Arsenate and phosphate as nucleophiles at the transition states of human purine nucleoside phosphorylase

Rafael G. Silva, Jennifer S. Hirschi, Mahmoud Ghanem, Andrew S. Murkin, Vern L. Schramm

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Abstract

Purine nucleoside phosphorylase (PNP) catalyzes the reversible phosphorolysis of 6-oxypurine (2′-deoxy)ribonucleosides, generating (2-deoxy)ribose 1-phosphate and the purine base. Transition-state models for inosine cleavage have been proposed with bovine, human, and malarial PNPs using arsenate as the nucleophile, since kinetic isotope effects (KIEs) are obscured on phosphorolysis due to high commitment factors. The Phe200Gly mutant of human PNP has low forward and reverse commitment factors in the phosphorolytic reaction, permitting the measurement of competitive intrinsic KIEs on both arsenolysis and phosphorolysis of inosine. The intrinsic 1′- 14C, 1′-3H, 2′-2H, 9- 15N, and 5′-3H2 KIEs for inosine were measured for arsenolysis and phosphorolysis. Except for the remote 5′-3H2, and some slight difference between the 2′-2H KIEs, all isotope effects originating in the reaction coordinate are the same within experimental error. Hence, arsenolysis and phosphorolysis proceed through closely related transition states. Although electrostatically similar, the volume of arsenate is greater than phosphate and supports a steric influence to explain the differences in the 5′- 3H2 KIEs. Density functional theory calculations provide quantitative models of the transition states for Phe200Gly human PNP-catalyzed arsenolysis and phosphorolysis, selected upon matching calculated and experimental KIEs. The models confirm the striking resemblance between the transition states for the two reactions.

Original languageEnglish (US)
Pages (from-to)2701-2709
Number of pages9
JournalBiochemistry
Volume50
Issue number13
DOIs
StatePublished - Apr 5 2011

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Purine-Nucleoside Phosphorylase
Nucleophiles
Isotopes
Phosphates
Inosine
Kinetics
Ribonucleosides
arsenic acid
Density functional theory

ASJC Scopus subject areas

  • Biochemistry

Cite this

Arsenate and phosphate as nucleophiles at the transition states of human purine nucleoside phosphorylase. / Silva, Rafael G.; Hirschi, Jennifer S.; Ghanem, Mahmoud; Murkin, Andrew S.; Schramm, Vern L.

In: Biochemistry, Vol. 50, No. 13, 05.04.2011, p. 2701-2709.

Research output: Contribution to journalArticle

Silva, Rafael G. ; Hirschi, Jennifer S. ; Ghanem, Mahmoud ; Murkin, Andrew S. ; Schramm, Vern L. / Arsenate and phosphate as nucleophiles at the transition states of human purine nucleoside phosphorylase. In: Biochemistry. 2011 ; Vol. 50, No. 13. pp. 2701-2709.
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