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Section 6: Future Outlook – Synthetic Biology & PHA Production

Synthetic biology is a fairly new field of research, only a decade old and has now been projected as the field of the future. This field calls together a broad range of scientists to collaborate in a multidisciplinary effort to better understand ways to manipulate the genetic parts of an organism in order to develop technologies and design biological organisms that can carry out targeted tasks for certain desired outcomes. With a great potential to revolutionize fields like biotechnology, its benefits could be invested towards improving upon the production of PHAs. One of these attempts has been successfully achieved through the targeted secretion of PHB from a bacterial cell but there is still plenty of room for more improvements. The section below looks at areas where synthetic biology techniques could be exploited to improve the PHA production process.


6.1    PHA Granule-Associated Proteins

After PHA biosynthesis, the PHA polymer formed aggregates into a spherical granule with its core containing the hydrophobic PHA polyester while attached to it surface are proteins, which include PHA synthase, PHA depolymerase, structural and regulatory proteins. As previously mentioned, PHA synthase (PhaC) is the key enzyme that catalyzes the conversion of 3-hydroxyacyl-CoA to the PHA polymer while PHA depolymerase (PhaZ) degrades the polymers either intracellularly by the accumulating bacteria or extracellularly by other bacteria in the environment capable of secreting this enzyme. Interestingly, the most abundant protein found on the surface of the PHA granule is the PHA phasin. These phasins are located on the interface between the hydrophobic polymer and the hydrophilic cytoplasm giving them an amphiphilic advantage, which they use to stabilize PHA polymer and hence preventing the granules from coalescing to each other [102]. Reports have also shown that the phasins accelerate the rate of PHA synthase activity, hence promoting the synthesis of PHB [103] while other studies have shown that phasins are needed to activate PHA synthase in some bacterial strains [104]. Phasins have also been demonstrated to affect the size and number of the granules formed in the cell. In one study, when PhaP mutant cells were induced for PHB accumulation, these researchers observed significantly low PHB synthesis rates and only one large granule was accumulated within the cell [105]. Another group of researchers later showed that when PhaP1 (one of the phasin genes) was overexpressed in the cell, multiple granules were formed within the cell [106]. This study gave more insight to the fact, unlike the other phasin genes (PhaP2-4), the PhaP1 gene specifically played a major role in PHB accumulation since the accumulation impacts were more obvious when PhaP1 was absent from the cell. Several studies have gone further the prove that phasins are indeed relevant for PHB accumulation and their stabilization alone can be a good technique to achieve high PHB accumulation rates [107,108]. It is important to highlight that the absence or presence of the phasins did not affect the molecular weight of the polymer, which means the mechanical properties of the polymer were intact [107]. With the growing knowledge base of PHA metabolism and its granule formation, we can leverage all the current information and use them to build genetic packages to develop organisms capable of producing PHA at high accumulation rates to achieve even higher PHA yields. However, the cost of the process will need to be further evaluated to make the investment more realistic and researchers will need to be trained extensively on ways to carry out these methods more efficiently. 



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