Structure and function of the N-terminal domain of Ralstonia eutropha polyhydroxyalkanoate synthase, and the proposed structure and mechanisms of the whole enzyme.

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. In an accompanying paper, the authors reported the crystal structure of the C-terminal domain of Ralstonia eutropha PHA synthase (PhaC1). Here, the authors report the 3D reconstructed model of full-length of R. eutropha PhaC1 (RePhaC1 ) by small angle X-ray scattering (SAXS) analysis. The catalytic C-terminal domain of RePhaC1 (RePhaC1 ) dimer is located at the center of RePhaC1 , and the N-terminal domain of RePhaC1 (RePhaC1 ) is located opposite the dimerization subdomain of RePhaC1 , indicating that RePhaC1 is not directly involved in the enzyme catalysis. The localization studies using RePhaC1 , RePhaC1 and RePhaC1 revealed that RePhaC1 plays important roles in PHA polymerization by localizing the enzyme to the PHA granules and stabilizing the growing PHA polymer near the active site of RePhaC1 . The serial truncation study on RePhaC1 suggested that the predicted five α-helices (N-α3 to N-α7) are required for proper folding and granule binding function of RePhaC1 . In addition, the authors also report the SAXS 3D reconstructed model of the RePhaC1 /RePhaM complex (RePhaM , PAKKA motif-truncated version of RePhaM). RePhaM forms a complex with RePhaC1 by interacting with RePhaC1 and activates RePhaC1 by providing a more extensive surface area for interaction with the growing PHA polymer.