Structure of polyhydroxyalkanoate (PHA) synthase PhaC from Chromobacterium sp USM2, producing biodegradable plastics

Polyhydroxyalkanoate (PHA) is a promising candidate for use as an alternative bioplastic to replace petroleum-based plastics. Our understanding of PHA synthase PhaC is poor due to the paucity of available three-dimensional structural information. Here we present a high-resolution crystal structure of the catalytic domain of PhaC from Chromobacterium sp. USM2, PhaC(Cs)-CAT. The structure shows that PhaC(Cs)-CAT forms an alpha/beta hydrolase fold comprising alpha/beta core and CAP subdomains. The active site containing Cys291, Asp447 and His477 is located at the bottom of the cavity, which is filled with water molecules and is covered by the partly disordered CAP subdomain. We designated our structure as the closed form, which is distinct from the recently reported catalytic domain from Cupriavidus necator (PhaC(Cn)-CAT). Structural comparison showed PhaC(Cn)-CAT adopting a partially open form maintaining a narrow substrate access channel to the active site, but no product egress. PhaC(Cs)-CAT forms a face-to- face dimer mediated by the CAP subdomains. This arrangement of the dimer is also distinct from that of the PhaC(Cn)-CAT dimer. These findings suggest that the CAP subdomain should undergo a conformational change during catalytic activity that involves rearrangement of the dimer to facilitate substrate entry and product formation and egress from the active site.