The three-dimensional (3D) protein structures of PHA synthases have been studied extensively using X-ray crystallography and other techniques. These studies have provided valuable insights into the molecular mechanisms of PHA synthesis and the structure-function relationships of these enzymes.

The crystal structures of several PHA synthases have been determined, including those from Ralstonia eutropha, Aeromonas hydrophila, and Allochromatium vinosum. These structures have revealed that PHA synthases have a complex quaternary structure, consisting of multiple subunits arranged in a ring-shaped structure. The active site of the enzyme is located at the interface between two subunits, and contains the catalytic residues responsible for the polymerization of β-hydroxyacyl-CoA monomers.

The crystal structures have also revealed the role of specific amino acid residues in substrate recognition and binding. For example, in Ralstonia eutropha PHA synthase, a conserved arginine residue has been shown to play a critical role in substrate recognition and binding.

More recently, computational methods such as molecular dynamics simulations have been used to study the dynamics of PHA synthases and their interactions with substrates and other molecules. These studies have provided insights into the mechanisms of substrate binding and catalysis in PHA synthases, and could be used to design new enzymes with improved activity and specificity.

Overall, the 3D protein structures of PHA synthases are providing valuable insights into the mechanisms of PHA synthesis and the design of new enzymes for biotechnology applications.