Structures of Polyhydroxyalkanoate Synthase PhaC from <i>Aeromonas caviae</i> , Producing Biodegradable Plastics

Polyhydroxyalkanoate (PHA) is a biodegradable polyester that can serve as a promising alternative to petrochemical plastics, which present a serious source of pollution. PHA synthase (PhaC) is a key enzyme responsible for producing a wide variety of PHAs in microorganisms. Here, we present crystal structures of full-length PhaC from Aeromonas caviae, a high-performance PhaC employed for industrial use. The structure reveals an N-terminal helical domain that mediates head-to-head dimerization and stabilizes the C-terminal α/β catalytic domain to form a tunnel that connects the catalytic center embedded inside the protein to the protein surface. We showed that this tunnel is a putative egress tunnel for the product PHA chain. Our results establish a fundamental understanding of the PhaC machinery that should lead to improvement of this enzyme in industrial applications.