title: The crystal structure of ADP-L-glycero-D-mannoheptose 6-epimerase: catalysis with a twist.
abstract: BACKGROUND
  ADP-L-glycero--mannoheptose 6-epimerase (AGME) is required for lipopolysaccharide (LPS) biosynthesis in most genera of pathogenic and non-pathogenic Gram-negative bacteria. It catalyzes the interconversion of ADP-D-glycero-D-mannoheptose and ADP-L-glycero-D-mannoheptose, a precursor of the seven-carbon sugar L-glycero-mannoheptose (heptose). Heptose is an obligatory component of the LPS core domain; its absence results in a truncated LPS structure resulting in susceptibility to hydrophobic antibiotics. Heptose is not found in mammalian cells, thus its biosynthetic pathway in bacteria presents a unique target for the design of novel antimicrobial agents.
  
  
  RESULTS
  The structure of AGME, in complex with NADP and the catalytic inhibitor ADP-glucose, has been determined at 2.0 A resolution by multiwavelength anomalous diffraction (MAD) phasing methods. AGME is a homopentameric enzyme, which crystallizes with two pentamers in the asymmetric unit. The location of 70 crystallographically independent selenium sites was a key step in the structure determination process. Each monomer comprises two domains: a large N-terminal domain, consisting of a modified seven-stranded Rossmann fold that is associated with NADP binding; and a smaller alpha/beta C-terminal domain involved in substrate binding.
  
  
  CONCLUSIONS
  The first structure of an LPS core biosynthetic enzyme leads to an understanding of the mechanism of the conversion between ADP-D-glycero--mannoheptose and ADP-L-glycero-D-mannoheptose. On the basis of its high structural similarity to UDP-galactose epimerase and the three-dimensional positions of the conserved residues Ser116, Tyr140 and Lys144, AGME was classified as a member of the short-chain dehydrogenase/reductase (SDR) superfamily. This study should prove useful in the design of mechanistic and structure-based inhibitors of the AGME catalyzed reaction.
authors.affiliation: Department ofChemistry and Chemical Biology, Cornell University, Ithaca, NY 14853, USA.
authors.forename: A M
authors.initials: AM
authors.lastname: Deacon
authors.forename: Y S
authors.initials: YS
authors.lastname: Ni
authors.forename: W G
authors.initials: WG
authors.lastname: Coleman
authors.forename: S E
authors.initials: SE
authors.lastname: Ealick
mesh_terms: D000245:Adenosine Diphosphate Glucose / Q000737:chemistry / Q000494:pharmacology; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D002238:Carbohydrate Epimerases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000378:metabolism; D002384:Catalysis; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology; D008070:Lipopolysaccharides / Q000096:biosynthesis; D008958:Models, Molecular; D009249:NADP / Q000378:metabolism; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D015203:Reproducibility of Results; D012643:Selenium / Q000737:chemistry / Q000378:metabolism; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D001426:Bacterial Proteins; D008070:Lipopolysaccharides; D000245:Adenosine Diphosphate Glucose; D009249:NADP; C037213:ADP-L-glycero-D-mannoheptose-6-epimerase; D002238:Carbohydrate Epimerases; D012643:Selenium