title: Constrained bonding environment in the Michaelis complex of Trypanosoma cruzi uridine phosphorylase.
abstract: The transition state for the Trypanosoma cruzi uridine phosphorylase (TcUP) reaction has an expanded S(N)2 character. We used binding isotope effects (BIE's) to probe uridine distortion in the complex with TcUP and sulfate to mimic the Michaelis complex. Inverse 1'-(3)H and 5'-(3)H BIE's indicate a constrained bonding environment of these groups in the complex. Quantum chemical modeling identified a uridine conformer whose calculated BIE's match the experimental values. This conformer differs in sugar pucker and uracil orientation from the unbound conformer and the transition-state structure. These results support ground-state stabilization in the Michaelis complex.
authors.affiliation: Department of Biochemistry, Albert Einstein College of Medicine of Yeshiva University, 1300 Morris Park Avenue, Bronx, NY 10461, USA.
authors.forename: Rafael G
authors.initials: RG
authors.lastname: Silva
authors.forename: D Randal
authors.initials: DR
authors.lastname: Kipp
authors.forename: Vern L
authors.initials: VL
authors.lastname: Schramm
mesh_terms: D002250:Carbon Radioisotopes / Q000737:chemistry / Q000378:metabolism; D007553:Isotope Labeling; D007700:Kinetics; D008956:Models, Chemical; D011485:Protein Binding; D015800:Protozoan Proteins / Q000737:chemistry* / Q000378:metabolism; D014316:Tritium / Q000737:chemistry / Q000378:metabolism; D014349:Trypanosoma cruzi / Q000737:chemistry / Q000201:enzymology*; D014529:Uridine / Q000737:chemistry / Q000378:metabolism; D014543:Uridine Phosphorylase / Q000737:chemistry* / Q000378:metabolism
chemical_list: D002250:Carbon Radioisotopes; D015800:Protozoan Proteins; D014316:Tritium; D014543:Uridine Phosphorylase; D014529:Uridine