PHA-Producing Organisms

Organic Acid and Solvent Production Acetic, Lactic, Gluconic, Succinic, and Polyhydroxyalkanoic Acids 

Production of polyhydroxyalkanoic acid by natural and recombinant strains of bacteria 

Bacterium PHA Carbon source Cell density (g/l) PHA content (%) References 

Alcaligenes latus PolyOHB) Sucrose 112 88 Wang and Lee (1997a, b) 

Azotobacter vinelandii PolyOHB) Glucose 40.1 79.8 Page and Cornish (1993) 

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From the MBM hydrolysis process that can provide a useful nitrogen source for PHA-producing organisms (see above), about 11% of lipids remain as surplus material after the degreasing step. 

The first step of the biochemical pathway of PHB synthesis consists in the conversion of a selected carbon source to acetate. Then, an enzyme cofactor is attached via the formation of a thioester bond. The enzyme, called coenzyme A (CoA), is a universal carrier of acyl groups in biosynthesis and acetyl-CoA is a basic metabolic molecule found in all PHA-producing organisms. A dimer acetoacetyl-CoA is formed via reversible condensation and subsequently reduced to a monomer unit (R)-3-hydroxybutyryl-CoA. PHB is formed via the polymerization of the latter, maintaining the asymmetric centre [5]. The basic simplified process is shown in Scheme 22.1. 

Intracellular degradation of PHAs in the PHA-producing organisms is a physiological process, and this activity can potentially influence the quality and yield of PHAs to be isolated... 

The production of PHA using residual oil from biotechnological rhamnose production as a carbon source for growth of C. necator H16 (the nomenclature in the article was "Ralstonia eutropha") andP. oleovorans-was described by Fiichtenbusch et al. (2000). The strains accumulated PHA at 41.3 and 38.9%, respectively, of the cell dry mass when they were cultivated in defined media with oil from the rhamnose production as the sole carbon source. The accumulated PHA isolated from C. necator was identified as PHB homopolyester, whereas the PHA isolated from P. oleovorans consisted, typically for this type of PHA-accumulating organism, of (P)-3-hydroxyhexanoicacid, (P)-3-hydroxyoctanoicacid, (/ )-3-hydroxydecanoic acid and (P)-3-hydroxydodecanoic acid. Approximately 20-25% of the carbon components of the residual oil were converted into PHA. Up to 80% of cell dry mass of PHB homopolyester from different plant oils was produced by C. necator DSM 545 (Fukui and Doi 1998). 

The large variability in the chemical structure and material properties of PHAs is due to the low substrate specificity of PHA synthases and the subsequent modifications by chemical reactions. The monomer composition, macromolecular structure and physical chemical properties of PHA vary depending on the producer organism as well as the carbon source used for the growth. Their properties span a wide range, including materials that resemble polypropylene and others that are elastomeric. 

Also of considerable interest to organic chemists, in both cases the substrates were fed as racemic mixtures, and with 5-methyloctanoate the PHA produced was enriched with one of the optical isomers by a ratio of approximately 7 1, as indicated by analysis of the copolymer by NMR spectroscopy. ... 

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