title: Structure of E. coli 5'-methylthioadenosine/S-adenosylhomocysteine nucleosidase reveals similarity to the purine nucleoside phosphorylases.
abstract: BACKGROUND
  5'-methylthioadenosine/S-adenosyl-homocysteine (MTA/AdoHcy) nucleosidase catalyzes the irreversible cleavage of 5'-methylthioadenosine and S-adenosylhomocysteine to adenine and the corresponding thioribose, 5'-methylthioribose and S-ribosylhomocysteine, respectively. While this enzyme is crucial for the metabolism of AdoHcy and MTA nucleosides in many prokaryotic and lower eukaryotic organisms, it is absent in mammalian cells. This metabolic difference represents an exploitable target for rational drug design.
  
  
  RESULTS
  The crystal structure of E. coli MTA/AdoHcy nucleosidase was determined at 1.90 A resolution with the multiwavelength anomalous diffraction (MAD) technique. Each monomer of the MTA/AdoHcy nucleosidase dimer consists of a mixed alpha/beta domain with a nine-stranded mixed beta sheet, flanked by six alpha helices and a small 3(10) helix. Intersubunit contacts between the two monomers present in the asymmetric unit are mediated primarily by helix-helix and helix-loop hydrophobic interactions. The unexpected presence of an adenine molecule in the active site of the enzyme has allowed the identification of both substrate binding and potential catalytic amino acid residues.
  
  
  CONCLUSIONS
  Although the sequence of E. coli MTA/AdoHcy nucleosidase has almost no identity with any known enzyme, its tertiary structure is similar to both the mammalian (trimeric) and prokaryotic (hexameric) purine nucleoside phosphorylases. The structure provides evidence that this protein is functional as a dimer and that the dual specificity for MTA and AdoHcy results from the truncation of a helix. The structure of MTA/AdoHcy nucleosidase is the first structure of a prokaryotic nucleoside N-ribohydrolase specific for 6-aminopurines.
authors.affiliation: Structural Biology and Biochemistry, Research Institute, Hospital for Sick Children, 555 University Avenue, Toronto, Ontario M5G 1X8, Canada.
authors.forename: J E
authors.initials: JE
authors.lastname: Lee
authors.forename: K A
authors.initials: KA
authors.lastname: Cornell
authors.forename: M K
authors.initials: MK
authors.lastname: Riscoe
authors.forename: P L
authors.initials: PL
authors.lastname: Howell
mesh_terms: D000225:Adenine / Q000378:metabolism; D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000201:enzymology*; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009699:N-Glycosyl Hydrolases / Q000737:chemistry*; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011683:Purine-Nucleoside Phosphorylase / Q000737:chemistry*; D012266:Ribose / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D012266:Ribose; D011683:Purine-Nucleoside Phosphorylase; C021784:5'-methylthioadenosine phosphorylase; D009699:N-Glycosyl Hydrolases; C019606:adenosylhomocysteine nucleosidase; D000225:Adenine