title: The N-glycosidase mechanism of ribosome-inactivating proteins implied by crystal structures of alpha-momorcharin.
abstract: BACKGROUND
  alpha-Momorcharin (alpha MMC) is a type I ribosome-inactivating protein. It inhibits protein synthesis by hydrolytically removing a specific adenine residue from a highly conserved, single-stranded loop of rRNA.
  
  
  RESULTS
  Here we describe the determination and refinement of the crystal structures of alpha MMC in the native state and in complexes with the product, adenine, and a substrate analogue, formycin 5'-monophosphate (FMP) at high resolution. Both adenine and the base of FMP are tightly bound; the ribose of bound FMP adopts a strained, high-energy conformation, which may mimic the structure of the transition state.
  
  
  CONCLUSIONS
  These structures indicate that residues Tyr70, Glu160 and Arg163 of alpha MMC are the most critical for catalysis. We propose that the strained conformation of the ribose in the target adenosine weakens the glycoside bond. Partial protonation mediated by Arg163 then facilitates N-glycoside bond cleavage, leading to the formation of an oxycarbonium ion intermediate which is stabilized by the negatively-charged Glu160. Tyr70 adopts subtly different conformations in the three structures implying that it may be important in substrate recognition and perhaps catalysis.
authors.affiliation: Oxford Centre for Molecular Sciences, UK.
authors.forename: J
authors.initials: J
authors.lastname: Ren
authors.forename: Y
authors.initials: Y
authors.lastname: Wang
authors.forename: Y
authors.initials: Y
authors.lastname: Dong
authors.forename: D I
authors.initials: DI
authors.lastname: Stuart
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D001665:Binding Sites; D018360:Crystallography, X-Ray / Q000379:methods; D006026:Glycoside Hydrolases / Q000378:metabolism*; D006027:Glycosides; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism / Q000633:toxicity*; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D012335:RNA, Ribosomal / Q000737:chemistry* / Q000378:metabolism*; D012269:Ribosomal Proteins; D054788:Ribosome Inactivating Proteins; D012270:Ribosomes / Q000187:drug effects*
chemical_list: D006027:Glycosides; D010940:Plant Proteins; D012335:RNA, Ribosomal; D012269:Ribosomal Proteins; C039582:MMC protein, Momordica charantia; D006026:Glycoside Hydrolases; D054788:Ribosome Inactivating Proteins