As of late 2007, 6 structures have been solved for this class of enzymes, all by the Herzberg group at the University of Maryland using PEPPM from the blue mussel, Mytilus edulis. The first structure (PDB accession code ) was solved in 1999 and featured a magnesium oxalate inhibitor. This structure identified the enzyme as consisting of identical beta barrel subunits (exhibiting the TIM barrel fold, which consists of eight parallel beta strands). Dimerization was observed in which a helix from each subunit interacts with the other subunit's barrel; the authors called this feature "helix swapping." The dimers can dimerize as well to form a homotetrameric enzyme. A double phosphoryl transfer mechanism was proposed on the basis of this study: this would involve breakage of PEP's phosphorus-oxygen bond to form a phosphoenzyme intermediate, followed by transfer of the phosphoryl group from the enzyme to carbon-3, forming PPR.
