Crystal structure of Ralstonia eutropha polyhydroxyalkanoate synthase C-terminal domain and reaction mechanisms

Polyhydroxyalkanoates (PHAs) are natural polyesters synthesized by numerous microorganisms as energy and reducing power storage materials, and have attracted much attention as substitutes for petroleum-based plastics. Here, we report the first crystal structure of Ralstonia eutropha PHA synthase (RePhaC1) at 1.8 angstrom resolution and structu-based mechanisms for PHA polymerization. RePhaC1 contains two distinct domains, the N-terminal (RePhaC1(ND)) and C-terminal (RePhaC1 (CD)) domains, and exists as a dimer. RePhaC1 (CD) catalyzes polymerization via non-processive ping-pong mechanism using a Cys-His-Asp catalytic triad. Molecular docking simulation of 3-hydroxybutyrylCoA to the active site of RePhaC1 (CD) reveals residues involved in the formation of 3-hydroxybutyrylCoA binding pocket and substrate binding tunnel. Comparative analysis with other polymerases elucidates how different classes of PHA synthases show different substrate specificities. Furthermore, we attempted structure-based protein engineering and developed a RePhaC1 mutant with enhanced PHA synthase activity.