title: Trypanosomal nucleoside hydrolase. A novel mechanism from the structure with a transition-state inhibitor.
abstract: Nucleoside N-ribohydrolases are targets for disruption of purine salvage in the protozoan parasites. The structure of a trypanosomal N-ribohydrolase in complex with a transition-state inhibitor is reported at 2.3 A resolution. The nonspecific nucleoside hydrolase from Crithidia fasciculata cocrystallized with p-aminophenyliminoribitol reveals tightly bound Ca2+ as a catalytic site ligand. The complex with the transition-state inhibitor is characterized by (1) large protein conformational changes to create a hydrophobic leaving group site (2) C3'-exo geometry for the inhibitor, typical of a ribooxocarbenium ion (3) stabilization of the ribooxocarbenium analogue between the neighboring group 5'-hydroxyl and bidentate hydrogen bonds to Asn168; and (4) octacoordinate Ca2+ orients a catalytic site water and is liganded to two hydroxyls of the inhibitor. The mechanism is ribooxocarbenium stabilization with weak leaving group activation and is a departure from glucohydrolases which use paired carboxylates to achieve the transition state.
authors.affiliation: Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461, USA.
authors.forename: M
authors.initials: M
authors.lastname: Degano
authors.forename: S C
authors.initials: SC
authors.lastname: Almo
authors.forename: J C
authors.initials: JC
authors.lastname: Sacchettini
authors.forename: V L
authors.initials: VL
authors.lastname: Schramm
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000032:analysis; D016831:Crithidia fasciculata / Q000201:enzymology*; D008024:Ligands; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009699:N-Glycosyl Hydrolases / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D008024:Ligands; D009699:N-Glycosyl Hydrolases; C096466:inosine-uridine hydrolase; D002118:Calcium