The Various Types of PHA

Microorganisms in nature are capable of synthesising various types of PHA depending on the types of carbon sources available and the biochemical pathways that are operating in the cell. Ever since monomers other than R-3-hydroxybutyrate (3HB) were identified in environmental samples [9,10], much effort has been directed to identify all the various types of monomers that can possibly be incorporated by PHA synthase. It is now possible to synthesise various PHA homopolymers and copolymers that have a certain monomer composition. 

P[3HB], they can be divided into three distinct groups, i.e., low MW P13HB] [14-16], high MW P[3HB] [1, 8, 12], and ultra-high molecular weight (UHMW) P[3HB] [17,18]. 

The low MW P[3HB] which is also known as complexed P[3HB] (cP[3HB]) is an ubiquitous cell constituent that exists in Eubacteria, archaebacteria, and eukaryotes [15, 16, 19, 20]. Recent studies have also revealed the presence of cP[3HB] in humans [21]. This cP[3HB] consists of about 120-200 3HB units and have a MW of about 12,000 Da [22]. Depending on the strength of their association with macromolecules, chloroform-soluble and chloroform-insoluble cP[3HB] have been identified [23]. The former forms a weakly bound (non-covalent) complex with polyphosphate salts while the latter is usually strongly bound (covalent) complex with proteins. These complexes are thought to function as ion (Ca ) transport channels across cell membranes and also may facilitate the uptake of extracellular deoxyribonucleic acid (DNA) material [23-25]. 

Recently, the production of UHMW P[3HB] (MW 3,000,000) has been achieved by using a recombinant Escherichia coli cultivated under specific fermentation conditions [17]. Unlike the high MW P[3HB] that is characterised by stiffness and brittleness, the UHMW P[3HB] seems to show improved characteristics [18]. In addition, it was also found that films prepared from this UHMW P[3HB] were completely degraded at 25 °C in a natural freshwater river within three weeks [27]. 

Interestingly, the recently found UHMW P[3HB] seems to possess better cbaracteristics [27]. Mechanical properties of the stretched and annealed UHMW P[3HB] films remained unchanged for six months at room temperature [38]. Further work is in progress to determine the applicability of this UHMW P[3HB]. At the moment, the P[3HB] homopolymer is yet to acquire any significant economic importance. 

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