Inhibition of human
purine nucleoside phosphorylase (PNP) stops growth of activated T-cells and the formation of 6-oxypurine bases, making it a target for
leukemia, autoimmune disorders, and
gout. Four generations of ribocation transition-state mimics bound to PNP are structurally characterized.
Immucillin-H (K*i(1/4) 58 pM, first generation)contains an iminoribitol
cation with four asymmetric carbons.
DADMe-Immucillin-H (K*i(1/4) 9 pM, second-generation),uses a methylene-bridged dihydroxypyrrolidine
cation with twoasymmetric centers.DATMe-
Immucillin-H (K*i(1/4)9 pM, third-generation) contains an open-chain amino alcohol
cation with two asymmetric carbons. SerMe-ImmH (K*i(1/4) 5 pM, fourth-generation) uses achiral dihydroxyaminoalcohol seramide as the ribocation mimic. Crystal structures of PNPs establish features of tight binding to be; 1) ion-pair formation between bound
phosphate (or its mimic) and inhibitor
cation, 2) leaving-group interactions to N1, O6, and N7 of 9-deazahypoxanthine, 3) interaction between
phosphate and inhibitor
hydroxyl groups, and 4) His257 interacting with the 5'-
hydroxyl group. The first generation analogue is an imperfect fit to the catalytic site with a long ion pair distance between the iminoribitol and bound
phosphate and weaker interactions to the leaving group. Increasing the ribocation to leaving-group distance in the second- to fourth-generation analogues provides powerful binding interactions and a facile synthetic route to powerful inhibitors. Despite chemical diversity in the four generations of transition-state analogues, the catalytic site geometry is almost the same for all analogues. Multiple solutions in transition-state analogue design are available to convert the energy of catalytic rate enhancement to binding energy in human PNP.