabolism. However, there are only a few studies on polyhydroxybutyrate (PHB) production using lactate, one of the major waste organic acids that could be implemented in the production of polylactic acid (PLA). Herein, we screened and characterized the PHA-producing microbial strains isolated from saltern soil from Docho Island (South Korea). Among the 24 identified microorganisms that can use lactate as a carbon source, Bacillus sp. YHY22, a newly reported strain, produced the highest amount of PHB: 4.05 g/L with 6.25 g/L dry cell weight, which is 64.7% PHB content under optimal production conditions. Bacillus sp. YHY22 could form the poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer with propionate addition. Moreover, Bacillus sp. YHY22 produced PHB in non-sterilized 2% lactate and 8% NaCl marine broth culture medium, suggesting that its production can occur in high salinity media without additional sterilization steps, rendering fermentation cost- and time-efficient.

Recent advances in the microbial synthesis of lactate-based copolymer

Due to the increasing environmental pollution of un-degradable plastics and the consumption of non-renewable resources, more attention has been attracted by new bio-degradable/based polymers produced from renewable resources. Polylactic acid (PLA) is one of the most representative bio-based materials, with obvious advantages and disadvantages, and has a wide range of applications in industry, medicine, and research. By copolymerizing to make up for its deficiencies, the obtained copolymers have more excellent properties. The development of a one-step microbial metabolism production process of the lactate (LA)-based copolymers overcomes the inherent shortcomings in the traditional chemical synthesis process. The most common lactate-based copolymer is poly(lactate-co-3-hydroxybutyrate) [P(LA-co-3HB)], within which the difference of LA monomer fraction will cause the change in the material properties. It is necessary to regulate LA monomer fraction by appropriate methods. Based on synthetic biology and systems metabolic engineering, this review mainly focus on how did the different production strategies (such as enzyme engineering, fermentation engineering, etc.) of P(LA-co-3HB) optimize the chassis cells to efficiently produce it. In addition, the metabolic engineering strategies of some other lactate-based copolymers are also introduced in this article. These studies would facilitate to expand the application fields of the corresponding materials.

Microbial Production of Biodegradable Lactate-Based Polymers and Oligomeric Building Blocks From Renewable and Waste Resources

Polyhydroxyalkanoates (PHAs) are naturally occurring biopolymers produced by microorganisms. PHAs have become attractive research biomaterials in the past few decades owing to their extensive potential industrial applications, especially as sustainable alternatives to the fossil fuel feedstock-derived products such as plastics. Among the biopolymers are the bioplastics and oligomers produced from the fermentation of renewable plant biomass. Bioplastics are intracellularly accumulated by microorganisms as carbon and energy reserves. The bioplastics, however, can also be produced through a biochemistry process that combines fermentative secretory production of monomers and/or oligomers and chemical synthesis to generate a repertoire of biopolymers. PHAs are particularly biodegradable and biocompatible, making them a part of today's commercial polymer industry. Their physicochemical properties that are similar to those of petrochemical-based plastics render them potential renewable plastic replacements. The design of efficient tractable processes using renewable biomass holds key to enhance their usage and adoption. In 2008, a lactate-polymerizing enzyme was developed to create new category of polyester, lactic acid (LA)-based polymer and related polymers. This review aims to introduce different strategies including metabolic and enzyme engineering to produce LA-based biopolymers and related oligomers that can act as precursors for catalytic synthesis of polylactic acid. As the cost of PHA production is prohibitive, the review emphasizes attempts to use the inexpensive plant biomass as substrates for LA-based polymer and oligomer production. Future prospects and challenges in LA-based polymer and oligomer production are also highlighted.

Biocatalytic synthesis of polylactate and its copolymers by engineered microorganisms

Poly(lactate), also called poly(lactic acid) or poly(lactide) [PLA], has been one of the most attractive bio-based polymers since it possesses desirable material properties for its use in general performance plastics in addition to biodegradability and biocompatibility. PLA has been produced by biological and chemical hybrid process comprising microbial fermentation for lactate (LA) production followed by purification and chemical polymerization process of LA. Recently, the direct one-step fermentative processes for production of PLA and several LA-containing polyesters have been developed by employing metabolically engineered microorganisms. Since natural microorganisms cannot produce the LA-containing polymers, several engineering strategies have been employed together based on the polyhydroxyalkanoate (PHA) biosynthesis system. In this chapter, we summarize strategies and procedures on developing the engineered microorganisms producing PLA and its copolymers, cultivating the cells, and extracting the polymers from the cells. Focuses were given on construction of enzymatic polymerization process of LA: design of metabolic pathway for PLA by mimicking PHA biosynthetic pathway, examination of possible enzymes, and engineering of the enzymes for better performances. This synthetic pathway has been established in a microorganism producing LA that enabled one-step fermentative production of LA-containing polyesters from carbohydrates derived from renewable biomass. Polymer production has been further enhanced by implementing strain engineering to concentrate the metabolic fluxes toward PLA formation. In addition, various monomers such as glycolate, 2-hydroxybutyrate, and phenyllactate have been copolymerized with LA by the microbial system. These fermentative production systems developed by using the engineered microorganisms can be versatile and sustainable platforms for the production of LA-containing polyesters and other non-natural polymers.

Screening, expression, purification and characterization of CoA-transferases for lactoyl-CoA generation

Lactoyl-CoA is critical for the biosynthesis of biodegradable and biocompatible lactate-based copolymers, which have wide applications. However, reports on acetyl-CoA: lactate CoA-transferases (ALCTs) are rare. To exploit novel ALCTs, amino acid sequence similarity searches based on the CoA-transferases from Clostridium propionicum and Megasphaera elsdenii were conducted. Two known and three novel enzymes were expressed, purified and characterized. Three novel ALCTs were identified, one each from Megasphaera sp. DISK 18, Clostridium lactatifermentans An75 and Firmicutes bacterium CAG: 466. ME-PCT from Megasphaera elsdenii had the highest catalytic efficiency for both acetyl-CoA (264.22 s^-1 mM^-1) and D-lactate (84.18 s^-1 mM^-1) with a broad temperature range for activity and good stability. This study, therefore, offers novel and efficient enzymes for lactoyl-CoA generation. To our best knowledge, this is the first report on the systematic mining of ALCTs, which offers valuable new tools for the engineering of pathways that rely on these enzymes.

Crystal structure and kinetic analyses of a hexameric form of (S)-3-hydroxybutyryl-CoA dehydrogenase from Clostridium acetobutylicum

(S)-3-Hydroxybutyryl-CoA dehydrogenase (HBD) has been gaining increased attention recently as it is a key enzyme in the enantiomeric formation of (S)-3-hydroxybutyryl-CoA [(S)-3HB-CoA]. It converts acetoacetyl-CoA to (S)-3HB-CoA in the synthetic metabolic pathway. (S)-3HB-CoA is further modified to form (S)-3-hydroxybutyrate, which is a source of biodegradable polymers. During the course of a study to develop biodegradable polymers, attempts were made to determine the crystal structure of HBD from Clostridium acetobutylicum (CacHBD), and the crystal structures of both apo and NAD⁺-bound forms of CacHBD were determined. The crystals belonged to different space groups: P2₁2₁2₁ and P2₁. However, both structures adopted a hexamer composed of three dimers in the asymmetric unit, and this oligomerization was additionally confirmed by gel-filtration column chromatography. Furthermore, to investigate the catalytic residues of CacHBD, the enzymatic activities of the wild type and of three single-amino-acid mutants were analyzed, in which the Ser, His and Asn residues that are conserved in the HBDs from C. acetobutylicum, C. butyricum and Ralstonia eutropha, as well as in the L-3-hydroxyacyl-CoA dehydrogenases from Homo sapiens and Escherichia coli, were substituted by alanines. The S117A and N188A mutants abolished the activity, while the H138A mutant showed a slightly lower K_m value and a significantly lower k_cat value than the wild type. Therefore, in combination with the crystal structures, it was shown that His138 is involved in catalysis and that Ser117 and Asn188 may be important for substrate recognition to place the keto group of the substrate in the correct position for reaction.

Microbial synthesis of a novel terpolyester P(LA-co-3HB-co-3HP) from low-cost substrates

Polylactide (PLA) is a bio-based plastic commonly synthesized by chemical catalytic reaction using lactic acid (LA) as a substrate. Here, novel LA-containing terpolyesters, namely, P[LA-co-3-hydroxybutyrate (3HB)-co-3-hydroxypropionate (3HP)], short as PLBP, were successfully synthesized for the first time by a recombinant Escherichia coli harbouring polyhydroxyalkanoate (PHA) synthase from Pseudomonas stutzeri (PhaC1_Ps ) with 4-point mutations at E130D, S325T, S477G and Q481K, and 3-hydroxypropionyl-CoA (3HP-CoA) synthesis pathway from glycerol, 3-hydroxybutyryl-CoA (3HB-CoA) as well as lactyl-CoA (LA-CoA) pathways from glucose. Combining these pathways with the PHA synthase mutant phaC1_Ps (E130D S325T S477G Q481K), the random terpolyester P(LA-co-3HB-co-3HP), or PLBP, was structurally confirmed by nuclear magnetic resonance to consist of 2 mol% LA, 90 mol% 3HB, and 8 mol% 3HP respectively. Remarkably, the PLBP terpolyester was produced from low-cost sustainable glycerol and glucose. Monomer ratios of PLBP could be regulated by ratios of glycerol to glucose. Other terpolyester thermal and mechanical properties can be manipulated by adjusting the monomer ratios. More PLBP applications are to be expected.

Consolidated bioprocessing of poly(lactate-co-3-hydroxybutyrate) from xylan as a sole feedstock by genetically-engineered Escherichia coli

Consolidated bioprocessing of lignocellulose is an attractive strategy for the sustainable production of petroleum-based alternatives. One of the underutilized sources of carbon in lignocellulose is the hemicellulosic fraction which largely consists of the polysaccharide xylan. In this study, Escherichia coli JW0885 (pyruvate formate lyase activator protein mutant, pflA(-)) was engineered to express recombinant xylanases and polyhydroxyalkanoate (PHA)-producing enzymes for the biosynthesis of poly(lactate-co-3-hydroxybutyrate) [P(LA-co-3HB)] from xylan as a consolidated bioprocess. The results show that E. coli JW0885 was capable of producing P(LA-co-3HB) when xylan was the only feedstock and different feeding and growth parameters were examined in order to improve upon initial yields. The highest yields of P(LA-co-3HB) copolymer obtained in this study occurred when xylan was added during mid-exponential growth after cells had been grown at high shaking-speeds (290 rpm). The results showed an inverse relationship between total PHA production and LA-monomer incorporation into the copolymer. Proton nuclear magnetic resonance ((1)H NMR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC) analyses corroborate that the polymers produced maintain physical properties characteristic of LA-incorporating PHB-based copolymers. The present study achieves the first ever engineering of a consolidated bioprocessing bacterial system for the production of a bioplastic from a hemicelluosic feedstock.

Advanced functionalization of polyhydroxyalkanoate via the UV-initiated thiol-ene click reaction

Polyhydroxyalkanoates (PHAs) incorporating vinyl-bearing 3-hydroxyalkanoates were prepared in 8.5-12.9 g L(-1) yield. The molar ratios (0-16 mol%) of the vinyl-bearing 3-hydroxyalkanoate derivatives were controlled by the continuous feeding of undecylenate at various concentrations. Subsequently, the PHAs were functionalized by UV-initiated thiol-ene click reaction and chemical modification. (1)H NMR spectra suggested that 3-mercaptopropionic acid and 2-aminoethanethiol were successfully introduced into the vinyl-bearing PHA. Subsequently, chemical modification using fluorescein or a fibronectin active fragment (GRGDS) was attempted. The former yielded a PHA derivative capable of emitting fluorescence under UV irradiation, which was useful for determining the miscibility of PHA in a composite film comprising poly-ʟ-lactic acid (PLLA) and PHA. In the latter case, PHA bearing GRGDS peptides exhibited cell adhesiveness, suggesting that its biocompatibility was improved upon peptide introduction. Taken together, the UV-initiated thiol-ene click reaction was demonstrated to be useful in PHA modification.

Indirect positive effects of a sigma factor RpoN deletion on the lactate-based polymer production in Escherichia coli

The production of bacterial polyesters, polyhydroxyalkanoates (PHAs), has been improved by several rational approaches such as overexpression and/or engineering of the enzymes directly related to PHA biosynthetic pathways. In this study, a new approach at transcription level has been applied to a new category of the copolymer of lactate (LA) and 3-hydroxybutyrate (3HB), P(LA-co-3HB). When the 4 disrupting mutants of sigma factors in Escherichia coli, rpoN, rpoS, fliA, fecI, were used as platforms for production of P(LA-co-3HB), increases in the production level and LA fraction of the copolymer were observed for the mutant strain with rpoN disruption. These positive impacts on the polymer production were caused in an "indirect manner" via changes in the multiple genes governed by RpoN. A genome-wide engineering by sigma factors would be a versatile approach for the production of value-added products of interest and available for combination with the other beneficial tools.

MtgA Deletion-Triggered Cell Enlargement of Escherichia coli for Enhanced Intracellular Polyester Accumulation

Bacterial polyester polyhydroxyalkanoates (PHAs) have been produced in engineered Escherichia coli, which turned into an efficient and versatile platform by applying metabolic and enzyme engineering approaches. The present study aimed at drawing out the latent potential of this organism using genome-wide mutagenesis. To meet this goal, a transposon-based mutagenesis was carried out on E. coli, which was transformed to produce poly(lactate-co-3-hydroxybutyrate) from glucose. A high-throughput screening of polymer-accumulating cells on Nile red-containing plates isolated one mutant that produced 1.8-fold higher quantity of polymer without severe disadvantages in the cell growth and monomer composition of the polymer. The transposon was inserted into the locus within the gene encoding MtgA that takes part, as a non-lethal component, in the formation of the peptidoglycan backbone. Accordingly, the mtgA-deleted strain E. coli JW3175, which was a derivate of superior PHA-producing strain BW25113, was examined for polymer production, and exhibited an enhanced accumulation of the polymer (7.0 g/l) compared to the control (5.2 g/l). Interestingly, an enlargement in cell width associated with polymer accumulation was observed in this strain, resulting in a 1.6-fold greater polymer accumulation per cell compared to the control. This result suggests that the increase in volumetric capacity for accumulating intracellular material contributed to the enhanced polymer production. The mtgA deletion should be combined with conventional engineering approaches, and thus, is a promising strategy for improved production of intracellularly accumulated biopolymers.

Development of rice bran treatment process and its use for the synthesis of polyhydroxyalkanoates from rice bran hydrolysate solution

Rice bran treatment process for the production of 43.7 kg of hydrolysate solution containing 24.41 g/L of glucose and small amount of fructose from 5 kg of rice bran was developed and employed to produce polyhydroxyalkanoates in recombinant Escherichia coli and Ralstonia eutropha strains. Recombinant E. coli XL1-Blue expressing R. eutropha phaCAB genes and R. eutropha NCIMB11599 could produce poly(3-hydroxybutyrate) with the polymer contents of 90.1 wt% and 97.2 wt%, respectively, when they were cultured in chemically defined MR medium and chemically defined nitrogen free MR medium containing 10 mL/L of rice bran hydrolysate solution, respectively. Also, recombinant E. coli XL1-Blue and recombinant R. eutropha 437-540, both of which express the Pseudomonas sp. phaC1437 gene and the Clostridium propionicum pct540 gene could produce poly(3-hydroxybutyrate-co-lactate) from rice bran hydrolysate solution. These results suggest that rice bran may be a good renewable resource for the production of biomass-based polymers by recombinant microorganisms.

Enhanced production of poly(lactate-co-3-hydroxybutyrate) from xylose in engineered Escherichia coli overexpressing a galactitol transporter

Poly(lactate-co-3-hydroxybutyrate) (P(LA-co-3HB)) was previously produced from xylose in engineered Escherichia coli. The aim of this study was to increase the polymer productivity and LA fraction in P(LA-co-3HB) using two metabolic engineering approaches: (1) deletions of competing pathways to lactate production and (2) overexpression of a galactitol transporter (GatC), which contributes to the ATP-independent xylose uptake. Engineered E. coli mutants (ΔpflA, Δpta, ΔackA, ΔpoxB, Δdld, and a dual mutant; ΔpflA + Δdld) and their parent strain, BW25113, were grown on 20 g l(-1) xylose for P(LA-co-3HB) production. The single deletions of ΔpflA, Δpta, and Δdld increased the LA fraction (58-66 mol%) compared to BW25113 (56 mol%). In particular, the ΔpflA + Δdld strain produced P(LA-co-3HB) containing 73 mol% LA. Furthermore, GatC overexpression increased both polymer yields and LA fractions in ΔpflA, Δpta, and Δdld mutants, and BW25113. The ΔpflA + gatC strain achieved a productivity of 8.3 g l(-1), which was 72 % of the theoretical maximum yield. Thus, to eliminate limitation of the carbon source, higher concentration of xylose was fed. As a result, BW25113 harboring gatC grown on 40 g l(-1) xylose reached the highest P(LA-co-3HB) productivity of 14.4 g l(-1). On the other hand, the ΔpflA + Δdld strain grown on 30 g l(-1) xylose synthesized 6.4 g l(-1) P(LA-co-3HB) while maintaining the highest LA fraction (73 mol%). The results indicated the usefulness of GatC for enhanced production of P(LA-co-3HB) from xylose, and the gene deletions to upregulate the LA fraction in P(LA-co-3HB). The polymers obtained had weight-averaged molecular weights in the range of 34,000-114,000.

Microbial production of lactate-containing polyesters

Due to our increasing concerns on environmental problems and limited fossil resources, biobased production of chemicals and materials through biorefinery has been attracting much attention. Optimization of the metabolic performance of microorganisms, the key biocatalysts for the efficient production of the desired target bioproducts, has been achieved by metabolic engineering. Metabolic engineering allowed more efficient production of polyhydroxyalkanoates, a family of microbial polyesters. More recently, non-natural polyesters containing lactate as a monomer have also been produced by one-step fermentation of engineered bacteria. Systems metabolic engineering integrating traditional metabolic engineering with systems biology, synthetic biology, protein/enzyme engineering through directed evolution and structural design, and evolutionary engineering, enabled microorganisms to efficiently produce natural and non-natural products. Here, we review the strategies for the metabolic engineering of microorganisms for the in vivo biosynthesis of lactate-containing polyesters and for the optimization of whole cell metabolism to efficiently produce lactate-containing polyesters. Also, major problems to be solved to further enhance the production of lactate-containing polyesters are discussed.

Protein design in systems metabolic engineering for industrial strain development

Accelerating the process of industrial bacterial host strain development, aimed at increasing productivity, generating new bio-products or utilizing alternative feedstocks, requires the integration of complementary approaches to manipulate cellular metabolism and regulatory networks. Systems metabolic engineering extends the concept of classical metabolic engineering to the systems level by incorporating the techniques used in systems biology and synthetic biology, and offers a framework for the development of the next generation of industrial strains. As one of the most useful tools of systems metabolic engineering, protein design allows us to design and optimize cellular metabolism at a molecular level. Here, we review the current strategies of protein design for engineering cellular synthetic pathways, metabolic control systems and signaling pathways, and highlight the challenges of this subfield within the context of systems metabolic engineering.

Enzyme and metabolic engineering for the production of novel biopolymers: crossover of biological and chemical processes

The development of synthetic biology has transformed microbes into useful factories for producing valuable polymers and/or their precursors from renewable biomass. Recent progress at the interface of chemistry and biology has enabled the production of a variety of new biopolymers with properties that substantially differ from their petroleum-derived counterparts. This review touches on recent trials and achievements in the field of biopolymer synthesis, including chemo-enzymatically synthesized aliphatic polyesters, wholly biosynthesized lactate-based polyesters, polyhydroxyalkanoates and other unusual bacterially synthesized polyesters. The expanding diversities in structure and the material properties of biopolymers are key for exploring practical applications. The enzyme and metabolic engineering approaches toward this goal are discussed by shedding light on the successful case studies.

A propionate CoA-transferase of Ralstonia eutropha H16 with broad substrate specificity catalyzing the CoA thioester formation of various carboxylic acids

In this study, we have investigated a propionate CoA-transferase (Pct) homologue encoded in the genome of Ralstonia eutropha H16. The corresponding gene has been cloned into the vector pET-19b to yield a histidine-tagged enzyme which was expressed in Escherichia coli BL21 (DE3). After purification, high-performance liquid chromatography/mass spectrometry (HPLC/MS) analyses revealed that the enzyme exhibits a broad substrate specificity for carboxylic acids. The formation of the corresponding CoA-thioesters of acetate using propionyl-CoA as CoA donor, and of propionate, butyrate, 3-hydroxybutyrate, 3-hydroxypropionate, crotonate, acrylate, lactate, succinate and 4-hydroxybutyrate using acetyl-CoA as CoA donor could be shown. According to the substrate specificity, the enzyme can be allocated in the family I of CoA-transferases. The apparent molecular masses as determined by gel filtration and detected by SDS polyacrylamide gel electrophoresis were 228 and 64 kDa, respectively, and point to a quaternary structure of the native enzyme (α4). The enzyme exhibited similarities in sequence and structure to the well investigated Pct of Clostridium propionicum. It does not contain the typical conserved (S)ENG motif, but the derived motif sequence EXG with glutamate 342 to be, most likely, the catalytic residue. Due to the homo-oligomeric structure and the sequence differences with the subclasses IA-C of family I CoA-transferases, a fourth subclass of family I is proposed, comprising - amongst others - the Pcts of R. eutropha H16 and C. propionicum. A markerless precise-deletion mutant R. eutropha H16∆pct was generated. The growth and accumulation behaviour of this mutant on gluconate, gluconate plus 3,3'-dithiodipropionic acid (DTDP), acetate and propionate was investigated but resulted in no observable phenotype. Both, the wild type and the mutant showed the same growth and storage behaviour with these carbon sources. It is probable that R. eutropha H16 is upregulating other CoA-transferase(s) or CoA-synthetase(s), thereby compensating for the lacking Pct. The ability of R. eutropha H16 to substitute absent enzymes by isoenzymes has been already shown in different other studies in the past.

Engineering of class I lactate-polymerizing polyhydroxyalkanoate synthases from Ralstonia eutropha that synthesize lactate-based polyester with a block nature

Class I polyhydroxyalkanoate (PHA) synthase from Ralstonia eutropha (PhaCRe) was engineered so as to acquire an unusual lactate (LA)-polymerizing activity. To achieve this, the site-directed saturation mutagenesis of PhaCRe was conducted at position 510, which corresponds to position 481 in the initially discovered class II LA-polymerizing PHA synthase (PhaC1PsSTQK), a mutation in which (Gln481Lys) was shown to be essential to its LA-polymerizing activity (Taguchi et al., Proc Natl Acad Sci USA 105(45):17323-17327, 2008). The LA-polymerizing activity of the PhaCReA510X mutants was evaluated based on the incorporation of LA units into the P[3-hydroxybutyrate(3HB)] backbone in vivo using recombinant Escherichia coli LS5218. Among 19 PhaCRe(A510X) mutants, 15 synthesized P (LA-co-3HB), indicating that the 510 residue plays a critical role in LA polymerization. The polymer synthesized by PhaCReA510S was fractionated using gel permeation chromatography in order to remove the low molecular weight fractions. The (13)C and (1)H NMR analyses of the high molecular weight fraction revealed that the polymer was a P(7 mol% LA-co-3HB) copolymer with a weight-averaged molecular weight of 3.2 × 10(5) Da. Interestingly, the polymer contained an unexpectedly high ratio of an LA-LA -LA triad sequence, suggesting that the polymer synthesized by PhaCRe mutant may not be a random copolymer, but presumably had a block sequence.

Engineered Corynebacterium glutamicum as an endotoxin-free platform strain for lactate-based polyester production

The first biosynthetic system for lactate (LA)-based polyesters was previously created in recombinant Escherichia coli (Taguchi et al. 2008). Here, we have begun efforts to upgrade the prototype polymer production system to a practical stage by using metabolically engineered Gram-positive bacterium Corynebacterium glutamicum as an endotoxin-free platform. We designed metabolic pathways in C. glutamicum to generate monomer substrates, lactyl-CoA (LA-CoA), and 3-hydroxybutyryl-CoA (3HB-CoA), for the copolymerization catalyzed by the LA-polymerizing enzyme (LPE). LA-CoA was synthesized by D: -lactate dehydrogenase and propionyl-CoA transferase, while 3HB-CoA was supplied by β-ketothiolase (PhaA) and NADPH-dependent acetoacetyl-CoA reductase (PhaB). The functional expression of these enzymes led to a production of P(LA-co-3HB) with high LA fractions (96.8 mol%). The omission of PhaA and PhaB from this pathway led to a further increase in LA fraction up to 99.3 mol%. The newly engineered C. glutamicum potentially serves as a food-grade and biomedically applicable platform for the production of poly(lactic acid)-like polyester.

Biosynthesis of lactate-containing polyesters by metabolically engineered bacteria

Due to increasing concerns about environmental problems, climate change and limited fossil resources, bio-based production of chemicals and polymers is gaining attention as one of the solutions to these problems. Polyhydroxyalkanoates (PHAs) are polyesters that can be produced by microbial fermentation. PHAs are synthesized using monomer precursors provided from diverse metabolic pathways and are accumulated as distinct granules inside the cells. On the other hand, most so-called bio-based polymers including polybutylene succinate, polytrimethylene terephthalate, and polylactic acid (PLA) are synthesized by a chemical process using monomers produced by fermentation. PLA, an attractive biomass-derived plastic, is currently synthesized by heavy metal-catalyzed ring opening polymerization of L-lactide that is made from fermentation-derived L-lactic acid. Recently, a complete biological process for the production of PLA and PLA copolymers from renewable resources has been developed by direct fermentation of recombinant bacteria employing PHA biosynthetic pathways coupled with a novel metabolic pathway. This could be accomplished by establishing a pathway for generating lactyl-CoA and engineering PHA synthase to accept lactyl-CoA as a substrate combined with systems metabolic engineering. In this article, we review recent advances in the production of lactate-containing homo- and co-polyesters. Challenges remaining to efficiently produce PLA and its copolymers and strategies to overcome these challenges through metabolic engineering combined with enzyme engineering are discussed.

Chemo-enzymatic synthesis of polyhydroxyalkanoate (PHA) incorporating 2-hydroxybutyrate by wild-type class I PHA synthase from Ralstonia eutropha

A previously established improved two-phase reaction system has been applied to analyze the substrate specificities and polymerization activities of polyhydroxyalkanoate (PHA) synthases. We first analyzed the substrate specificity of propionate coenzyme A (CoA) transferase and found that 2-hydroxybutyrate (2HB) was converted into its CoA derivative. Then, the synthesis of PHA incorporating 2HB was achieved by a wild-type class I PHA synthase from Ralstonia eutropha. The PHA synthase stereoselectively polymerized (R)-2HB, and the maximal molar ratio of 2HB in the polymer was 9 mol%. The yields and the molecular weights of the products were decreased with the increase of the (R)-2HB concentration in the reaction mixture. The weight-average molecular weight of the polymer incorporating 9 mol% 2HB was 1.00 × 10(5), and a unimodal peak with polydispersity of 3.1 was observed in the GPC chart. Thermal properties of the polymer incorporating 9 mol% 2HB were analyzed by DSC and TG-DTA. T (g), T (m), and T (d) (10%) were observed at -1.1°C, 158.8°C, and 252.7°C, respectively. In general, major components of PHAs are 3-hydroxyalkanoates, and only engineered class II PHA synthases have been reported as enzymes having the ability to polymerize HA with the hydroxyl group at C2 position. Thus, this is the first report to demonstrate that wild-type class I PHA synthase was able to polymerize 2HB.

Lactate fraction dependent mechanical properties of semitransparent poly(lactate-co-3-hydroxybutyrate)s produced by control of lactyl-CoA monomer fluxes in recombinant Escherichia coli

In order to evaluate the mechanical properties of poly(lactate-co-3-hydroxybutyrate) [P(LA-co-3HB)] and its correlation with the LA fraction, P(LA-co-3HB)s with a variety of LA fractions were prepared using recombinant Escherichia coli expressing the LA-polymerizing enzyme and monomer supplying enzymes. The LA-overproducing mutant E. coli JW0885 with a pflA gene disruption was used for the LA-enriched polymer production. The LA fraction was also varied by jar-fermentor based fine-regulation of the anaerobic status of the culture conditions, resulting in LA fractions ranging from 4 to 47 mol%. In contrary to the opaque P(3HB) film, the copolymer films attained semitransparency depending on the LA fraction. Young's modulus values of the P(LA-co-3HB)s (from 148 to 905 MPa) were lower than those of poly(lactic acid) (PLA) (1020 MPa) and P(3HB) (1079 MPa). In addition, the value of elongation at break of the copolymer with 29 mol% LA reached 150%. In conclusion, P(LA-co-3HB)s were found to be a comparatively pliable and flexible material, differing from both of the rigid homopolymers.