Medium-Chain-Length PHA

Medium-Chain-Length PHAs [poly(HAMCL)s]. 58... 

Medium-Chain-Length-PHAs Containing only 3-Hydroxy-n-... 

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. 

Second, some organisms are able to incorporate longer pendent chains yielding another class of PHA medium chain length PHA, poly(HAMCL). Poly (HAmcl) is specifically accumulated by fluorescent pseudomonads. When aliphatic hydrocarbons like n-alkane, n-alkanoate, or n-alkanol serve as feedstocks for Pseudomonas oleovorans the resulting PHA is a random copolymer... 

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

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)... 

In addition to these rigid materials, the elastomeric medium-chain-length PHAs produced by fluorescent Pseudomonas are now emerging which can be cross-linked by conventional techniques to yield a biodegradable rubber. A typical application of this material would be a paper coating. 

Polyhydroxybutyrate [PHB] is the most common type of polymer that falls in the category of polyhydroxyalkanoates. PHAs are generally classified into short-chain-length PHA (sCL-PHA) and medium-chain-length PHA (mCL-PHA) by the different number of carbons in their repeating units. 

The second pathway involves the production of PHAs via the fatty acid degradation pathway. In this case, the resulting monomers in the polymer chain were similar in structure to the carbon source or shortened by 2, 4 or 6 carbon atoms [29]. In this pathway the fatty acids are first converted to the corresponding acyl-CoA which are then oxidised by the /1-oxidation pathway via enoyl-CoA, (5)-3-hydroxyacyl-CoA and 3-ketoacyl-CoA precursors. Finally, enzymes like the enoyl-CoA hydratase, hydroxyacyl-CoA epimerase, and /3-ketoacyl-CoA reductase connect the /1-oxidation pathway with the medium-chain length PHA biosynthesis through the PHA synthase [30]. 

Notes PHB Poly-3-hydroxybutyrate 3HB 3-hydroxybutyrate 3HV 3-hydroxyvalerate 4HB 4-hydroxybutyrate 3HH 3-hydroxyhexanoate 3HO 3-hydroxyoctanoate sc/-PHA short chain length PHA mc/-PHA medium chain length PHA. 

Fig. 6 Genetic organization of the clusterpfcjC7ZC2DF/involved in the synthesis of medium-chain-length PHAs. The putative functions of some medium-chain-length PHA proteins are indicated...

De Roo G, Ren Q, Witholt B, Kessler B (2000) Development of an improved in vitro activity assay for medium chain length PHA polymerase based on coenzyme A release measurements. J Microbiol Methods 41 1—8... 

Romano A, Van der Plas LHW, Witholt B, Eggink G, Mooibroek H (2005) Expression of poly-3-(R)-hydroxyalkanoate (PHA) polymerase and acyl-CoA-transacylase in plastids of transgenic potato leads to the synthesis of a hydrophobic polymer, presumably medium-chain-length PHAs. Planta 220 455-464... 

Such copol5mers can be produced under controlled growth conditions as a defined mixture of substrates for a certain type of microorganisms is offered. In this way, a well defined and reproducible copolymer will be formed (22). PHAs can be produced by both wild-t e and recombinant bacteria. The production of medium chain length PHAs occurs in pseudomonas (23). 

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... 

PHAs demonstrate a wide range of physical and mechanical properties depending on the number of carbon atoms in the constituent monomer units of the polymer chain. There are two main types of PHAs. SCL-PHAs have three to five carbon atoms in their monomer units whereas medium chain length PHAs or MCL-PHAs have 6-14 carbon atoms in their monomer units. SCL-PHAs tend to be hard, crystalline, and brittle polymers with high melting points whereas MCL- LCL-PHAs are usually soft and elastomeric with lower melting points compared to SCL-PHAs. 

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