Producers of PHA
PHA biosynthesis represented by Ralstonia eutropha
R. eutropha is among the bacteria that have been extensively studied for the production of PHA. In R. eutropha , two acetyl-CoA moieties are condensed to acetoacetyl-CoA by a ß-ketothiolase (PhaA). The product then undergoes reduction by an NADPH-dependent reductase (PhaB) which produces the (R)-isomer of 3-hydroxybutyryl-CoA. R. eutropha was also capable of producing the P(3HB) homopolymer from even carbon numbered n-alkanoates while odd-carbon numbered n-alkanoates resulted in the accumulation of copolymers of 3HB and 3HV .
Based on the various types of PHA that were synthesized by R. eutropha, one common observation is that the incorporated monomers always contained only 3 to 5 carbon atoms. This had lead to the conclusion that in R. eutropha, the PHA synthase enzyme which polymerizes the monomers is only active towards SCL HA.
PHA biosynthesis represented by the pseudomonads
The pseudomonads belonging to the rRNA-homology-group I can synthesize PHAMCL from various alkanes, alkanols, or alkanoates . In contrast to R. eutropha, most fluorescent pseudomonads belonging to rRNA-homology-group I generally do not synthesize PHA containing SCL monomers (PHASCL). Bacteria in this group, derives the 3-hydroxyacyl-CoA substrates of C6 to C14 for PHAMCL synthase, from the intermediates of fatty acid ß-oxidation pathway. Most of the rRNA-homology-group I pseudomonads except P. oleovorans, can also synthesize PHA containing MCL monomers (PHAMCL) from unrelated carbon sources such as carbohydrates. These bacteria accumulate PHA containing 3-hydroxydecanoate (3HD) as the predominant monomer from various carbon sources such as gluconate, fructose, acetate, glycerol, and lactate. Some Pseudomonas strains are also capable of accumulating both 3HB and MCL monomers from various carbon sources.
Recombinant Escherichia coli
E. coli is considered a better commercial producer of PHA because it can use a wider range of cheap carbon sources, and also because it is easier and less costly to purify the polymer from this bacteria. Besides that, since E. coli do not have an intracellular PHA depolymerase (because E. coli is not a natural PHA producer), the synthesized PHA will not be degraded.
Ever since the first successful expression of the R. eutropha PHA biosynthetic genes [11, 12, 13], various other heterologous genes have been introduced into E. coli resulting in the ability to produce both PHA homopolymers and copolymers. In recombinant microorganisms, plasmid stability is of crucial importance for continued PHA production.
Ever since the first successful expression of the PHA biosynthetic enzymes in plant, much knowledge about the potential of this system has been obtained. It is possible to produce various kinds of PHA homopolymers and copolymers in transgenic plant and this system appears to be the most cost-effective PHA producer because in principle the PHAs are produced from CO2, H2O and sunlight. If transgenic plants were to become the major approach for producing PHAs cost-effectively, competition between the use of fertile agricultural land for food and for plastics production would be inevitable.