Enzymatic Background PHA Synthases

The type of PHA produced biotechnologically is highly dependent on the microbial production strain. Cupriavidus necator can only polymerize 3HAs consisting of 3-5 C-atoms 

Characteristic Thermoplastic Thermoplastic Thermoplastic -elastomeric Elastomeric Thermoplastic -elastomeric Thermoplastic -Elastomeric Elastomeric Latex 

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. 

Due to this high speciflcity of PHA synthases, it was believed for a long time that one strain can either produce cZ-PHAs or mcZ-PHAs. Some exceptions were recently reported [86-88]. Today it is known that PHA synthases of the strains Thiocapsia pfennigii and Aeromonas caviae constitute exceptions within class III they are able to incorporate as weU scl- as mcl-hydroxyacyl-CoAs [87]. Another exception of the three-class system described above seems to be the polymerizing enzyme isolated from a Bacillus megaterium strain, which, by some scientists, is proposed as a fourth class of PHA synthases [89]. This enzyme type requires phaC genes that are similar to those in class III, but demands phaR instead of phaE [87]. 

At the moment, PHA synthesis genes of extremophiUc archaea are under detailed investigation. Haloferax mediterranei, a representative of the extreme halophilic group of the archaea branch, displays the rare ability to synthesize copolyesters of 3HB and 3HV (PHBHV) from unrelated carbon sources like sugars, hence without the need of a structurally related 3HV precursor [36, 90]. This ability saves costs normally needed for 3HV precursors. 

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