PHA Inclusions Self-Assembly and Structure

The soluble polyester synthase converts into an amphipathic molecule upon availability of substrate and subsequent hydrophobic polyester chain elongation [15]. This leads to self-assembly of so-called PHA granules with the hydrophobic PHA in the core and the active polyester synthases at the surface, representing the water-polyester interface. Analysis of the granule-associated polyester synthase from R. eutropha showed about 40-fold increased enzyme activity, as compared with the soluble enzyme [46]. This data suggests that interfacial activation occurred and a lid-like structure as found in lipases and also found in the R. eutropha polyester synthase model may also play a role in polyester synthases [37]. 

Gerngross and Martin [46] were the first to demonstrate in vitro synthesis of PHA and self-assembly of spherical granules by only using purified polyester synthase and substrate. Their work fully defined that the polyester synthase possesses 

These PHA granules grow in size, while the attached polyester synthases constantly converts precursor from the cytosol and into constituents of the growing polyester chain. However, it is unclear whether larger granules occur because of fusion events or whether simple increase in size on the basis of continuous polymerization takes place. Approximately 5-8 PHA granules are formed intracellularly comprising almost the entire cell volume, when maximum PHA accumulation is obtained [22]. 

When PHA granules are heterologously produced in recombinant . Coli, a few specific E. coli proteins attach to the granule surface, presumably functionally replacing the phasin proteins. 

The non-covalently attached proteins are not vital for PHA granule formation, however, they serve various biological functions, for example, PHA granule structure, PHA biosynthesis gene regulation, and PHA mobilization. Yet, only the covalently attached polyester synthases possess all the inherent properties needed for PHA granule formation. 

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