PHAmcl

Synthesis of PHAMCL from fatty acids such as octanoic acid or from the corresponding alkanes such as octane was first detected in P. oleovorans [119]. The alkanes are oxidized to the fatty acids the latter are activated by thiokinases and then degraded via the fatty acid /1-oxidation pathway. Obviously intermediates of this pathway accumulate under conditions favorable for the synthesis of PHA and are subsequently converted into substrates for the PHA synthase. Many reactions for the conversion of an intermediate of the -oxidation cycle into R-(-)-3-hydroxyacyl-CoA were considered. These were ... 

It is worth mentioning that metabolic engineering of E. coli recently provided recombinant strains which synthesized PHAMCL from gluconate. For this, beside phaC2Po or phaClPa> the thioesterase I from E. coli (TesA) [128] or the acyl-ACP thioesterase from Umbellularia californica [129], respectively, were expressed in E. coli. However, the amounts of PHAMCL accumulated in the cells were rather low, and this artificial pathway was not very efficient. 

Separation of PHASCL and PHAMCL into two separate granules within the same cell often seems to occur if two different PHA synthases with non-overlapping substrate ranges are present in the cell. Obviously, each type of PHA synthase participates separately in the initiation of micelle formation of nascent PHA granules, because only PHA molecules of similar structure, which remain bound to the PHA synthase protein, can contribute to the formation of a micelle. When the granules become larger, further PHA synthase molecules can only bind to the surface of the granules if they represent the same type of PHA synthase. 

Phages, display of peptides and proteins on, 12 474-475 Phagocytosis, 18 253 PHAmcl biosynthetic genes, 20 258. See also Medium-chain-length PHA (PHAmcl)... 

PHAmcl (medium length side chains monomer unit has more than five carbon atoms)... 

Fig. 11 Compounding technologies of PHAmcl and PHB copolymers in dependence of composition and molecular weight [48, 49]...

Polyhydroxyalkanoates can be synthesized in vitro in cell-free systems with purified PHA synthase protein. This has been demonstrated several times not only with different PHASCL synthases [34-37] but also with PHAMCL synthases [38]. 

Several PHA producing microorganisms as well as several types of PHA were discovered. " The whole PHA family can be sub-divided into three main categories the short-chain length PHA (PHAscl), the medium-chain length PHA (PHAmcl) and rarer PHA (Figure 1.1). The structural differences inside the PHA family imply deep differences in their thermo-mechanical... 

PHAmcl are mainly produced by the Pseudomonads.They are usually synthesized as copolymers of two or three or even more monomers, obtained by p-oxidation of fatty acids used as feeding substrates, the monomeric parts usually bear n 2 carbons. One noticeable exception to this general rule is the recently reported Pseudomonas mendocina strain able to produce pure homopolymers of poly-3-hydro q octanoates (PHO). ... 

In contrast to this, a much more complex model of the surface of PHA granules was proposed for bacteria accumulating PHAmcl- Electron microscopical studies suggested that the surface consists of two distinct crystalhne protein lattice layers which are presumably separated by a phospholipid layer [76]. These studies also revealed the presence of two major proteins of 20 kDa and 43 kDa as grana-associated proteins in P. oleovorans [76] that were also found in our laboratory [ 148]. The larger protein was localized in the outer protein layer [76]. In P. aeruginosa we identified proteins of 15 kDa and 41 kDa as the major grana-associated proteins [148]. 

PHAs such as PHB and PHBV are brittle, which is related to their high crystalline degree and they may lack the superior mechanieal properties required for biomedical and packaging applications. The physical properties of P(3HB-co-4HB) and P(3HB-co-3HP) show some improved flexibility as biodegradable thermoplastics compared to those of PHB, as the 4HB and 3HP monomer fractions in the copolymer increase, respeetively. On the other hand, PHAmcl may be elastomeric but have very low mechanieal strength. These properties are a consequence of PHAs ehemical strueture. Therefore, since these different types of PHAs have various struetural and physical chemical properties, they should be classified according to their properties and modified in order to be easy to use for target applications. 

The biosynthesis of PHAscl and PHAmcl is performed following different metabolic pathways (Figure 2.2). Thus, PHAscl are synthesized in three main steps starting with acetyl-CoA, whereas PHAmcl can be synthesized by two different pathways. The first pathway consists of the p-oxidation of fatty acids prior to their incorporation in the polyester chain, the second consists of the de novo synthesis of fatty acids. ... 

These different metabolic pathways show the importance of different carbon sources. Indeed, different carbon sources can be used for the production of PHAscl, whereas the choice of the carbon source deeply influences the structure of the resulting PHAmcl, therefore this category of PHAs is discussed in the section on structure. 

PHAmcl are usually more biocompatible than PHAscl- Indeed, recent studies on PHA degradation products have shown that the cytotoxicity of the oligo-hydrojyalkanoates (OHA) decreased with the increasing OHA side chain length. However, the most abundant studies have concerned P4HA and the PHAscl-co-PHAmcl due to their larger accessibility and specific properties. 

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