Eukaryot Cell. 2014 Feb 28. [Epub ahead of print]
INHIBITION AND STRUCTURE OF TOXOPLASMA GONDII PURINE NUCLEOSIDE PHOSPHORYLASE
Donaldson TM1, Cassera MB, Ho MC, Zhan C, Merino EF, Evans GB, Tyler PC, Almo SC, Schramm VL, Kim K.
The intracellular pathogen Toxoplasma gondii is a purine auxotroph that relies on purine salvage for proliferation. We have optimized T. gondii purine nucleoside phosphorylase (TgPNP) stability and crystallized TgPNP with phosphate and Immucillin-H, a transition-state analogue that has high affinity for the enzyme. Immucillin-H bound to TgPNP with a dissociation constant of 370 pM, the highest affinity of eleven immucillins selected to probe the catalytic site. The specificity for transition-state analogues indicated an early dissociative transition state for TgPNP. Compared with PfPNP, large substituents surrounding the 5' -hydroxyl group of inhibitors demonstrate reduced capacity for TgPNP inhibition. Catalytic discrimination against large 5' groups is consistent with the inability of TgPNP to catalyze the phosphorylysis of 5' -methylthioinosine to hypoxanthine. In contrast to mammalian PNP, the 2' -hydroxyl group is crucial for inhibitor binding in the catalytic site of TgPNP. This first crystal structure of TgPNP describes the basis for discrimination against 5' -methylthioinosine and similarly 5' -hydroxy-substituted immucillins; structural differences reflect the unique adaptations of purine salvage pathways of Apicomplexa.
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