The amorphous state of bacterial poly[(R)-3-hydroxyalkanoate]in vivo

Microbial Polyhydroxyalkanoates and Nonnatural Polyesters

Microorganisms produce diverse polymers for various purposes such as storing genetic information, energy, and reducing power, and serving as structural materials and scaffolds. Among these polymers, polyhydroxyalkanoates (PHAs) are microbial polyesters synthesized and accumulated intracellularly as a storage material of carbon, energy, and reducing power under unfavorable growth conditions in the presence of excess carbon source. PHAs have attracted considerable attention for their wide range of applications in industrial and medical fields. Since the first discovery of PHA accumulating bacteria about 100 years ago, remarkable advances have been made in the understanding of PHA biosynthesis and metabolic engineering of microorganisms toward developing efficient PHA producers. Recently, nonnatural polyesters have also been synthesized by metabolically engineered microorganisms, which opened a new avenue toward sustainable production of more diverse plastics. Herein, the current state of PHAs and nonnatural polyesters is reviewed, covering mechanisms of microbial polyester biosynthesis, metabolic pathways, and enzymes involved in biosynthesis of short-chain-length PHAs, medium-chain-length PHAs, and nonnatural polyesters, especially 2-hydroxyacid-containing polyesters, metabolic engineering strategies to produce novel polymers and enhance production capabilities and fermentation, and downstream processing strategies for cost-effective production of these microbial polyesters. In addition, the applications of PHAs and prospects are discussed.

Novel unexpected functions of PHA granules

Polyhydroxyalkanoates (PHA), polyesters accumulated by numerous prokaryotes in the form of intracellular granules, have been for decades considered being predominantly storage molecules. However, numerous recent discoveries revealed and emphasized their complex biological role for microbial cells. Most of all, it was repeatedly reported and confirmed that the presence of PHA granules in prokaryotic cells enhances stress resistance and robustness of microbes against various environmental stress factors such as high or low temperature, freezing, oxidative, and osmotic pressure. It seems that protective mechanisms of PHA granules are associated with their extraordinary architecture and biophysical properties as well as with the complex and deeply interconnected nature of PHA metabolism. Therefore, this review aims at describing novel and unexpected properties of PHA granules with respect to their contribution to stress tolerance of various prokaryotes including common mesophilic heterotrophic bacteria, but also extremophiles or photo-autotrophic cyanobacteria. KEY POINTS: • PHA granules present in bacterial cells reveal unique properties and functions. • PHA enhances stress robustness of bacterial cells.

Quantum scale biomimicry of low dimensional growth: An unusual complex amorphous precursor route to TiO2 band confinement by shape adaptive biopolymer-like flexibility for energy applications

Crystallization via an amorphous pathway is often preferred by biologically driven processes enabling living species to better regulate activation energies to crystal formation that are intrinsically linked to shape and size of dynamically evolving morphologies. Templated ordering of 3-dimensional space around amorphous embedded non-equilibrium phases at heterogeneous polymer─metal interfaces signify important routes for the genesis of low-dimensional materials under stress-induced polymer confinement. We report the surface induced catalytic loss of P=O ligands to bond activated aromatization of C−C C=C and Ti=N resulting in confinement of porphyrin-TiO₂ within polymer nanocages via particle attachment. Restricted growth nucleation of TiO₂ to the quantum scale (≤2 nm) is synthetically assisted by nitrogen, phosphine and hydrocarbon polymer chemistry via self-assembly. Here, the amorphous arrest phase of TiO₂ is reminiscent of biogenic amorphous crystal growth patterns and polymer coordination has both a chemical and biomimetic significance arising from quantum scale confinement which is atomically challenging. The relative ease in adaptability of non-equilibrium phases renders host structures more shape compliant to congruent guests increasing the possibility of geometrical confinement. Here, we provide evidence for synthetic biomimicry akin to bio-polymerization mechanisms to steer disorder-to-order transitions via solvent plasticization-like behaviour. This challenges the rationale of quantum driven confinement processes by conventional processes. Further, we show the change in optoelectronic properties under quantum confinement is intrinsically related to size that affects their optical absorption band energy range in DSSC.

Chloroform induces outstanding crystallization of poly(hydroxybutyrate) (PHB) vesicles within bacteria

Poly[(R)-3-hydroxyalkanoate]s or PHAs are aliphatic polyesters produced by numerous microorganisms. They are accumulated as energy and carbon reserve in the form of small intracellular vesicles. Poly[(R)-3-hydroxybutyrate] (PHB) is the most ubiquitous and simplest PHA. An atomic force microscope coupled with a tunable infrared laser (AFM-IR) was used to record highly spatially resolved infrared spectra of commercial purified PHB and native PHB within bacteria. For the first time, the crystallinity degree of native PHB within vesicle has been directly evaluated in situ without alteration due to the measure or extraction and purification steps of the polymer: native PHB is in crystalline state at 15% whereas crystallinity degree reaches 57% in commercial PHB. Chloroform addition on native PHB induces crystallization of the polymer within bacteria up to 60%. This possibility of probing and changing the physical state of polymer in situ could open alternative ways of production for PHB and others biopolymers. Graphical abstract An atomic force microscope coupled with a tunable infrared laser (AFM-IR) has been used to record local infrared spectra of biopolymer PHB within bacteria. Deconvolution of those spectra has allowed to determine in situ the crystallinity degree of native PHB.

A new biological recovery approach for PHA using mealworm, Tenebrio molitor

Bacterial polyhydroxyalkanoates (PHA) are expensive partly due to the recovery and purification processes. Thus, many studies have been carried out in order to minimize the cost. Here we report on the use of mealworm, which is the larva of mealworm beetle (Tenebrio molitor) to recover PHA granules from Cupriavidus necator. Mealworms were shown to readily consume the freeze-dried C. necator cells and excrete the PHA granules in the form of whitish feces. Further purification using water, detergent and heat resulted in almost 100% pure PHA granules. Comparison with chloroform extraction showed no signs of reduction in the molecular weight and dispersion of the PHA molecules. Scanning electron microscopy and dynamic light scattering measurements revealed that the biologically recovered PHA granules retained their native spherical morphology. The PHA granules were subjected to a battery of tests to determine their purity and properties in comparison to the chloroform extracted PHA. This study has demonstrated the possibility of using mealworms as a biological agent to partially purify the PHA granules.

Self-Assembled Protein-Coated Polyhydroxyalkanoate Beads: Properties and Biomedical Applications

Polyhydroxyalkanoates (PHAs) are biological polyesters that can be naturally produced by a range of bacteria as water-insoluble inclusions composed of a PHA core coated with PHA synthesis, structural, and regulatory proteins. These naturally self-assembling shell-core particles have been recently conceived as biomaterials that can be bioengineered as biologically active beads for medical applications. Protein engineering of PHA-associated proteins enabled the production of PHA-protein assemblies exhibiting biologically active protein-based functions relevant for applications as vaccines or diagnostics. Here we provide an overview of the recent advances in bioengineering of PHA particles toward the display of biomedically relevant protein functions such as selected disease-specific antigens as diagnostic tools or for the design of particulate subunit vaccines against infectious diseases such as tuberculosis, meningitis, pneumonia, and hepatitis C.

Feasibility study of an alkaline-based chemical treatment for the purification of polyhydroxybutyrate produced by a mixed enriched culture

This study focused on investigating the feasibility of purifying polyhydroxybutyrate (PHB) from mixed culture biomass by alkaline-based chemical treatment. The PHB-containing biomass was enriched on acetate under non-sterile conditions. Alkaline treatment (0.2 M NaOH) together with surfactant SDS (0.2 w/v% SDS) could reach 99% purity, with more than 90% recovery. The lost PHB could be mostly attributed to PHB hydrolysis during the alkaline treatment. PHB hydrolysis could be moderated by increasing the crystallinity of the PHB granules, for example, by biomass pretreatment (e.g. freezing or lyophilization) or by effective cell lysis (e.g. adjusting alkali concentration). The suitability of the purified PHB by alkaline treatment for polymer applications was evaluated by molecular weight and thermal stability. A solvent based purification method was also performed for comparison purposes. As result, PHB produced by mixed enriched cultures was found suitable for thermoplastic applications when purified by the solvent method. While the alkaline method resulted in purity, recovery yield and molecular weight comparable to values reported in literature for PHB produced by pure cultures, it was found unsuitable for thermoplastic applications. Given the potential low cost and favorable environmental impact of this method, it is expected that PHB purified by alkaline method may be suitable for other non-thermal polymer applications, and as a platform chemical.

Fourier-transform infrared (FTIR) spectroscopy for monitoring and determining the degree of crystallisation of polyhydroxyalkanoates (PHAs)

FTIR spectroscopy has been used to monitor and determine the degree of crystallisation in a sample of polyhydroxybutyrate-co-14%valerate (PHB-co-14%HV). Time series spectra of solution-cast films of the polymer revealed spectral changes attributed to the onset of crystallisation. Curve fitting was used to obtain an absolute measure of crystallinity. Mean centred principal-component analysis (PCA) revealed that 99.9% of the spectral variance could be attributed to factor 1. The loadings plot for factor 1 contained features attributable to crystalline and amorphous phases. These features were opposite in sign, indicating that changes in the spectra with the onset of crystallisation are simultaneous and opposite in direction, i.e. as the crystalline band increases the amorphous band decreases. Cross-peaks in asynchronous 2D correlation maps indicate there are likely to be very minor components that are changing out of phase. The presence of these minor components is supported by examination of the loadings of higher factors in the PCA model. PCA has been shown to be suitable for determining the number of dynamic spectral features and has enabled relative and objective monitoring of crystallisation kinetics.

Peculiarities of PHA granules preparation and PHA depolymerase activity determination

An extensive amount of knowledge on biochemistry of poly(3-hydroxyalkanoic acid) (PHA) synthesis and on its biodegradation has accumulated during the last two decades. Numerous genes encoding enzymes involved in the formation of PHA and in PHA degradation (PHA depolymerases) were cloned and characterized from many microorganisms. A large variety of methods exists for determination of PHA depolymerase activity and for preparation of the polymeric substrate (PHA). Unfortunately, results obtained with these different methods cannot be compared directly because they highly depend on the assay method applied and on the history of PHA granules preparation. In this contribution, the peculiarities, advantages, disadvantages and limitations of existing PHA depolymerase assay methods are described.

Assay of poly(3-hydroxybutyrate) depolymerase activity and product determination

Two methods for accurate poly(3-hydroxybutyrate) (PHB) depolymerase activity determination and quantitative and qualitative hydrolysis product determination are described. The first method is based on online determination of NaOH consumption rates necessary to neutralize 3-hydroxybutyric acid (3HB) and/or 3HB oligomers produced during the hydrolysis reaction and requires a pH-stat apparatus equipped with a software-controlled microliter pump for rapid and accurate titration. The method is universally suitable for hydrolysis of any type of polyhydroxyalkanoate or other molecules with hydrolyzable ester bonds, allows the determination of hydrolysis rates of as low as 1 nmol/min, and has a dynamic capacity of at least 6 orders of magnitude. By applying this method, specific hydrolysis rates of native PHB granules isolated from Ralstonia eutropha H16 were determined for the first time. The second method was developed for hydrolysis product identification and is based on the derivatization of 3HB oligomers into bromophenacyl derivates and separation by high-performance liquid chromatography. The method allows the separation and quantification of 3HB and 3HB oligomers up to the octamer. The two methods were applied to investigate the hydrolysis of different types of PHB by selected PHB depolymerases.

Microbial degradation of polyhydroxyalkanoates

Polyesters such as poly(3-hydroxybutyrate) (PHB) or other polyhydroxyalkanoates (PHA) have attracted commercial and academic interest as new biodegradable materials. The ability to degrade PHA is widely distributed among bacteria and fungi and depends on the secretion of specific extracellular PHA depolymerases (e-PHA depolymerases), which are carboxyesterases (EC 3.1.1.75 and EC 3.1.1.76), and on the physical state of the polymer (amorphous or crystalline). This contribution provides a summary of the biochemical and molecular biological characteristics of e-PHA depolymerases and focuses on the intracellular mobilization of storage PHA by intracellular PHA depolymerases (i-PHA depolymerases) of PHA-accumulating bacteria. The importance of different assay systems for PHA depolymerase activity is also discussed.

Bacterial poly(3-hydroxybutyrate): an optical microscopy and microfocus X-ray diffraction study

Two-dimensional spatially resolved microfocus X-ray diffraction has been used to investigate spherulites of pure bacterial poly(3-hydroxybutyrate) (PHB) and of a blend of natural and synthetic atactic PHB (a-PHB) crystallized at a relatively high temperature (Tc = 140 degrees C). Both samples investigated contained practically two-dimensional spherulites, characterized by wide extinction bands (band spacing > 80 microns). The X-ray diffraction patterns confirmed that the unit cell alpha-axis is oriented along the spherulite radius in PHB and that the same is true for the a-PHB containing blend. Comparison of the matrix of diffraction patterns with the polarized optical micrograph of the scanned sample area indicated a very clear correlation between pattern changes and banding, yielding a straightforward picture of the structural variations within the spherulite.

Properties, modifications and applications of biopolyesters

Poly(hydroxyalkanoates) (PHAs), of which poly(hydroxybutyrate) (PHB) is the most common, can be accumulated by a large number of bacteria as energy and carbon reserve. Due to their biodegradability and biocompatibility these optically active biopolyesters may find industrial applications. A general overview of the physical and material properties of PHAs, alongside with accomplished applications and new developments in this field is presented in this chapter. The properties of PHAs are dependent on their monomer composition and therefore it is of great interest that recent research has revealed that, in addition to PHB, a large variety of PHAs can be synthesized microbially. The monomer composition of PHAs depends on the nature of the carbon source and microorganism used. PHB is a typical highly crystalline thermoplastic whereas medium chain length PHAs are elastomers with low melting points and a relatively lower degree of crystallinity. By (chemical) modification of the PHAs, the ultimate properties of the materials can be adjusted even further, when necessary. Applications that have been developed from PHB and related materials (e.g. Biopol) can be found in very different application areas and cover packaging, hygienic, agricultural and biomedical products. Recent application developments based on medium chain length PHAs range from high solid alkyd-like paints to pressure sensitive adhesives, biodegradable cheese coatings and biodegradable rubbers. Technically, the prospects for PHAs are very promising. When the price of these materials can be further reduced, application of biopolyesters will also become economically very attractive.

Polyhydroxyalkanoates, biopolyesters from renewable resources: physiological and engineering aspects

Polyhdroxyalkanoates (PHAs), stored as bacterial reserve materials for carbon and energy, are biodegradable substitutes to fossil fuel plastics that can be produced from renewable raw materials. PHAs can be produced under controlled conditions by biotechnological processes. By varying the producing strains, substrates and cosubstrates, a number of polyesters can be synthesized which differ in monomer composition. By this means, PHAs with tailored interesting physical features can be produced. All of them are completely degradable to carbon dioxide and water through natural microbiological mineralization. Consequently, neither their production nor their use or degradation have a negative ecological impact. After a historical review, possibilities for the synthesis of novel PHAs applying different micro-organisms are discussed, and pathways of PHA synthesis and degradation are shown in detail for important PHA producers. This is followed by a discussion of the physiological role of the accumulation product in different micro-organisms. Detection, analysis, and extraction methods of PHAs from microbial biomass are shown, in addition to methods for polyester characterization. Strategies for PHA production under discontinuous and continuous regimes are discussed in detail in addition to the use of different cheap carbon sources from the point of view of different PHA producing strains. An outlook on PHA production by transgenic plants closes the review.

Physical properties of bacterial poly((R)-3-hydroxyalkanoates)

Poly((R)-3-hydroxyalkanoates) are bacterial storage polyesters, currently receiving much attention because of their potential application as biodegradable and biocompatible plastics. Methods and skills from both microbiology and polymer science can be used to manipulate these materials to make their physical properties meet the requirements for specific applications. The present paper reviews the physical properties of the most promising poly((R)-3-hydroxyalkanoates) as well as the opportunities offered by polymer technology to improve them, emphasizing the mechanical properties in relation to structure and processing.Key words: biopolymers, poly(hydroxyalkanoates), PHB, PHA.

Biomimetic, amorphous granules of polyhydroxyalkanoates: composition, mobility, and stabilization in vitro by proteins

It is a remarkable feature of poly(3-hydroxybutyrate) (PHB) that although the isolated polymer is highly crystalline, native PHB storage granules in cells are only found in an amorphous, mobile state. It has recently been proposed that the failure of PHB granules to crystallize is simply the result of slow nucleation kinetics that are operative for small, isolated particles. In support of this new model, we present here a straightforward procedure by which pure crystalline PHB can be reconstituted into submicron-size, detergent-coated artificial granules. The artificial granules are amorphous and stable in suspension, and they are essentially indistinguishable from their native counterparts in terms of size, morphology, molecular mobility, and density. Furthermore, when the surfactant coating is removed from the artificial granules by dialysis, the granules crystallize, verifying the nucleation hypothesis. In vivo, the PHB granule surface is likely to consist of both polypeptide and lipid; in vitro it is possible to prepare amorphous PHB granules that are stabilized solely by phospholipids or by any of several common proteins (serum albumin, casein, or ovalbumin). Artificial amorphous granules may be prepared from a variety of different bacterial PHAs and from blends of incompatible polyesters.Key words: polyhydroxyalkanoate, poly(3-hydroxybutyrate), granules, nucleation, latex.

Nuclear magnetic resonance relaxation studies of poly(hydroxybutyrate) in whole cells and in artificial granules

The physical state of poly(hydroxybutyrate) (PHB) in whole cells and in the form of artificial biomimetic granules has been probed using 13C nuclear magnetic resonance (NMR) spectroscopy. Studies on varying concentrations of whole cells of Alcaligenes eutrophus show that changes in the line widths of PHB in whole cells do not correlate with changes in transverse relaxation times. Solid-state magic-angle spinning NMR studies demonstrate that the line broadening results from a reduction in the static field homogeneity rather than from intrinsic properties of the PHB within the cells. Transverse and longitudinal relaxation times of PHB in whole cells and in artificial granules are similar, indicating similarities in structure and mobility.

Immunocytochemical analysis of poly-beta-hydroxybutyrate (PHB) synthase in Alcaligenes eutrophus H16: localization of the synthase enzyme at the surface of PHB granules

Antibodies raised against the Alcaligenes eutrophus poly-beta-hydroxybutyrate (PHB) synthase polypeptide were used for immunocytochemical localization of the synthase enzyme in whole cells and purified PHB granules. The data presented demonstrate for the first time that the synthase enzyme is located on the surface of the PHB granule rather than being incorporated inside the granule during its formation. From these basic observations and data from the recent literature, a model of granule assembly is proposed.