Transition-state analysis of Trypanosoma cruzi uridine phosphorylase-catalyzed arsenolysis of uridine.

J Am Chem Soc. 2011 Jun 29;133(25):9923-31. 
Silva RG, Vetticatt MJ, Merino EF, Cassera MB, Schramm VL. 
Department of Biochemistry, Albert Einstein College of Medicine of Yeshiva University, 1300 Morris Park Avenue, Bronx, New York 10461, USA. 
Abstract - Uridine phosphorylase catalyzes the reversible phosphorolysis of uridine and 2'-deoxyuridine to generate uracil and (2-deoxy)ribose 1-phosphate, an important step in the pyrimidine salvage pathway. The coding sequence annotated as a putative nucleoside phosphorylase in the Trypanosoma cruzi genome was overexpressed in Escherichia coli , purified to homogeneity, and shown to be a homodimeric uridine phosphorylase, with similar specificity for uridine and 2'-deoxyuridine and undetectable activity toward thymidine and purine nucleosides. Competitive kinetic isotope effects (KIEs) were measured and corrected for a forward commitment factor using arsenate as the nucleophile. The intrinsic KIEs are: 1'-(14)C = 1.103, 1,3-(15)N(2) = 1.034, 3-(15)N = 1.004, 1-(15)N = 1.030, 1'-(3)H = 1.132, 2'-(2)H = 1.086, and 5'-(3)H(2) = 1.041 for this reaction. Density functional theory was employed to quantitatively interpret the KIEs in terms of transition-state structure and geometry. Matching of experimental KIEs to proposed transition-state structures suggests an almost synchronous, S(N)2-like transition-state model, in which the ribosyl moiety possesses significant bond order to both nucleophile and leaving groups. Natural bond orbital analysis allowed a comparison of the charge distribution pattern between the ground-state and the transition-state models.