title: Helix swapping between two alpha/beta barrels: crystal structure of phosphoenolpyruvate mutase with bound Mg(2+)-oxalate.
abstract: BACKGROUND
  Phosphonate compounds are important secondary metabolites in nature and, when linked to macromolecules in eukaryotes, they might play a role in cell signaling. The first obligatory step in the biosynthesis of phosphonates is the formation of a carbon-phosphorus bond by converting phosphoenolpyruvate (PEP) to phosphonopyruvate (P-pyr), a reaction that is catalyzed by PEP mutase. The PEP mutase functions as a tetramer and requires magnesium ions (Mg2+).
  
  
  RESULTS
  The crystal structure of PEP mutase from the mollusk Mytilus edulis, bound to the inhibitor Mg(2+)-oxalate, has been determined using multiwavelength anomalous diffraction, exploiting the selenium absorption edge of a selenomethionine-containing protein. The structure has been refined at 1.8 A resolution. PEP mutase adopts a modified alpha/beta barrel fold, in which the eighth alpha helix projects away from the alpha/beta barrel instead of packing against the beta sheet. A tightly associated dimer is formed, such that the two eighth helices are swapped, each packing against the beta sheet of the neighboring molecule. A dimer of dimers further associates into a tetramer. Mg(2+)-oxalate is buried close to the center of the barrel, at the C-terminal ends of the beta strands.
  
  
  CONCLUSIONS
  The tetramer observed in the crystal is likely to be physiologically relevant. Because the Mg(2+)-oxalate is inaccessible to solvent, substrate binding and dissociation might be accompanied by conformational changes. A mechanism involving a phosphoenzyme intermediate is proposed, with Asp58 acting as the nucleophilic entity that accepts and delivers the phosphoryl group. The active-site architecture and the chemistry performed by PEP mutase are different from other alpha/beta-barrel proteins that bind pyruvate or PEP, thus the enzyme might represent a new family of alpha/beta-barrel proteins.
authors.affiliation: Center for Advanced Research in Biotechnology, University of Maryland Biotechnology Institute, Rockville 20850, USA.
authors.forename: K
authors.initials: K
authors.lastname: Huang
authors.forename: Z
authors.initials: Z
authors.lastname: Li
authors.forename: Y
authors.initials: Y
authors.lastname: Jia
authors.forename: D
authors.initials: D
authors.lastname: Dunaway-Mariano
authors.forename: O
authors.initials: O
authors.lastname: Herzberg
mesh_terms: D000818:Animals; D049872:Bivalvia / Q000201:enzymology; D002384:Catalysis; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D010070:Oxalates / Q000378:metabolism*; D017875:Phosphotransferases (Phosphomutases) / Q000737:chemistry* / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation
chemical_list: D010070:Oxalates; D017875:Phosphotransferases (Phosphomutases); C057167:phosphoenolpyruvate mutase