Medium chain length PHAs MCL-PHA

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. 

Two major classes of PHAs are known to exist. PHAs whose polymeric units are predominantly five carbons or less are classified as short-chain-length PHAs (SCL PHAs) while all others are referred to as medium-chain-length PHAs (MCL PHAs). The term MCL was coined because the number of carbons in the monomers roughly corresponds to those of medium-chain-length carboxylic acids. PHA nomenclatme may still be in a state of flux as new stnictmes continue to be discovered. 

Conversion of grass biomass into fermentable sugars and its utilization for medium chain length polyhydroxyalkanoate (mcl-PHA) production by Pseudomonas strains. Bioresour. Technol, 150, 202 - 209. 

Medium chain length polyhydroxyalkanoate (mcl-PHA) production from volatile fatty acids derived from the anaerobic digestion of grass. Appl. Microbiol. Biotechnol, 98 (2), 611-620. 

Kellerhals MB, Kessler B, Witholt B, Tchouboukov A, Brandi H (2000) Renewable long-chain fatty acids for production of biodegradable medium-chain-length polyhydroxyalkanoates (mcl-PHAs) at laboratory and pilot plant scales. Macromolecules 33 4690-4698 Kessler B, Witholt B (1998) Synthesis, recovery and possible application of medium-chain-length polyhydroxyalkanoates a short overview. Macromol Symp 130 245-260 Kessler B, Witholt B (2001) Factors involved in the regulatory network of polyhydroxyalkanoate metabolism. J Biotechnol 86 97-104... 

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. 

The wide substrate range of PHA synthase has resulted in the versatility of the monomer compositions, which is a clear advantage because the monomer variation provides PHAs an extended spectrum of associated properties. PHAs are classified into three classes according to their monomer compositions Short-chain-length (SCL)-PHAs, medium-chain-length (MCL)-PHAs and short-chain-length-medium-chain-length (SCL-MCL)-PHAs. ... 

Bhatt et al. created blends of medium chain length PHA (mcl-PH A) with NR as well as other types of rubber, and they found that in addition to altering the thermal properties, the blending can also affect the biodegradable properties of PHA and NR. The blend films were effectively degraded by Pseudomonas sp. 202, the degradation ratio of which can be tailored by adjusting the rubber/ PHA ratio. ... 

Kellerrhals M.B., Kessler B., Witholt B., Tchouboukov, Brandi H. Renewable long-chain fatty acids for production of biodegradable medium-chain-length polyhydroxyalkanoates (mcl-PHAs) at laboratory and pilot plant scales. Macromolecules 33 (2000) 4690. 

Medium chain length polyhydroxyalkanoates, MCL-PHAs, produced by bacteria as inclusion bodies or granules were analyzed in situ by differential scanning calorimetry (DSC) without isolation from the cells. The kinetic DSC study of PHA granules, which contained mostly 3-hydroxyoctanoate units (PHO), in Pseudomonas putida BMOl cells showed that the polymer within the granules existed in an amorphous state, but it crystallized after dehydration of the cells under freeze-drying condition (below -50°C) followed by annealing at ambient temperature. In this manner, PHO within the cells... 

Mcl-PHAs are also classified according to the type of substituent in the alkyl in the side chains so that poly(HAMCL)s containing unsubstituted n-alkyl groups are identified as medium-chain-length poly(3-hydroxy-n-alkanoate) [poly (nHAMCL)]. Both poly(nHAMCL) and PHAs containing unusual groups are described in Sect. 3. 

PHAs are generally classified into short chain length (sCL) and medium chain length (mCL) molecules. For example, sCL-PHAs are based on 4 or 5 carbon atoms in their repeating patterns, while mCL-PHAs contain 6 or more carbon atoms... 

The second PHA synthesis pathway (pathway II) is related to fatty acid uptake by microorganisms. After fatty acid P-oxidation, acyl-CoA enters the PHA monomer synthesis process. Enzymes including 3-ketoacyl-CoA reductase, epimerase, (I )-enoyI-CoA hydratase/enoyl-CoA hydratase I, acyl-CoA oxidase (putative), and enoyl-CoA hydratase I (putative) were found to be involved in supplying the PHA precursor 3-hydroxyacyl-CoA for PHA synthesis. Pseudomonas putida, Pesudomonas aeruginosa, and A. hydrophila are able to use pathway n to synthesize medium-chain-length (mcl) PHA or copolymers of (/ )-3-hydroxybutyrate (R3HB) and (R)-3-hydroxyhexanoate (PHBHHx). 

PHAs containing (R)-3-hydroxyalkanoates longer than 3HV, so-called medium-chain-length (mcl) PHAs, constitute elastomers with lower melting points, low glass-transition temperatures and low crystallinity in comparison with scl-PHAs. Table 2 shows the characteristics of representative PHAs with different compositions. 

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