Polyester bacterial

Polyhydroxyalkanoates (PHAs) such as poly(4-hydroxybutyric acid) (P(4HB)), poly(4-hydroxyvalerate) (P(4HV)), and their copolymers are bacterial polyesters synthesized by microbial fermentation. They have been described in detail in the earlier section on hydrolytically resorbable biopolymers. They are also found to be resorbable by enzymatic action in vivo. This particular family of bacterial polyesters is one of the most promising biomaterials currently under investigation. 

The enzymatic hydrolysis of poly-p-hydroxybutyrate, PHB, by several different bacteria, which are known to secrete active esterases, has been studied in some detail by several research groups [7, 8]. As with the polysaccharides, the final products of these degradation reactions are the monomers, dimers and trimers, which are removed by hydrolysis only from hydroxyl-end of the polymer chain, as follows  

These products are water soluble and can diffuse through the surrounding aqueous environment until they come in contact with, and are taken up by, the cells and used as nutrients. 

The exoeellular enzymes produced by three different polyester-degrading bacteria have been characterized using native granules of PHB as the substrate. The bacteria studied were the following (1) two Pseudomonas strains which 

As can be seen from these data, the order of activity was At A2 Bi B2 for PHB as the substrate, and for the oligomer substrates, the order was Ms M4 M3 (M2 unreactive) for enzymes At and B2. However, the hydrolysis products from PHB with both A, and B2 were only the monomer and the M2 and M3 oligomers, and no higher oligomers were formed. With Al the product distribution was 23% Mt + 57% M2 + 20% M3, and with B2 34% Mi + 19% M2 + 47% M3. For each of the oligomers as substrates, the relative frequencies of attack (relative rates of hydrolysis) at the internal ester groups with each of these two enzymes, Ai and B2, is shown schematically below  

The depolymerase secreted by A. faecalis in the presence of PHB also had a molecular weight of 50 kDa, but it was much more active than the enzymes from P. lemoignei as indicated by a much lower KM value for the former with the native PHB granules [17a]. Furthermore, the hydrolysis of PHB catalyzed by this enzyme produced almost entirely the dimer, and the dimer and monomer were the only products obtained from the hydrolysis of the M3, M4 and Ms oligomers with this enzyme. 

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Poly(3-hydroxybutyrate) (1.8) is a bacterial polyester that behaves as an acceptable thermoplastic, yet can be produced from renewable agricultural feedstocks and is biodegradable. It is typically produced not in the pure state. 

RA Gross. Bacterial polyesters Structural variability in microbial synthesis. In SW Shalaby, ed. Biomedical Polymers Designed-to-Degrade Systems. Cincinnati, OH Hanser/Gardner, 1994, pp 173-188. 

Keywords. Bacterial polyester, Medium-chain-length poly(3-hydroxyalkanoates), Pseudomonas oleovorans, Pseudomonas putida, Functional poly(3-hydroxyalkanoates), Short-chain-length poly(3-hydroxyalkanoates)... 

S. Y. Lee, Y. Lee, and F. Wang, Chiral compounds from bacterial polyesters Sugars to plastics to fine chemicals, Biotechnol. Bioeng., 65 (1999) 363-368. 

The copolymer poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-co-PHV) produced by A eutrophus has generated more interest than poly-(R)-3-hydroxybutyrate (PHB) homopolymer. Since these bacterial polyesters are biodegradable thermoplastics, their mechanical and physical properties have received much attention. PHB is a relatively stiff and brittle material because of its high crystallinity. However, the physiochemi-cal and mechanical properties of [P(HB-HV)] vary widely and depend on the molar percentage of 3-hydroxyvalerate (HV) in the copolymer (4,5) as shown inTable 1. Propionic acid is converted by a synthetase to propionyl-CoA, and the biosynthetic P-ketothiolase catalyzes the condensation of propionyl-CoA with acetyl-CoA to 3-ketovaleryl-CoA by the acetoacetyl-CoA reductase. The hydroxyvaleryl moiety is finally covalently linked to the polyester by the PHA synthase (6). 

It is clear that green polymers, as defined by their biodegradability, are almost exclusively biopolymers. The major classes of biopolymer of interest here are proteins and polysaccharides, naturally occurring biopolymers, and these are subdivided into various sub-classes, with different applications, as described above. Other polymers of interest are the bacterial polyesters and polylactides. All of these polymers have the potential to be processed into new materials, but clearly not all of these will have either attractive properties or be economically viable materials. 

Chemical structure and some physical properties of bacterial polyesters... 

Poly(3-hydroxybutyrate) (1.8) is a bacterial polyester that behaves as an acceptable thermoplastic, yet can be prodticed from renewable agricultural feedstocks and is biodegradable. It is tyjiically produced not in the pure state, but formed alongside minor amounts of poly(3-hydroxyv alerate). The ratio of these two polymers in a given sample is determined by the ratio of glucose and propionic acid in the medium in which the bacteria live and carry out their metabolic processes. 

Lenz, R. W. and Marchessault, R. m.. Biomacromole. 2005, 6(1), Bacterial Polyesters Biosynthesis, Biodegradable Plastics and Biotechnol. 

Marois, Y Zhang, Z. Vert, M. Deng, X. Lenz, R. and Guidoin, R. J. Biomater. Sci. Polym. Ed. 1999,10, 483 99. Hydrolytic and Enzymatic Incubation ofPolyhy-droxyoctanoate (PHO) A Short-Term in Vitro Study of a Degradable Bacterial Polyester. 

Bacterial polyesters (polyhydroxy alkanoates) Poly(3-hydroxy butyrate), poly(3-hydroxy valerate) Some micro-organisms, especially bacteria, can synthesise biocompatible aliphatic polyesters, which have potential in medical applications. 

Arostegui SM, Aponte MA, Diaz E (1999) Bacterial polyesters produced by Pseudomonas oleovorans containing nitrophenyl groups. Macromolecules 32 2889-2895 Arpigny JL, Jaeger K-E (1999) Bacterial hpolytic enzymes classification and properties. Biochem J 343 177-183... 

Ewering C, Lutke-Eversloh T, Luftmann H, Steinbuchel A (2002) Identification of novel sulfur-containing bacterial polyesters Biosynthesis of poly(3-hydroxy-S-propyl-w-thioalkanoates) containing thioether linkages in the side chains. Microbiol Mol Biol Rev 148 1397-1406... 

Fritzsche K, Lenz RW, Fuller R (1990a) An unusutil bacterial polyester with a phenyl pendant group. Macromol Chem 191 1957-1965... 

Geller BE (1996) Bacterial polyesters. Synthesis, properties, and application. Russ Chem Rev 65 725-734... 

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