Different Types of PHA

PHA synthases are the key enzymes of PHA biosynthesis. These enzymes catalyze the covalent linkage between the hydroxyl group of one and the carboxyl group of another hydroxyalkanoic acid. The substrates of PHA synthases are the coenzyme A thioesters of hydroxyalkanoic acids there is no evidence that PHA synthases can utilize either free hydroxyalkanoic acids or other derivatives of hydroxyalkanoic acids. With respect to size, structure, and substrate specificity, three different types of PHA synthases (I, II, and III) can be distinguished (see below). 

There are many different types of PHA, distinctly characterised by chain length, type of functional group and degree of unsaturated bonds. A higher degree of unsaturation increases the rubber qualities of a polymer, and different functional groups change the physical and chemical properties of a polymer. 

Another terpolymer poly(3-hydroxybutyrate-co-4-hydroxybutyrate-co-3-hydroxyhexanoate) [P(3HB-co-4HB-co-3HHx)] was found to have better thermal stability due to the introduction of 4HB and 3HHx monomers into P(3HB) (Xie and Chen 2008). It has lower crystalUnity and better flexibility compared to P(3HB) homopolymer and copolymers. Poly(3-hydroxybutyrate-C( -3-hydroxyvalerate-co-4-hydroxybutyrate) [P(3HB-co-3HV-co-4HB)] also showed superior properties over those of 3HB and its copolymer (Madden et al. 2000 Chanprateep and Kulpreecha 2006). Table 2.3 shows the comparison of different types of PHA polymer. 

Varieties of blends with different types of PHAs have been produced. P(3HB) is the most common, lowest cost and commercially available member of the PHAs. Although P(3HB) can be considered as a polymer of high potential for numerous applications, its low crystallization rate and brittle nature is a disadvantage in some cases [69]. Several smdies were carried out to enhance P(3HB) mechanical and... 

The use of nuclear magnetic resonance (NMR) in quantifying PHA was fully explored starting from 1980 onwards. NMR is a non-invasive approach, which can be accurately used to determine the different types of PHA consisting of a variety of chemical structures. 

PHA Block copolymerization and grafting reactions, chlorination, cross-linking, epoxidation, and hydroxyl and carboxylic acid functionalization of the PHA Chen et al. (2009), Wu et al. (2008), Li et al. (2003), Loh et al. (2007), Baki and Steinbuchel (2007) Blends with natural fibers, PLA, PCL, and difference types of PHA, even with inorganic particles, such as bioglass and tricalcium phosphate Avella et al. (2000), Urakami et al. (2000), Misra et al. (2006), Luo etal. (2007)... 

Blending two different types of PHAs or PHAs with synthetic or other natural polymers have generated novel biomaterials to suit specific biomedical applications. 

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. 

Different metabolic pathways guide the production of different types of PHAs. Thus the choice of carbon source for the production of these PHAs has unequal importance for PHAscl and PHAmcl- For PHAmcl. the choice of the carbon source is cmcial for the final structure of the biopolymer. On the contraiy, for PHAscl. almost any carbon source can be used. Thus, more in depth studies in this field were performed for PHAscl- K was thus shown that the Cupriviadus genus has great potential for this type of production. Recent work on mixed microbial cultures has also shown an interesting perspective, especially knowing that these systems are usually cheaper to establish. Also, for a long time, the assumed hypothesis of the necessity of... 

Polyhydroxyalkanoates (PHAs) are biodegradable and biocompatible polyesters with versatile structural compositions. Bacterial PHAs are produced using a combination of renewable feedstock and biological methods mostly via a fermentation process. Native and recombinant microorganisms have been generally used to produce different types of PHAs, such as homo-polymers and copolymers of diverse morphology. Alternative production schemes of PHAs in vitro based on cell-free enzymatic catalysis are gaining momentum and may become the preferred route to some specialty products. ... 

With respect to size and structure, type I- and type II-PHA synthases are distinguished from type III-PHA synthases. Whereas type I- and type II-PHA synthases consist of only one type of subunit, type III-PHA synthases consist of two different types of subunits. The differences between type I- and type II-PHA synthases regarding the substrate specificity will be described in the next section. 

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. 

Table 1. Chemical Composition of Medium Chain Length PHAs Synthesized by Pseudomonas putida KT2442 Diming Growth on Different Types of Carbon Sources3... 

Whereas class-I and class-II PHA synthases consist of only one type of subunit exhibiting MWs in the range of about 60 to 65kDa in most cases, class-III and class-IV PHA synthases consist of two different types of subunits each exhibiting a size of about 40 plus 40kDa (class-III) or of about 40 plus 22kDa (class-IV), respectively. 

Biotechnology studies have shown that PHA is produced in controlled, balanced growing conditions, by feeding and then starving the micro-organisms of nutrients [ALB 11, ESC 11], Depending on which hydrocarbon substrates and microorganisms are used, different types of (co)polymers and macromolecular architectures can be obtained [POL 11],... 

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