Uncommon PHA Constituents

The PHA synthases of the Pseudomonads are probably the most versatile with broad substrate specificity. In Pseudomonas oleovorans for example, it is possible to synthesise PHA consisting of saturated, unsaturated, halogenated, branched and aromatic 3-hydroxyalkanoates with 6-14 carbon atoms [89-92]. 

1 Conditions that Promote the Biosynthesis and Accumuiation of PHA in Microorganisms 

An attractive feature of the microbial PHA is the ability to produce them using renewable carbon sources. The plastic materials widely in use today are synthesised from fossil fuels 

Although it was known that the intermediates of the p-oxidation cycle are channelled towards PHA biosynthesis, only recently the precursor sources were identified. In A. caviae, the p-oxidation intermediate, trans-2-tnoy -CoK is converted to (R)-3-hydroxyacyl-CoA via (R)-specific hydration catalysed by a (R)-specific enoyl-CoA hydratase [122, 123]. Subsequently, Tsuge and co-workers [124] reported the identification of similar enoyl-CoA hydratases in F. aeruginosa. In the latter case, two different enoyl-CoA hydratases with different substrate specificities channelled both SCL and MCL enoyl-CoA towards PHA biosynthesis. In recombinant . coli it was further shown that 3-ketoacyl-CoA intermediates in the p-oxidation cycle can also be channelled towards PHA biosynthesis by a NADPH-dependent 3-ketoacyl-acyl carrier protein (ACP) reductase [125]. A similar pathway was also identified in F. aeruginosa [126]. In addition, it was also reported that the acetoacetyl-CoA reductase (PhaB) of R. eutropha can also carry out the conversion of 3-ketoacyl-CoA intermediates in Pathway II to the corresponding (R)-3-hydroxyacyl-CoA in E. coli [127]. The results clearly indicate that several channelling pathways are available to supply substrates from p-oxidation cycle to the PHA synthase. This explains why it was not possible to obtain mutants that completely lack PHA accumulation ability, unless the mutation occurred in the PHA synthase gene [128]. 

Among the various metabolic pathways that are involved in PHA biosynthesis, the fatty acid de novo biosynthesis pathway (Pathway III) is of particular interest because of its 

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