Properties of a novel intracellular poly(3-hydroxybutyrate) depolymerase with high specific activity (PhaZd) in Wautersia eutropha H16

Poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHAs), green alternatives to petroleum-based plastics: a review

In spite of the fact that petroleum-based plastics are convenient in terms of fulfilling the performance requirements of many applications, they contribute significantly to a number of ecological and environmental problems. Recently, the public awareness of the negative effects of petroleum-based plastics on the environment has increased. The present utilization of natural resources cannot be sustained forever. Furthermore, oil is often subjected to price fluctuations and will eventually be depleted. The increase in the level of carbon dioxide due to the combustion of fossil fuel is causing global warming. Concerns about preservation of natural resources and climate change are considered worldwide motivations for academic and industrial researchers to reduce the consumption and dependence on fossil fuel. Therefore, bio-based polymers are moving towards becoming the favorable option to be utilized in polymer manufacturing, food packaging, and medical applications. This paper represents an overview of the feasibility of both Poly Lactic Acid (PLA) and polyhydroxyalkanoates (PHAs) as alternative materials that can replace petroleum-based polymers in a wide range of industrial applications. Physical, thermal, rheological, and mechanical properties of both polymers as well as their permeability and migration properties have been reviewed. Moreover, PLA's recyclability, sustainability, and environmental assessment have been also discussed. Finally, applications in which both polymers can replace petroleum-based plastics have been explored and provided.

A sensitive pH indicator-based spectrophotometric assay for PHB depolymerase activity on microtiter plates

A continuous spectrophotometric assay for the screening of PHB depolymerase activity in microtiter plates was developed. We evaluated crystalline PHB in the suspension and coated it with the addition of a pH indicator to detect the breakage of the ester bond by proton titration. The reaction rate and the concentration of the recombinant PhaZ1 from Paucimonas lemoignei PHB depolymerase presented a linear correlation. A comparison of the proposed method with the turbidimetric method adapted to the microtiter plates revealed that the use of indicators increases the response signal by at least 5-fold, resulting in increased sensitivity and better signal-to-noise ratio. Furthermore, the proposed method offers a wide range of pH from 5.0 to 9.2 by using different buffer-indicator pairs and was employed for the screening of PHB-depolymerase activity on 140 bacterial strains isolated from Lake Chapala. Eleven strains were positive for PHB-depolymerase activity, which were ACSLRF-27, ACPLRF-6, and ACPLRF-5 (16S rRNA sequence alignment revealed 99-100% similarity with Actinomadura geliboluensis strain A8036, Streptomyces cavourensis strain NRRL 2740, and Streptomyces coelicolor strain DSM 40233, respectively); these that showed the highest activities. In conclusion, the method was successfully applied for finding new strains and for quantifying the PHB depolymerases activity with crystalline PHB.

The Puzzling Conservation and Diversification of Lipid Droplets from Bacteria to Eukaryotes

Membrane compartments are amongst the most fascinating markers of cell evolution from prokaryotes to eukaryotes, some being conserved and the others having emerged via a series of primary and secondary endosymbiosis events. Membrane compartments comprise the system limiting cells (one or two membranes in bacteria, a unique plasma membrane in eukaryotes) and a variety of internal vesicular, subspherical, tubular, or reticulated organelles. In eukaryotes, the internal membranes comprise on the one hand the general endomembrane system, a dynamic network including organelles like the endoplasmic reticulum, the Golgi apparatus, the nuclear envelope, etc. and also the plasma membrane, which are linked via direct lateral connectivity (e.g. between the endoplasmic reticulum and the nuclear outer envelope membrane) or indirectly via vesicular trafficking. On the other hand, semi-autonomous organelles, i.e. mitochondria and chloroplasts, are disconnected from the endomembrane system and request vertical transmission following cell division. Membranes are organized as lipid bilayers in which proteins are embedded. The budding of some of these membranes, leading to the formation of the so-called lipid droplets (LDs) loaded with hydrophobic molecules, most notably triacylglycerol, is conserved in all clades. The evolution of eukaryotes is marked by the acquisition of mitochondria and simple plastids from Gram-positive bacteria by primary endosymbiosis events and the emergence of extremely complex plastids, collectively called secondary plastids, bounded by three to four membranes, following multiple and independent secondary endosymbiosis events. There is currently no consensus view of the evolution of LDs in the Tree of Life. Some features are conserved; others show a striking level of diversification. Here, we summarize the current knowledge on the architecture, dynamics, and multitude of functions of the lipid droplets in prokaryotes and in eukaryotes deriving from primary and secondary endosymbiosis events.

Ability of T1 Lipase to Degrade Amorphous P(3HB): Structural and Functional Study

An enzyme with broad substrate specificity would be an asset for industrial application. T1 lipase apparently has the same active site residues as polyhydroxyalkanoates (PHA) depolymerase. Sequences of both enzymes were studied and compared, and a conserved lipase box pentapeptide region around the nucleophilic serine was detected. The alignment of 3-D structures for both enzymes showed their active site residues were well aligned with an RMSD value of 1.981 Å despite their sequence similarity of only 53.8%. Docking of T1 lipase with P(3HB) gave forth high binding energy of 5.4 kcal/mol, with the distance of 4.05 Å between serine hydroxyl (OH) group of TI lipase to the carbonyl carbon of the substrate, similar to the native PhaZ7 _Pl . This suggests the possible ability of T1 lipase to bind P(3HB) in its active site. The ability of T1 lipase in degrading amorphous P(3HB) was investigated on 0.2% (w/v) P(3HB) plate. Halo zone was observed around the colony containing the enzyme which confirms that T1 lipase is indeed able to degrade amorphous P(3HB). Results obtained in this study highlight the fact that T1 lipase is a versatile hydrolase enzyme which does not only record triglyceride degradation activity but amorphous P(3HB) degradation activity as well.

A patatin-like protein associated with the polyhydroxyalkanoate (PHA) granules of Haloferax mediterranei acts as an efficient depolymerase in the degradation of native PHA

The key enzymes and pathways involved in polyhydroxyalkanoate (PHA) biosynthesis in haloarchaea have been identified in recent years, but the haloarchaeal enzymes for PHA degradation remain unknown. In this study, a patatin-like PHA depolymerase, PhaZh1, was determined to be located on the PHA granules in the haloarchaeon Haloferax mediterranei. PhaZh1 hydrolyzed the native PHA (nPHA) [including native polyhydroxybutyrate (nPHB) and native poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (nPHBV) in this study] granules in vitro with 3-hydroxybutyrate (3HB) monomer as the primary product. The site-directed mutagenesis of PhaZh1 indicated that Gly16, Ser47 (in a classical lipase box, G-X-S47-X-G), and Asp195 of this depolymerase were essential for its activity in nPHA granule hydrolysis. Notably, phaZh1 and bdhA (encoding putative 3HB dehydrogenase) form a gene cluster (HFX_6463 to _6464) in H. mediterranei. The 3HB monomer generated from nPHA degradation by PhaZh1 could be further converted into acetoacetate by BdhA, indicating that PhaZh1-BdhA may constitute the first part of a PHA degradation pathway in vivo. Interestingly, although PhaZh1 showed efficient activity and was most likely the key enzyme in nPHA granule hydrolysis in vitro, the knockout of phaZh1 had no significant effect on the intracellular PHA mobilization, implying the existence of an alternative PHA mobilization pathway(s) that functions effectively within the cells of H. mediterranei. Therefore, identification of this patatin-like depolymerase of haloarchaea may provide a new strategy for producing the high-value-added chiral compound (R)-3HB and may also shed light on the PHA mobilization in haloarchaea.

Comparative proteome analysis reveals four novel polyhydroxybutyrate (PHB) granule-associated proteins in Ralstonia eutropha H16

Identification of proteins that were present in a polyhydroxybutyrate (PHB) granule fraction isolated from Ralstonia eutropha but absent in the soluble, membrane, and membrane-associated fractions revealed the presence of only 12 polypeptides with PHB-specific locations plus 4 previously known PHB-associated proteins with multiple locations. None of the previously postulated PHB depolymerase isoenzymes (PhaZa2 to PhaZa5, PhaZd1, and PhaZd2) and none of the two known 3-hydroxybutyrate oligomer hydrolases (PhaZb and PhaZc) were significantly present in isolated PHB granules. Four polypeptides were found that had not yet been identified in PHB granules. Three of the novel proteins are putative α/β-hydrolases, and two of those (A0671 and B1632) have a PHB synthase/depolymerase signature. The third novel protein (A0225) is a patatin-like phospholipase, a type of enzyme that has not been described for PHB granules of any PHB-accumulating species. No function has been ascribed to the fourth protein (A2001), but its encoding gene forms an operon with phaB2 (acetoacetyl-coenzyme A [CoA] reductase) and phaC2 (PHB synthase), and this is in line with a putative function in PHB metabolism. The localization of the four new proteins at the PHB granule surface was confirmed in vivo by fluorescence microscopy of constructed fusion proteins with enhanced yellow fluorescent protein (eYFP). Deletion of A0671 and B1632 had a minor but detectable effect on the PHB mobilization ability in the stationary growth phase of nutrient broth (NB)-gluconate cells, confirming the functional involvement of both proteins in PHB metabolism.

Impact of Ralstonia eutropha's poly(3-Hydroxybutyrate) (PHB) Depolymerases and Phasins on PHB storage in recombinant Escherichia coli

The model organism for polyhydroxybutyrate (PHB) biosynthesis, Ralstonia eutropha H16, possesses multiple isoenzymes of granules coating phasins as well as of PHB depolymerases, which degrade accumulated PHB under conditions of carbon limitation. In this study, recombinant Escherichia coli BL21(DE3) strains were used to study the impact of selected PHB depolymerases of R. eutropha H16 on the growth behavior and on the amount of accumulated PHB in the absence or presence of phasins. For this purpose, 20 recombinant E. coli BL21(DE3) strains were constructed, which harbored a plasmid carrying the phaCAB operon from R. eutropha H16 to ensure PHB synthesis and a second plasmid carrying different combinations of the genes encoding a phasin and a PHB depolymerase from R. eutropha H16. It is shown in this study that the growth behavior of the respective recombinant E. coli strains was barely affected by the overexpression of the phasin and PHB depolymerase genes. However, the impact on the PHB contents was significantly greater. The strains expressing the genes of the PHB depolymerases PhaZ1, PhaZ2, PhaZ3, and PhaZ7 showed 35% to 94% lower PHB contents after 30 h of cultivation than the control strain. The strain harboring phaZ7 reached by far the lowest content of accumulated PHB (only 2.0% [wt/wt] PHB of cell dry weight). Furthermore, coexpression of phasins in addition to the PHB depolymerases influenced the amount of PHB stored in cells of the respective strains. It was shown that the phasins PhaP1, PhaP2, and PhaP4 are not substitutable without an impact on the amount of stored PHB. In particular, the phasins PhaP2 and PhaP4 seemed to limit the degradation of PHB by the PHB depolymerases PhaZ2, PhaZ3, and PhaZ7, whereas almost no influence of the different phasins was observed if phaZ1 was coexpressed. This study represents an extensive analysis of the impact of PHB depolymerases and phasins on PHB accumulation and provides a deeper insight into the complex interplay of these enzymes.

To be or not to be a poly(3-hydroxybutyrate) (PHB) depolymerase: PhaZd1 (PhaZ6) and PhaZd2 (PhaZ7) of Ralstonia eutropha, highly active PHB depolymerases with no detectable role in mobilization of accumulated PHB

The putative physiological functions of two related intracellular poly(3-hydroxybutyrate) (PHB) depolymerases, PhaZd1 and PhaZd2, of Ralstonia eutropha H16 were investigated. Purified PhaZd1 and PhaZd2 were active with native PHB granules in vitro. Partial removal of the proteinaceous surface layer of native PHB granules by trypsin treatment or the use of PHB granules isolated from ΔphaP1 or ΔphaP1-phaP5 mutant strains resulted in increased specific PHB depolymerase activity, especially for PhaZd2. Constitutive expression of PhaZd1 or PhaZd2 reduced or even prevented the accumulation of PHB under PHB-permissive conditions in vivo. Expression of translational fusions of enhanced yellow fluorescent protein (EYFP) with PhaZd1 and PhaZd2 in which the active-site serines (S190 and Ser193) were replaced with alanine resulted in the colocalization of only PhaZd1 fusions with PHB granules. C-terminal fusions of inactive PhaZd2(S193A) with EYFP revealed the presence of spindle-like structures, and no colocalization with PHB granules was observed. Chromosomal deletion of phaZd1, phaZd2, or both depolymerase genes had no significant effect on PHB accumulation and mobilization during growth in nutrient broth (NB) or NB-gluconate medium. Moreover, neither proteome analysis of purified native PHB granules nor lacZ fusion studies gave any indication that PhaZd1 or PhaZd2 was detectably present in the PHB granule fraction or expressed at all during growth on NB-gluconate medium. In conclusion, PhaZd1 and PhaZd2 are two PHB depolymerases with a high capacity to degrade PHB when artificially expressed but are apparently not involved in PHB mobilization in the wild type. The true in vivo functions of PhaZd1 and PhaZd2 remain obscure.

New insights in the formation of polyhydroxyalkanoate granules (carbonosomes) and novel functions of poly(3-hydroxybutyrate)

The metabolism of polyhydroxybutyrate (PHB) and related polyhydroxyalkanoates (PHAs) has been investigated by many groups for about three decades, and good progress was obtained in understanding the mechanisms of biosynthesis and biodegradation of this class of storage molecules. However, the molecular events that happen at the onset of PHB synthesis and the details of the initiation of PHB/PHA granule formation, as well as the complex composition of the proteinaceous surface layer of PHB/PHA granules, have only recently come into the focus of research and were not reviewed yet. In this contribution, we summarize the progress in understanding the initiation and formation of the PHA granule complex at the example of Ralstonia eutropha H16 (model organism of PHB-accumulating bacteria). Where appropriate, we include information on PHA granules of Pseudomonas putida as a representative species for medium-chain-length PHA-accumulating bacteria. We suggest to replace the previous micelle mode of PHB granule formation by the Scaffold Model in which the PHB synthase initiation complex is bound to the bacterial nucleoid. In the second part, we highlight data on other forms of PHB: oligo-PHB with ≈100 to 200 3-hydroxybutyrate (3HB) units and covalently bound PHB (cPHB) are unrelated in function to storage PHB but are presumably present in all living organisms, and therefore must be of fundamental importance.

Poly(3-hydroxybutyrate) degradation in Ralstonia eutropha H16 is mediated stereoselectively to (S)-3-hydroxybutyryl coenzyme A (CoA) via crotonyl-CoA

Degradation of poly(3-hydroxybutyrate) (PHB) by the thiolytic activity of the PHB depolymerase PhaZ1 from Ralstonia eutropha H16 was analyzed in the presence of different phasins. An Escherichia coli strain was constructed that harbored the genes for PHB synthesis (phaCAB), the phasin PhaP1, and the PHB depolymerase PhaZ1. PHB was isolated in the native form (nPHB) from this recombinant E. coli strain, and the in vitro degradation of the polyester was examined. Degradation resulted in the formation of the expected 3-hydroxybutyryl coenzyme A (3HB-CoA) and in the formation of a second product, which occurred in significantly higher concentrations than 3HB-CoA. This second product was identified by liquid chromatography mass spectrometry (LC-MS) as crotonyl-CoA. Replacement of PhaP1 by PhaP2 or PhaP4 resulted in a lower degradation rate, whereas the absence of the phasins prevented the degradation of nPHB by the PHB depolymerase PhaZ1 almost completely. In addition, the in vitro degradation of nPHB granules isolated from R. eutropha H16 (wild type) and from the R. eutropha ΔphaP1 and ΔphaP1-4 deletion mutants was examined. In contrast to the results obtained with nPHB granules isolated from E. coli, degradation of nPHB granules isolated from the wild type of R. eutropha yielded high concentrations of 3HB-CoA and low concentrations of crotonyl-CoA. The degradation of nPHB granules isolated from the ΔphaP1 and ΔphaP1-4 deletion mutants of R. eutropha was significantly reduced in comparison to that of nPHB granules isolated from wild-type R. eutropha. Stereochemical analyses of 3HB-CoA revealed that the (R) stereoisomer was collected after degradation of granules isolated from E. coli, whereas the (S) stereoisomer was collected after degradation of granules isolated from R. eutropha. Based on these results, a newly observed mechanism in the degradation pathway for PHB in R. eutropha is proposed which is connected by crotonyl-CoA to the β-oxidation cycle. According to this model, the NADPH-dependent synthesis of PHB with (R)-3HB-CoA as the intermediate and the PHB degradation yielding (S)-3HB-CoA, which is further converted in an NAD-dependent reaction, are separated.

PHB granules are attached to the nucleoid via PhaM in Ralstonia eutropha

BACKGROUND

Poly(3-hydroxybutyrate) (PHB) granules are important storage compounds of carbon and energy in many prokaryotes which allow survival of the cells in the absence of suitable carbon sources. Formation and subcellular localization of PHB granules was previously assumed to occur randomly in the cytoplasm of PHB accumulating bacteria. However, contradictionary results on subcellular localization of PHB granules in Ralstonia eutropha were published, recently.

RESULTS

Here, we provide evidence by transmission electron microscopy that PHB granules are localized in close contact to the nucleoid region in R. eutropha during growth on nutrient broth. Binding of PHB granules to the nucleoid is mediated by PhaM, a PHB granule associated protein with phasin-like properties that is also able to bind to DNA and to phasin PhaP5. Over-expression of PhaM resulted in formation of many small PHB granules that were always attached to the nucleoid region. In contrast, PHB granules of ∆phaM strains became very large and distribution of granules to daughter cells was impaired. Association of PHB granules to the nucleoid region was prevented by over-expression of PhaP5 and clusters of several PHB granules were mainly localized near the cell poles.

CONCLUSION

Subcellular localization of PHB granules is controlled in R. eutropha and depends on the presence and concentrations of at least two PHB granule associated proteins, PhaM and PhaP5.

Localization of poly(3-hydroxybutyrate) (PHB) granule-associated proteins during PHB granule formation and identification of two new phasins, PhaP6 and PhaP7, in Ralstonia eutropha H16

Poly(3-hydroxybutyrate) (PHB) granules are covered by a surface layer consisting of mainly phasins and other PHB granule-associated proteins (PGAPs). Phasins are small amphiphilic proteins that determine the number and size of accumulated PHB granules. Five phasin proteins (PhaP1 to PhaP5) are known for Ralstonia eutropha. In this study, we identified three additional potential phasin genes (H16_B1988, H16_B2296, and H16_B2326) by inspection of the R. eutropha genome for sequences with "phasin 2 motifs." To determine whether the corresponding proteins represent true PGAPs, fusions with eYFP (enhanced yellow fluorescent protein) were constructed. Similar fusions of eYFP with PhaP1 to PhaP5 as well as fusions with PHB synthase (PhaC1), an inactive PhaC1 variant (PhaC1-C319A), and PhaC2 were also made. All fusions were investigated in wild-type and PHB-negative backgrounds. Colocalization with PHB granules was found for all PhaC variants and for PhaP1 to PhaP5. Additionally, eYFP fusions with H16_B1988 and H16_B2326 colocalized with PHB. Fusions of H16_B2296 with eYFP, however, did not colocalize with PHB granules but did colocalize with the nucleoid region. Notably, all fusions (except H16_B2296) were soluble in a ΔphaC1 strain. These data confirm that H16_B1988 and H16_B2326 but not H16_B2296 encode true PGAPs, for which we propose the designation PhaP6 (H16_B1988) and PhaP7 (H16_B2326). When localization of phasins was investigated at different stages of PHB accumulation, fusions of PhaP6 and PhaP7 were soluble in the first 3 h under PHB-permissive conditions, although PHB granules appeared after 10 min. At later time points, the fusions colocalized with PHB. Remarkably, PHB granules of strains expressing eYFP fusions with PhaP5, PhaP6, or PhaP7 localized predominantly near the cell poles or in the area of future septum formation. This phenomenon was not observed for the other PGAPs (PhaP1 to PhaP4, PhaC1, PhaC1-C319A, and PhaC2) and indicated that some phasins can have additional functions. A chromosomal deletion of phaP6 or phaP7 had no visible effect on formation of PHB granules.

Examination of PHB Depolymerases in Ralstonia eutropha: Further Elucidation of the Roles of Enzymes in PHB Homeostasis

Polyhydroxyalkanoates (PHA) are biodegradable polymers that are attractive materials for use in tissue engineering and medical device manufacturing. Ralstonia eutropha is regarded as the model organism for PHA biosynthesis. We examined the effects of PHA depolymerase (PhaZ) expression on PHA homeostasis in R. eutropha strains. In order to analyze the impact of PhaZs on R. eutropha granule architecture, we performed electron microscopy on several phaZ knockout strains and the wild type strain grown under PHA production conditions. Analysis of the acquired micrographs was based on stereology: the ratio of granule area and cell area was determined, along with total granule count per full-size cell image. Cells bearing a phaZ2 knockout mutation alone or in conjunction with a phaZ1 mutation were found to have a high granule volume per cell volume and a higher granule count compared to wild type. A phaZ quadruple knockout strain appeared to have a low granule volume per cell volume and a low granule count per cell. Cells bearing a phaZ3 knockout were found to have a higher granule count than the wild type, whereas granule volume per cell volume was similar. Accordingly, we hypothesize that PhaZs have not only an impact on PHA degradation but also on the 3-dimensional granule architecture. Based on our data, PhaZ2 is postulated to affect granule density. This work increased our knowledge about PHA depolymerases in R. eutropha, including enzymes that had previously been uncharacterized.

Interaction between poly(3-hydroxybutyrate) granule-associated proteins as revealed by two-hybrid analysis and identification of a new phasin in Ralstonia eutropha H16

A large number of polypeptides are attached to poly(3-hydroxybutyrate) (PHB) granules of Ralstonia eutropha, such as PHB synthase (PhaC1), several PHB depolymerases (PhaZs) and phasins (PhaPs), the regulator protein PhaR Reu , and possibly others. In this study we used the bacterial adenylate cyclase-based two-hybrid assay to investigate interactions between known PHB granule-associated proteins (PGAPs) and to screen for new PGAPs. The utility of the system was tested by the in vivo verification of previously postulated interactions of the PHB synthase subunits of R. eutropha (PhaC1 homo-oligomerization) and of Bacillus megaterium (PhaC Bmeg –PhaR Bmeg hetero-oligomerization). Nine proteins (PhaA, PhaB1, PhaC1, PhaP1–PhaP4, PhaZ1 and PhaR), with established functions in PHB metabolism of R. eutropha, were tested for interaction in all combinations. While no significant interaction was detected between the PHB synthase PhaC1 and any of the other eight tested Pha proteins, strong interactions were found between all phasin proteins, in particular between PhaP2 and PhaP4. When PhaP2 was used as bait in a two-hybrid screening experiment with a genomic library of R. eutropha, the B1934 gene product was identified in 24 out of 53 isolated clones. B1934 encodes a hypothetical protein (15.7 kDa) with similarity to phasins of PHB-accumulating bacteria. A fusion protein of eYfp and the B1934 gene product colocalized with PHB granules, confirming that B1934 represents a new phasin (PhaP5). PhaP5 was not essential for PHB granule formation, but overexpression of PhaP5 increased the number of cells with PHB granules at the cell poles.

Versatile metabolic adaptations of Ralstonia eutropha H16 to a loss of PdhL, the E3 component of the pyruvate dehydrogenase complex

A previous study reported that the Tn5-induced poly(3-hydroxybutyric acid) (PHB)-leaky mutant Ralstonia eutropha H1482 showed a reduced PHB synthesis rate and significantly lower dihydrolipoamide dehydrogenase (DHLDH) activity than the wild-type R. eutropha H16 but similar growth behavior. Insertion of Tn5 was localized in the pdhL gene encoding the DHLDH (E3 component) of the pyruvate dehydrogenase complex (PDHC). Taking advantage of the available genome sequence of R. eutropha H16, observations were verified and further detailed analyses and experiments were done. In silico genome analysis revealed that R. eutropha possesses all five known types of 2-oxoacid multienzyme complexes and five DHLDH-coding genes. Of these DHLDHs, only PdhL harbors an amino-terminal lipoyl domain. Furthermore, insertion of Tn5 in pdhL of mutant H1482 disrupted the carboxy-terminal dimerization domain, thereby causing synthesis of a truncated PdhL lacking this essential region, obviously leading to an inactive enzyme. The defined ΔpdhL deletion mutant of R. eutropha exhibited the same phenotype as the Tn5 mutant H1482; this excludes polar effects as the cause of the phenotype of the Tn5 mutant H1482. However, insertion of Tn5 or deletion of pdhL decreases DHLDH activity, probably negatively affecting PDHC activity, causing the mutant phenotype. Moreover, complementation experiments showed that different plasmid-encoded E3 components of R. eutropha H16 or of other bacteria, like Burkholderia cepacia, were able to restore the wild-type phenotype at least partially. Interestingly, the E3 component of B. cepacia possesses an amino-terminal lipoyl domain, like the wild-type H16. A comparison of the proteomes of the wild-type H16 and of the mutant H1482 revealed striking differences and allowed us to reconstruct at least partially the impressive adaptations of R. eutropha H1482 to the loss of PdhL on the cellular level.

Genome-wide transcriptome analyses of the ‘Knallgas’ bacterium Ralstonia eutropha H16 with regard to polyhydroxyalkanoate metabolism

Ralstonia eutropha H16 is probably the best-studied ‘Knallgas’ bacterium and producer of poly(3-hydroxybutyrate) (PHB). Genome-wide transcriptome analyses were employed to detect genes that are differentially transcribed during PHB biosynthesis. For this purpose, four transcriptomes from different growth phases of the wild-type H16 and of the two PHB-negative mutants PHB−4 and ΔphaC1 were compared: (i) cells from the exponential growth phase with cells that were in transition to stationary growth phase, and (ii) cells from the transition phase with cells from the stationary growth phase of R. eutropha H16, as well as (iii) cells from the transition phase of R. eutropha H16 with those from the transition phase of R. eutropha PHB−4 and (iv) cells from the transition phase of R. eutropha ΔphaC1 with those from the transition phase of R. eutropha PHB−4. Among a large number of genes exhibiting significant changes in transcription level, several genes within the functional class of lipid metabolism were detected. In strain H16, phaP3, accC2, fabZ, fabG and H16_A3307 exhibited a decreased transcription level in the stationary growth phase compared with the transition phase, whereas phaP1, H16_A3311, phaZ2 and phaZ6 were found to be induced in the stationary growth phase. Compared with PHB−4, we found that phaA, phaB1, paaH1, H16_A3307, phaP3, accC2 and fabG were induced in the wild-type, and phaP1, phaP4, phaZ2 and phaZ6 exhibited an elevated transcription level in PHB−4. In strain ΔphaC1, phaA and phaB1 were highly induced compared with PHB−4. Additionally, the results of this study suggest that mutant strain PHB−4 is defective in PHB biosynthesis and fatty acid metabolism. A significant downregulation of the two cbb operons in mutant strain PHB−4 was observed. The putative polyhydroxyalkanoate (PHA) synthase phaC2 identified in strain H16 was further investigated by several functional analyses. Mutant PHB−4 could be phenotypically complemented by expression of phaC2 from a plasmid; on the other hand, in the mutant H16ΔphaC1, no PHA production was observed. PhaC2 activity could not be detected in any experiment.

The structure of PhaZ7 at atomic (1.2 A) resolution reveals details of the active site and suggests a substrate-binding mode

Poly-(R)-hydroxyalkanoates (PHAs) are bacterial polyesters that are degraded by a group of enzymes known as PHA depolymerases. Paucimonas lemoignei PhaZ7 depolymerase is the only extracellular depolymerase that has been described as being active towards amorphous PHAs. A previously determined crystal structure of PhaZ7 revealed an alpha/beta-hydrolase fold and a Ser-His-Asp catalytic triad. In order to address questions regarding the catalytic mechanism and substrate binding, the atomic resolution structure of PhaZ7 was determined after cocrystallization with the protease inhibitor PMSF. The reported structure has the highest resolution (1.2 A) of currently known depolymerase structures and shows a sulfur dioxide molecule covalently attached to the active-site residue Ser136. Structural comparison with the free PhaZ7 structure (1.45 A resolution) revealed no major changes in the active site, suggesting a preformed catalytic triad. The oxyanion hole was found to be formed by the amide groups of Met137 and Asn49. Nine well ordered water molecules were located in the active site. Manual docking of a substrate trimer showed that the positions of these water molecules coincide well with the substrate atoms. It is proposed that these water molecules are displaced upon binding of the substrate. Furthermore, conformational changes were identified after comparison with a previously determined PhaZ7 dimer structure in a different space group. The changes were located in surface loops involved in dimer formation, indicating some flexibility of these loops and their possible involvement in polyester binding.

Identification and characterization of the intracellular poly-3-hydroxybutyrate depolymerase enzyme PhaZ of Sinorhizobium meliloti

Background

S. meliloti forms indeterminate nodules on the roots of its host plant alfalfa (Medicago sativa). Bacteroids of indeterminate nodules are terminally differentiated and, unlike their non-terminally differentiated counterparts in determinate nodules, do not accumulate large quantities of Poly-3-hydroxybutyrate (PHB) during symbiosis. PhaZ is in intracellular PHB depolymerase; it represents the first enzyme in the degradative arm of the PHB cycle in S. meliloti and is the only enzyme in this half of the PHB cycle that remains uncharacterized.

Results

The S. meliloti phaZ gene was identified by in silico analysis, the ORF was cloned, and a S. meliloti phaZ mutant was constructed. This mutant exhibited increased PHB accumulation during free-living growth, even when grown under non-PHB-inducing conditions. The phaZ mutant demonstrated no reduction in symbiotic capacity; interestingly, analysis of the bacteroids showed that this mutant also accumulated PHB during symbiosis. This mutant also exhibited a decreased capacity to tolerate long-term carbon starvation, comparable to that of other PHB cycle mutants. In contrast to other PHB cycle mutants, the S. meliloti phaZ mutant did not exhibit any decrease in rhizosphere competitiveness; however, this mutant did exhibit a significant increase in succinoglycan biosynthesis.

Conclusions

S. meliloti bacteroids retain the capacity to synthesize PHB during symbiosis; interestingly, accumulation does not occur at the expense of symbiotic performance. phaZ mutants are not compromised in their capacity to compete for nodulation in the rhizosphere, perhaps due to increased succinoglycan production resulting from upregulation of the succinoglycan biosynthetic pathway. The reduced survival capacity of free-living cells unable to access their accumulated stores of PHB suggests that PHB is a crucial metabolite under adverse conditions.

Identification and characterization of a novel intracellular poly(3-hydroxybutyrate) depolymerase from Bacillus megaterium

A gene that codes for a novel intracellular poly(3-hydroxybutyrate) (PHB) depolymerase, designated PhaZ1, has been identified in the genome of Bacillus megaterium. A native PHB (nPHB) granule-binding assay showed that purified soluble PhaZ1 had strong affinity for nPHB granules. Turbidimetric analyses revealed that PhaZ1 could rapidly degrade nPHB granules in vitro without the need for protease pretreatment of the granules to remove surface proteins. Notably, almost all the final hydrolytic products produced from the in vitro degradation of nPHB granules by PhaZ1 were 3-hydroxybutyric acid (3HB) monomers. Unexpectedly, PhaZ1 could also hydrolyze denatured semicrystalline PHB, with the generation of 3HB monomers. The disruption of the phaZ1 gene significantly affected intracellular PHB mobilization during the PHB-degrading stage in B. megaterium, as demonstrated by transmission electron microscopy and the measurement of the PHB content. These results indicate that PhaZ1 is functional in intracellular PHB mobilization in vivo. Some of these features, which are in striking contrast with those of other known nPHB granule-degrading PhaZs, may provide an advantage for B. megaterium PhaZ1 in fermentative production of the biotechnologically valuable chiral compound (R)-3HB.

The PHA Depolymerase Engineering Database: A systematic analysis tool for the diverse family of polyhydroxyalkanoate (PHA) depolymerases

Background

Polyhydroxyalkanoates (PHAs) can be degraded by many microorganisms using intra- or extracellular PHA depolymerases. PHA depolymerases are very diverse in sequence and substrate specificity, but share a common α/β-hydrolase fold and a catalytic triad, which is also found in other α/β-hydrolases.

Results

The PHA Depolymerase Engineering Database (DED, ) has been established as a tool for systematic analysis of this enzyme family. The DED contains sequence entries of 587 PHA depolymerases, which were assigned to 8 superfamilies and 38 homologous families based on their sequence similarity. For each family, multiple sequence alignments and profile hidden Markov models are provided, and functionally relevant residues are annotated.

Conclusion

The DED is a valuable tool which can be applied to identify new PHA depolymerase sequences from complete genomes in silico, to classify PHA depolymerases, to predict their biochemical properties, and to design enzyme variants with improved properties.

Ralstonia eutropha H16 flagellation changes according to nutrient supply and state of poly(3-hydroxybutyrate) accumulation

Two-dimensional polyacrylamide gel electrophoresis (2D PAGE), in combination with matrix-assisted laser desorption ionization-time of flight analysis, and the recently revealed genome sequence of Ralstonia eutropha H16 were employed to detect and identify proteins that are differentially expressed during different phases of poly(3-hydroxybutyric acid) (PHB) metabolism. For this, a modified protein extraction protocol applicable to PHB-harboring cells was developed to enable 2D PAGE-based proteome analysis of such cells. Subsequently, samples from (i) the exponential growth phase, (ii) the stationary growth phase permissive for PHB biosynthesis, and (iii) a phase permissive for PHB mobilization were analyzed. Among several proteins exhibiting quantitative changes during the time course of a cultivation experiment, flagellin, which is the main protein of bacterial flagella, was identified. Initial investigations that report on changes of flagellation for R. eutropha were done, but 2D PAGE and electron microscopic examinations of cells revealed clear evidence that R. eutropha exhibited further significant changes in flagellation depending on the life cycle, nutritional supply, and, in particular, PHB metabolism. The results of our study suggest that R. eutropha is strongly flagellated in the exponential growth phase and loses a certain number of flagella in transition to the stationary phase. In the stationary phase under conditions permissive for PHB biosynthesis, flagellation of cells admittedly stagnated. However, under conditions permissive for intracellular PHB mobilization after a nitrogen source was added to cells that are carbon deprived but with full PHB accumulation, flagella are lost. This might be due to a degradation of flagella; at least, the cells stopped flagellin synthesis while normal degradation continued. In contrast, under nutrient limitation or the loss of phasins, cells retained their flagella.

Thiolysis of poly(3-hydroxybutyrate) — A novel biochemical reaction

An enzyme involved in the degradation of poly(3-hydroxybutyrate) (PHB), PhaZa1 from Ralstonia eutropha H16, catalyzes the thiolysis of PHB with coenzyme A. The properties of this novel reaction were investigated in vitro. The active center’s cysteine residue was point-mutated to prepare C183S and C183A. The wild type and the two mutants were compared in terms of hydrolytic and thiolytic activity. C183S was as active as the wild type in the hydrolysis, but less active in the thiolysis, of PHB. The cysteine residue was important for the thiolysis of PHB, which may be why it has been conserved in almost all PhaZa1 homologs in various bacteria.Key words: PHB, thiolysis, PHB depolymerase

Poly(3-hydroxybutyrate) (PHB) depolymerase PhaZa1 is involved in mobilization of accumulated PHB in Ralstonia eutropha H16

The recently finished genome sequence of Ralstonia eutropha H16 harbors nine genes that are thought to encode functions for intracellular depolymerization (mobilization) of storage poly(3-hydroxybutyrate) (PHB). Based on amino acid similarities, the gene products belong to four classes (PhaZa1 to PhaZa5, PhaZb, PhaZc, and PhaZd1/PhaZd2). However, convincing direct evidence for the in vivo roles of the gene products is poor. In this study, we selected four candidate genes (phaZa1, phaZb, phaZc, and phaZd1) representing the four classes and investigated the physiological function of the gene products (i) with recombinant Escherichia coli strains and (ii) with R. eutropha null mutants. Evidence for weak but significant PHB depolymerase activity was obtained only for PhaZa1. The physiological roles of the other potential PHB depolymerases remain uncertain.

Purification and molecular cloning of an intracellular 3-hydroxybutyrate-oligomer hydrolase from Paucimonas lemoignei

An intracellular 3-hydroxybutyrate-oligomer hydrolase was purified from a poly(3-hydroxybutyrate)-degrading bacterium, Paucimonas lemoignei. It hydrolyzed the 3-hydroxybutyrate dimer with the highest specific activity of any of the enzymes reported so far. The gene was cloned and sequenced. The deduced amino acid sequence showed that the enzyme is a homolog of the PhaZc of Ralstonia eutropha H16.

Isolated poly(3-hydroxybutyrate) (PHB) granules are complex bacterial organelles catalyzing formation of PHB from acetyl coenzyme A (CoA) and degradation of PHB to acetyl-CoA

Poly(3-hydroxybutyrate) (PHB) granules isolated in native form (nPHB granules) from Ralstonia eutropha catalyzed formation of PHB from (14)C-labeled acetyl coenzyme A (CoA) in the presence of NADPH and concomitantly released CoA, revealing that PHB biosynthetic proteins (acetoacetyl-CoA thiolase, acetoacetyl-CoA reductase, and PHB synthase) are present and active in isolated nPHB granules in vitro. nPHB granules also catalyzed thiolytic cleavage of PHB in the presence of added CoA, resulting in synthesis of 3-hydroxybutyryl-CoA (3HB-CoA) from PHB. Synthesis of 3HB-CoA was also shown by incubation of artificial (protein-free) PHB with CoA and PhaZa1, confirming that PhaZa1 is a PHB depolymerase catalyzing the thiolysis reaction. Acetyl-CoA was the major product detectable after incubation of nPHB granules in the presence of NAD(+), indicating that downstream mobilizing enzyme activities were also present and active in isolated nPHB granules. We propose that intracellular concentrations of key metabolites (CoA, acetyl-CoA, 3HB-CoA, NAD(+)/NADH) determine whether a cell accumulates or degrades PHB. Since the degradation product of PHB is 3HB-CoA, the cells do not waste energy by synthesis and degradation of PHB. Thus, our results explain the frequent finding of simultaneous synthesis and breakdown of PHB.

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.

Biochemical Evidence That phaZ Gene Encodes a Specific Intracellular Medium Chain Length Polyhydroxyalkanoate Depolymerase in Pseudomonas putida KT2442

Polyhydroxyalkanoates (PHAs) can be catabolized by many microorganisms using intra- or extracellular PHA depolymerases. Most of our current knowledge of these intracellular enzyme-coding genes comes from the analysis of short chain length PHA depolymerases, whereas medium chain length PHA (mcl-PHA) intracellular depolymerization systems still remained to be characterized. The phaZ gene of some Pseudomonas putida strains has been identified only by mutagenesis and complementation techniques as putative intracellular mcl-PHA depolymerase. However, none of their corresponding encoded PhaZ enzymes have been characterized in depth. In this study the PhaZ depolymerase from P. putida KT2442 has been purified and biochemically characterized after its overexpression in Escherichia coli. To facilitate these studies we have developed a new and very sensitive radioactive method for detecting PHA hydrolysis in vitro. We have demonstrated that PhaZ is an intracellular depolymerase that is located in PHA granules and that hydrolyzes specifically mcl-PHAs containing aliphatic and aromatic monomers. The enzyme behaves as a serine hydrolase that is inhibited by phenylmethylsulfonyl fluoride. We have modeled the three-dimensional structure of PhaZ complexed with a 3-hydroxyoctanoate dimer. Using this model, we found that the enzyme appears to be built up from a corealpha/beta hydrolase-type domain capped with a lid structure with an active site containing a catalytic triad buried near the connection between domains. All these data constitute the first biochemical characterization of PhaZ and allow us to propose this enzyme as the paradigmatic representative of intracellular endo/exo-mcl-PHA depolymerases.

Microscopical investigation of poly(3-hydroxybutyrate) granule formation inAzotobacter vinelandii

Poly(3-hydroxybutyrate) (PHB) granule formation in Azotobacter vinelandii was investigated by laser scanning fluorescence microscopy after staining the cells with Nilered and Baclight. Cells that had been starved for a carbon source for > or =3 days were almost free of PHB granules. Formation of visible PHB granules started within 1-2 h after transfer of the cells to a medium permissive for PHB accumulation. Fluorescent PHB granules at the early stages of formation were exclusively found in the cell periphery of the 2-3 mum ovoid-shaped cells. After 3 h of PHB accumulation or later, PHB granules were also found to be detached from the cell periphery. Our results indicate that PHB granule formation apparently begins at the inner site of the cytoplasmic membrane. This finding is different from previous assumptions that PHB granule formation occurs randomly in the cytoplasm of PHB-accumulating bacteria.

Fermentative production of (R)-(−)-3-hydroxybutyrate using 3-hydroxybutyrate dehydrogenase null mutant of Ralstonia eutropha and recombinant Escherichia coli

Two systems, one using an (R)-(-)-3-hydroxybutyrate dehydrogenase (BDH) null mutant of Ralstonia eutropha and the other using a recombinant Escherichia coli strain containing a synthetic poly[(R)-(-)-3-hydroxybutyrate] (PHB) operon and an extracellular PHB depolymerase gene, were used for the fermentative production of (R)-(-)-3-hydroxybutyrate (3HB). The concentration of 3HB in the culture supernatant of the mutant R. eutropha system reached about 30 mM after 5 d under anaerobic conditions, although it was about 4-10 mM under aerobic conditions. On the other hand, the 3HB concentration in the culture supernatant of the recombinant E. coli system reached about 70 mM after 4 d, indicating that about 70% of the glucose added was converted to 3HB.

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.

Poly(3-hydroxybutyrate) granules at the early stages of formation are localized close to the cytoplasmic membrane in Caryophanon latum

Localization of newly synthesized poly(3hydroxybutyrate) (PHB) granules was determined by confocal laser scanning fluorescence microscopy of Nile red-stained cells and by transmission electron microscopy (TEM). PHB granules of Nile red-stained living cells of Caryophanon latum at the early stages of PHB accumulation were frequently found at or close to the cytoplasmic membrane. TEM analysis of the same culture revealed electron-translucent globular structures resembling PHB granules that were nonrandomly distributed in the cell lumen but were frequently found at or close to the cytoplasmic membrane. Immunogold labeling using PHB-specific antiserum confirmed that the electron-translucent structures represented PHB granules. Electron microscopy examination of PHB granules after cell lysis revealed that PHB granules were often associated with membrane vesicles. Nonrandom localization of PHB granules was also found in Beijerinckia indica. Cells of this species harbored one PHB granule at each cell pole. Our results show that newly synthesized PHB granules often are close to or even in physical contact with the cytoplasmic membrane. Possible explanations for this unexpected finding and a hypothetical model of PHB granule formation in C. latum are discussed.

Identification and characterization of the Bacillus thuringiensis phaZ gene, encoding new intracellular poly-3-hydroxybutyrate depolymerase

A gene that codes for a novel intracellular poly-3-hydroxybutyrate (PHB) depolymerase has now been identified in the genome of Bacillus thuringiensis subsp. israelensis ATCC 35646. This gene, previously annotated as a hypothetical 3-oxoadipate enol-lactonase (PcaD) gene and now designated phaZ, encodes a protein that shows no significant similarity with any known PHB depolymerase. Purified His-tagged PhaZ could efficiently degrade trypsin-activated native PHB granules as well as artificial amorphous PHB granules and release 3-hydroxybutyrate monomer as a hydrolytic product, but it could not hydrolyze denatured semicrystalline PHB. In contrast, purified His-tagged PcaD of Pseudomonas putida was unable to degrade trypsin-activated native PHB granules and artificial amorphous PHB granules. The B. thuringiensis PhaZ was inactive against p-nitrophenylpalmitate, tributyrin, and triolein. Sonication supernatants of the wild-type B. thuringiensis cells exhibited a PHB-hydrolyzing activity in vitro, whereas those prepared from a phaZ mutant lost this activity. The phaZ mutant showed a higher PHB content than the wild type at late stationary phase of growth in a nutrient-rich medium, indicating that this PhaZ can function as a PHB depolymerase in vivo. PhaZ contains a lipase box-like sequence (G-W-S(102)-M-G) but lacks a signal peptide. A purified His-tagged S102A variant had lost the PHB-hydrolyzing activity. Taken together, these results indicate that B. thuringiensis harbors a new type of intracellular PHB depolymerase.