In vitro synthesis of poly(3-hydroxybutyric acid) by using an enzymatic coenzyme A recycling system

Coenzyme A Thioester Intermediates as Platform Molecules in Cell-Free Chemical Biomanufacturing

The in vitro synthesis of Coenzyme A (CoA)-thioester intermediates opens new avenues to transform simple molecules into more complex and multifunctional ones by assembling cell-free biosynthetic cascades. In this review, we have systematically cataloged known CoA-dependent enzyme reactions that have been successfully implemented in vitro. To faciliate their identification, we provide their UniProt ID when available. Based on this catalog, we have organized enzymes into three modules: activation, modification, and removal. i) The activation module includes enzymes capable of fusing CoA with organic molecules. ii) The modification module includes enzymes capable of catalyzing chemical modifications in the structure of acyl-CoA intermediates. And iii) the removal module includes enzymes able to remove the CoA and release an organic molecule different from the one activated in the upstream. Based on these reactions, we constructed a reaction network that summarizes the most relevant CoA-dependent biosynthetic pathways reported until today. From the information available in the articles, we have plotted the total turnover number of CoA as a function of the product titer, observing a positive correlation between both parameters. Therefore, the success of a CoA-dependent in vitro pathway depends on its ability to regenerate CoA, but also to regenerate other cofactors such as NAD(P)H and ATP.

The PHB Granule Biogenesis Pathway in Caulobacter

PHB granules are bacterial organelles that store excess carbohydrates in the form of water-insoluble polyhydroxybutyrate (PHB). The PHB polymerase, phasin (a small amphipathic protein), and active PHB synthesis are essential for the formation of mature PHB granules in Caulobacter crescentus. Granule formation was found to be initiated by the condensation of self-associating PHB polymerase-GFP into foci, closely followed by the recruitment and condensation of phasin-mCherry. Following the active synthesis of PHB and granule maturation, the polymerase dissociates from mature granules and the PHB depolymerase is recruited to the granule. The polymerase directly binds phasin in vitro through its intrinsically disordered N-terminal domain. Thus, granule biogenesis is initiated and controlled by the action of a PHB polymerase and an associated helper protein, phasin, that together synthesize the hydrophobic granule's content while forming the granules protein boundary.

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

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

ATP-Independent and Cell-Free Biosynthesis of β-Hydroxy Acids Using Vinyl Esters as Smart Substrates

In vitro biosynthetic pathways that condense and reduce molecules through coenzyme A (CoASH) activation demand energy and redox power in the form of ATP and NAD(P)H, respectively. These coenzymes must be orthogonally recycled by ancillary reactions that consume chemicals, electricity, or light, impacting the atom economy and/or the energy consumption of the biosystem. In this work, we have exploited vinyl esters as dual acyl and electron donor substrates to synthesize β-hydroxy acids through a non-decarboxylating Claisen condensation, reduction and hydrolysis stepwise cascade, including a NADH recycling step, catalyzed by a total of 4 enzymes. Herein, the chemical energy to activate the acyl group with CoASH and the redox power for the reduction are embedded into the vinyl esters. Upon optimization, this self-sustaining cascade reached a titer of (S)-3-hydroxy butyrate of 24 mM without requiring ATP and simultaneously recycling CoASH and NADH. This work illustrates the potential of in vitro biocatalysis to transform simple molecules into multi-functional ones.

Prospects of Using Biocatalysis for the Synthesis and Modification of Polymers

Trends in the dynamically developing application of biocatalysis for the synthesis and modification of polymers over the past 5 years are considered, with an emphasis on the production of biodegradable, biocompatible and functional polymeric materials oriented to medical applications. The possibilities of using enzymes not only as catalysts for polymerization but also for the preparation of monomers for polymerization or oligomers for block copolymerization are considered. Special attention is paid to the prospects and existing limitations of biocatalytic production of new synthetic biopolymers based on natural compounds and monomers from biomass, which can lead to a huge variety of functional biomaterials. The existing experience and perspectives for the integration of bio- and chemocatalysis in this area are discussed.

Round, round we go - strategies for enzymatic cofactor regeneration

Covering: up to the beginning of 2020Enzymes depending on cofactors are essential in many biosynthetic pathways of natural products. They are often involved in key steps: catalytic conversions that are difficult to achieve purely with synthetic organic chemistry. Hence, cofactor-dependent enzymes have great potential for biocatalysis, on the condition that a corresponding cofactor regeneration system is available. For some cofactors, these regeneration systems require multiple steps; such complex enzyme cascades/multi-enzyme systems are (still) challenging for in vitro biocatalysis. Further, artificial cofactor analogues have been synthesised that are more stable, show an altered reaction range, or act as inhibitors. The development of bio-orthogonal systems that can be used for the production of modified natural products in vivo is an ongoing challenge. In light of the recent progress in this field, this review aims to provide an overview of general strategies involving enzyme cofactors, cofactor analogues, and regeneration systems; highlighting the current possibilities for application of enzymes using some of the most common cofactors.

Beyond Intracellular Accumulation of Polyhydroxyalkanoates: Chiral Hydroxyalkanoic Acids and Polymer Secretion

Polyhydroxyalkanoates (PHAs) are ubiquitous prokaryotic storage compounds of carbon and energy, acting as sinks for reducing power during periods of surplus of carbon source relative to other nutrients. With close to 150 different hydroxyalkanoate monomers identified, the structure and properties of these polyesters can be adjusted to serve applications ranging from food packaging to biomedical uses. Despite its versatility and the intensive research in the area over the last three decades, the market share of PHAs is still low. While considerable rich literature has accumulated concerning biochemical, physiological, and genetic aspects of PHAs intracellular accumulation, the costs of substrates and processing costs, including the extraction of the polymer accumulated in intracellular granules, still hampers a more widespread use of this family of polymers. This review presents a comprehensive survey and critical analysis of the process engineering and metabolic engineering strategies reported in literature aimed at the production of chiral (R)-hydroxycarboxylic acids (RHAs), either from the accumulated polymer or by bypassing the accumulation of PHAs using metabolically engineered bacteria, and the strategies developed to recover the accumulated polymer without using conventional downstream separations processes. Each of these topics, that have received less attention compared to PHAs accumulation, could potentially improve the economy of PHAs production and use. (R)-hydroxycarboxylic acids can be used as chiral precursors, thanks to its easily modifiable functional groups, and can be either produced de-novo or be obtained from recycled PHA products. On the other hand, efficient mechanisms of PHAs release from bacterial cells, including controlled cell lysis and PHA excretion, could reduce downstream costs and simplify the polymer recovery process.

In vitro synthesis of polyhydroxyalkanoates using thermostable acetyl-CoA synthetase, CoA transferase, and PHA synthase from thermotorelant bacteria

Thermostable enzymes are required for the rapid and sustainable production of polyhydroxyalkanoate (PHA) in vitro. The in vitro synthesis of PHA using the engineered thermostable synthase PhaC1_SG(STQK) has been reported; however, the non-thermostable enzymes acetyl-CoA synthetase (ACS) and CoA transferase (CT) from mesophilic strains were used as monomer-supplying enzymes in this system. In the present study, acs and ct were cloned from the thermophilic bacteria Pelotomaculum thermopropionicum JCM10971 and Thermus thermophilus JCM10941 to construct an in vitro PHA synthesis system using only thermostable enzymes. ACS from P. thermopropionicum (ACS_Pt) and CT from T. thermophilus (CT_Tt) were confirmed to have high thermostability, and their optimal temperatures were around 60°C and 75°C, respectively. The in vitro PHA synthesis was successfully performed by ACS_Pt, CT_Tt, PhaC1_SG(STQK), and poly(3-hydroxybutyrate) [P(3HB)] was synthesized at 45°C. Furthermore, the yields of P(3HB) and P(lactate-co-3HB) at 37°C were 1.4-fold higher than those of the in vitro synthesis system with non-thermostable ACS and CT from mesophilic strains. Overall, the thermostable ACS and CT were demonstrated to be useful for the efficient in vitro PHA synthesis at relatively high temperatures.

Quantitative target analysis and kinetic profiling of acyl-CoAs reveal the rate-limiting step in cyanobacterial 1-butanol production

Cyanobacterial 1-butanol production is an important model system for direct conversion of CO₂ to fuels and chemicals. Metabolically-engineered cyanobacteria introduced with a heterologous Coenzyme A (CoA)-dependent pathway modified from Clostridium species can convert atmospheric CO₂ into 1-butanol. Efforts to optimize the 1-butanol pathway in Synechococcus elongatus PCC 7942 have focused on the improvement of the CoA-dependent pathway thus, probing the in vivo metabolic state of the CoA-dependent pathway is essential for identifying its limiting steps. In this study, we performed quantitative target analysis and kinetic profiling of acyl-CoAs in the CoA-dependent pathway by reversed phase ion-pair liquid chromatography-triple quadrupole mass spectrometry. Using ¹³C-labelled cyanobacterial cell extract as internal standard, measurement of the intracellular concentration of acyl-CoAs revealed that the reductive reaction of butanoyl-CoA to butanal is a possible rate-limiting step. In addition, improvement of the butanoyl-CoA to butanal reaction resulted in an increased rate of acetyl-CoA synthesis by possibly compensating for the limitation of free CoA species. We inferred that the efficient recycling of free CoA played a key role in enhancing the conversion of pyruvate to acetyl-CoA.

Influence of the operon structure on poly(3-hydroxypropionate) synthesis in Shimwellia blattae

Glycerol has become a cheap and abundant carbon source due to biodiesel production at a large scale, and it is available for several biotechnological applications. We recently established poly(3-hydroxypropionate) [poly(3HP)] synthesis in a recombinant Shimwellia blattae strain (Heinrich et al. Appl Environ Microbiol 79:3582-3589, 2013). The major drawbacks of the current strains are (i) low poly(3HP) yields, (ii) low plasmid stability and (iii) insufficient conversion rates. In this study, we demonstrated the influence of alterations of the operon structure, consisting of 1,3-propanediol dehydrogenase (dhaT) and aldehyde dehydrogenase (aldD) of Pseudomonas putida KT2442, propionate:coenzyme A (propionate-CoA) transferase (pct) of Clostridium propionicum X2 and polyhydroxyalkanoate (PHA) synthase (phaC1) of Ralstonia eutropha H16. It was shown that S. blattae ATCC33430/pBBR1MCS-2::dhaT::pct::aldD::phaC1 synthesized up to 14.5 % (wtPHA/wtCDW) in a 2-L fed-batch fermentation process. Furthermore, we overcame the problem of plasmid losses during the fermentation period by engineering a carbon source-dependent plasmid addiction system in a triose phosphate isomerase knockout mutant. An assumed poly(3-hydroxyalkanoic acid) degrading activity of the lipase/esterase YbfF could not be confirmed.

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.

Insight in the role of bovine serum albumin for promoting the in situ surface growth of polyhydroxybutyrate (PHB) on patterned surfaces via enzymatic surface-initiated polymerization

Polyhydroxyalkanoates (PHAs) are a family of aliphatic polyesters produced by a variety of microorganisms as a reserve of carbon and energy. Enzymes involved in the synthesis of PHAs can be utilized to produce polymers in vitro, both in bulk and on solid surfaces. Here, site-specific attachment of the key catalytic enzyme, PHA synthase, on lithographically patterned surfaces and subsequent addition of (R)-3-hydroxybutyryl-CoA substrate allowed us to fabricate spatially ordered polyhydroxybutyrate (PHB) polymeric structures via an in situ enzymatic surface-initiated polymerization (ESIP). By varying the reaction conditions, we enhanced the growth of PHB on solid surfaces and analyzed the resulting structures by fluorescence microscopy, atomic force microscopy (AFM), attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and gel permeation chromatography (GPC). We found that stabilization of smaller PHB granule structures by an addition of bovine serum albumin (BSA) was the most important factor for a successful synthesis of a PHB layer up to 1mum in thickness, consisting mainly of larger cluster assemblies of PHB granules that cover the entire patterned area. Immunofluorescence detection and surface contact angle analysis revealed that BSA was physically bound to the PHB polymer all through the cluster, and reduced the overall hydrophobicity of the polymer surface. Based on information obtained from AFM, kinetic measurements and various polymer characterization methods, a plausible model for roles of BSA in the enhancement of PHB formation on surfaces is discussed. Furthermore, by using biotinylated BSA conjugates, we were able to incorporate biotin groups into the PHB polymer matrix, thus generating a bioactive surface that can be used for displaying other functional biomolecules through streptavidin-biotin interaction on the PHB structures. Because of its versatility, our fabrication strategy is expected to be a useful surface modification tool for numerous biomedical and biotechnological applications.

Real-Time Analysis of Enzymatic Surface-Initiated Polymerization Using Surface Plasmon Resonance (SPR)

The kinetics of enzymatic surface-initiated polymerization of PHB on gold surface has been examined by SPR and the resultant polymer layers characterized by AFM and FT-IR spectrometry. The immobilized enzyme catalyzed surface-initiated polymerization of 3HB-CoA, resulting in the formation of a polymer brush on the surface. The rate of polymer growth from the surface was monitored by SPR in real-time. Polymer growth as measured by the increase in the resonance angle showed no apparent lag phase during the polymerization reaction. SPR analysis also revealed that the thickness of the polymer film could be controlled by varying the initial enzyme density on the surface. The average thicknesses of the PHB film after polymerization reaction were 95, 45 and 15 nm for the surfaces that were treated with 0.5, 0.3 and 0.1*10(-6) M of enzyme, respectively. The binding of PHA synthase at different concentration to the mixed SAMs and subsequent polymerization.

Genetics and Biochemistry of Polyhydroxyalkanoate Granule Self-assembly: The Key Role of Polyester Synthases

PHAs (polyhydroxyalkanoates = biopolyester) composed of hydroxy fatty acids represent a rather complex class of storage polymers synthesized by various bacteria and archaea and are deposited as water-insoluble cytoplasmic nano-sized inclusions. These spherical particles are composed of a polyester core surrounded by phospholipids and proteins. The key enzymes of polyester biosynthesis and polyester particle formation are the polyester synthases, which catalyze the formation of polyesters. Various metabolic routes have been identified and established in bacteria to provide substrate for polyester synthases. Although not essential for particle formation, non-covalently attached proteins, the so-called phasins, can be found at the particle surface and are considered as structural proteins. Protein engineering of polyester synthases and phasins was used to shed light into the topology of these granule attached proteins. Biopolyesters and the respective micro-/nano-structures are currently considered as biocompatible and biodegradable biomaterials with numerous potential applications particularly in the medical field.

Enzymatic synthesis of poly(3-hydroxybutyrate-co-4-hydroxybutyrate) with CoA recycling using polyhydroxyalkanoate synthase and acyl-CoA synthetase

We succeeded in developing a novel method for in vitro poly(3-hydroxybutyrate-co-4-hydroxybutyrate) [P(3 HB-co-4 HB)] synthesis with CoA recycling using polyhydroxyalkanoate synthase and an acyl-CoA synthetase. Using this method, the monomer compositions in P(3 HB-co-4 HB)s could be controlled strictly by the ratios of the monomers in the reaction mixtures.

Enzyme-catalyzed poly(3-hydroxybutyrate) synthesis from acetate with CoA recycling and NADPH regeneration in Vitro

We established a novel enzyme-catalyzed poly(3-hydroxybutyrate) [P(3HB)] synthesis system capable of recycling CoA on the basis of the P(3HB) biosynthetic pathway in Ralstonia eutropha. The system includes purified beta-ketothiolase (PhaA), NADPH-dependent acetoacetyl-CoA reductase (PhaB), PHA synthase (PhaC), acetyl-CoA synthetase (Acs) and glucose dehydrogenase (GDH). In this system, acetyl-CoA was synthesized from acetate and CoA by Acs and ATP, and then two molecules of acetyl-CoA were condensed by PhaA to synthesize acetoacetyl-CoA, which was converted to (R)-3-hydroxybutyryl-CoA (3HBCoA) by PhaB and NADPH. The 3HBCoA was polymerized by PhaC and converted to P(3HB). In this system, the CoA molecules that were released during the condensation and polymerization reactions catalyzed by PhaA and PhaC, respectively, were reused successfully for the synthesis of acetyl-CoA. In addition, NADPH, which was consumed in the reduction of acetoacetyl-CoA, was regenerated by the action of GDH. In this system, the yield of P(3HB) synthesized from acetate as the substrate was 5.6 mg in a 5-ml reaction mixture, and the weight-average molecular weight and polydispersity were 6.64 x 10(6) and 1.36, respectively. Furthermore, CoA was reused at least 26 times, and NADPH was also regenerated at least 26 times during 24 h of reaction.

Poly(3-hydroxybutyrate) granule-associated proteins: impacts on poly(3-hydroxybutyrate) synthesis and degradation

Polyhydroxyalkanoates (PHAs) represent a group of biopolymers that are synthesized by many bacteria as storage compounds and deposited as insoluble cytoplasmic inclusions. Because they have many putative technical and medical applications, PHAs may play an important role in human life in the future. Therefore, for academic interest the bacterial PHA metabolism has been studied in much detail. In the past decade much new and unexpected information about the metabolism of PHA in bacteria became available. Aspects of the biogenesis of PHA granules in bacteria become more and more important in the literature. Several enzymes, proteins, and mechanisms of regulation are involved in PHA biosynthesis and PHA granule biogenesis. The intention of this review is to give an overview about our current knowledge of the structure of the PHA granule surface and the PHA granule-associated proteins involved in biogenesis and degradation. The focus is on the PHA synthases, the intracellular PHA depolymerases, the phasins, and the transcriptional regulator PhaR, which are the main actors in biosynthesis and intracellular degradation of PHAs and formation of PHA granules. In addition, putative applications of PHA granules and PHA granule-associated proteins in nanotechnology are discussed.

Enzymatic Surface-Initiated Polymerization: A Novel Approach for the in Situ Solid-Phase Synthesis of Biocompatible Polymer Poly(3-hydroxybutyrate)

A novel system for surface-initiated enzymatic polymerization of a film of polyhydroxyalkanoate (PHA) on solid surfaces has been developed and characterized. PHAs are aliphatic polyesters produced by a variety of microorganisms as a reserve of carbon and energy, and their properties range from elastomers to thermoplastics, depending on their monomeric composition. The PHA synthase from Ralstonia eutropha H16 was expressed as a poly-histidine fusion in Escherichia coli and immobilized onto several solid substrates through a transition-metal complex, Ni(2+)-nitrilotriacetic acid. The immobilized PHA synthase catalyzed the surface-initiated polymerization of 3-(R)-hydroxybutyryl-CoA, forming a polymer film with a uniform thickness on the surface. In this work, we describe the patterned immobilization of the intact enzyme on silicon and subsequent enzymatic polymerization. The immobilized enzyme had a lower specific activity and did not exhibit a lag phase as compared to the soluble enzyme.

Polyester synthases: natural catalysts for plastics

Polyhydroxyalkanoates (PHAs) are biopolyesters composed of hydroxy fatty acids, which represent a complex class of storage polyesters. They are synthesized by a wide range of different Gram-positive and Gram-negative bacteria, as well as by some Archaea, and are deposited as insoluble cytoplasmic inclusions. Polyester synthases are the key enzymes of polyester biosynthesis and catalyse the conversion of (R)-hydroxyacyl-CoA thioesters to polyesters with the concomitant release of CoA. These soluble enzymes turn into amphipathic enzymes upon covalent catalysis of polyester-chain formation. A self-assembly process is initiated resulting in the formation of insoluble cytoplasmic inclusions with a phospholipid monolayer and covalently attached polyester synthases at the surface. Surface-attached polyester synthases show a marked increase in enzyme activity. These polyester synthases have only recently been biochemically characterized. An overview of these recent findings is provided. At present, 59 polyester synthase structural genes from 45 different bacteria have been cloned and the nucleotide sequences have been obtained. The multiple alignment of the primary structures of these polyester synthases show an overall identity of 8-96% with only eight strictly conserved amino acid residues. Polyester synthases can been assigned to four classes based on their substrate specificity and subunit composition. The current knowledge on the organization of the polyester synthase genes, and other genes encoding proteins related to PHA metabolism, is compiled. In addition, the primary structures of the 59 PHA synthases are aligned and analysed with respect to highly conserved amino acids, and biochemical features of polyester synthases are described. The proposed catalytic mechanism based on similarities to alpha/beta-hydrolases and mutational analysis is discussed. Different threading algorithms suggest that polyester synthases belong to the alpha/beta-hydrolase superfamily, with a conserved cysteine residue as catalytic nucleophile. This review provides a survey of the known biochemical features of these unique enzymes and their proposed catalytic mechanism.

Biochemical and enzymological properties of the polyhydroxybutyrate synthase from the extremely halophilic archaeon strain 56

Some members of the archaebacterial family Halobacteriaceae have been determined to accumulate polyhydroxyalkanoate (PHA) and poly(3-hydroxybutyrate) (PHB). The extremely halophilic archaebacterium strain 56 is capable of accumulating large amounts of PHB. Since measurements of enzyme activities related to archaebacterial PHB biosynthesis have never been achieved, we investigated the enzymology of PHB biosynthesis in strain 56. Crude extracts of strain 56 cultivated under accumulating conditions showed PHB synthase activity, whereas neither beta-ketothiolase nor NADH/NADPH-dependent acetoacetyl-CoA reductase activity was detectable. An 80-kDa protein, cross-reacting with the anti-PHB synthase antibodies raised against the PHB synthase from Ralstonia eutropha, was identified in the crude extract and was strongly enriched by purification of PHB granules. The granule-associated PHB synthase was enzymologically characterized. Enzyme kinetics showed a specific activity of about 4.6U/mg and Hill plot analysis revealed a K(0.5) of 56 microM with (R)-3-hydroxybutyryl-CoA employed as substrate. A Hill coefficient of 1.75 indicated that the PHB synthase exhibited positive cooperativity. The thioesters 3-hydroxyvaleryl-CoA, 4-hydroxybutyryl-CoA, and 3-hydroxydecanoyl-CoA were not accepted as substrates. Moreover, the PHB synthase was found to be competitively inhibited by CoA, showing an IC(50) of 160 microM. The PHB synthase was stable up to 60 degrees C and still exhibited about 90% of the maximum enzyme activity, which was obtained at 40 degrees C. In contrast to the soluble PHB synthase, the granule-bound PHB synthase was almost independent of the salt concentration. The PHB synthase could not be released from the PHB granules, indicating a covalent attachment to the PHB core. This is the first description of an archaebacterial PHA synthase.

In vitro biosynthesis of polyesters

In vitro synthesis of polyesters using isolated enzymes as catalyst via non-biosynthetic pathways is reviewed. In most cases, lipase was used as catalyst and various monomer combinations, typically oxyacids or their esters, dicarboxylic acids or their derivatives/glycols, and lactones, afforded the polyesters. The enzymatic polymerization often proceeded under mild reaction conditions in comparison with chemical processes. By utilizing characteristic properties of lipases, regio- and enantioselective polymerizations proceeded to give functional polymers, most of which are difficult to synthesize by conventional methodologies.

Biochemical and molecular basis of microbial synthesis of polyhydroxyalkanoates in microorganisms

Intensive research on the physiology, biochemistry, and molecular genetics of the metabolism of polyhydroxyalkanoates (PHA) during the last 15 years has revealed a dramatic increase of our knowledge on the biosynthesis of these polyesters in bacteria. This mainly very basic research has revealed several new, hitherto not described enzymes and pathways. In addition, many genes encoding the enzymes of these pathways and in particular the key enzyme of PHA biosynthesis, PHA synthase, were cloned and characterized at a molecular level. This knowledge was utilized to establish PHA biosynthesis in many prokaryotic and eukaryotic organisms, which were unable to synthesize PHAs, and to apply the methodology of metabolic engineering, thus opening new perspectives for the production of various PHAs by fermentation biotechnology or agriculture in economically feasible processes. This contribution summarizes the properties of PHA synthases and gives an overview on the genes for these enzymes and other enzymes of PHA biosynthesis that have been cloned and are available. It also summarizes our current knowledge on the regulation at the enzyme and gene level of PHA biosynthesis in bacteria.

Biochemical and genetic analysis of PHA synthases and other proteins required for PHA synthesis

Polyhydroxyalkanoic acids (PHA) represent a complex class of storage polyesters that are synthesized by a wide range of different gram-positive and gram-negative bacteria as well as by some Archaea and that are deposited as insoluble cytoplasmic inclusions. PHA synthases, which are the key enzymes for PHA biosynthesis, have been characterized in much detail. At present 42 PHA synthase structural genes from 38 different bacteria have been cloned, and from 30 genes the nucleotide sequences were obtained. The strategies successfully employed to clone these genes and the current knowledge on the organization of the PHA synthase genes and other genes encoding proteins related to PHA metabolism will be compiled. In addition, the primary structures of the 30 PHA synthases were aligned and analyzed with respect to highly conserved amino acids and biochemical features. The direction, in which research should proceed, in order to increase our knowledge on biosynthesis of PHAs and to utilize this knowledge for the development of technically and economically feasible processes for the production of these polyesters will be outlined.