STRUCTURE-BASED DESIGN OF INHIBITORS OF PURINE NUCLEOSIDE PHOSPHORYLASE .4. A STUDY OF PHOSPHATE MIMICS

9-(3,3-Dimethyl-5-phosphonopentyl)guanine was synthesized and found to be a potent inhibitor of purine nucleoside phosphorylase (PNP) (IC50 = 44 nM). A number of other functional end groups were investigated as phosphate mimics attached to the 9-position of guanine by this same a lkyl side chain, which provided a sensitive method for the detection o f any interaction of these groups with the phosphate binding site of P NP. Both the sulfonic acid (compound 13) and the carboxylic acid (comp ound 15) end groups interact significantly with the phosphate binding site, but in different ways, as determined by X-ray crystallographic a nalysis of the complexes. The sulfonic acid of 13, which binds about o ne-fourth as tightly as the phosphonate 12, binds in the phosphate sub site much like the phosphonic acid. The carboxylic acid, the interacti on of which is much weaker, turns away from the center of the phosphat e binding site to form hydrogen bonds with Ser 200 and Met 219. Thus, the only phosphate mimics that bind like phosphate itself are themselv es highly ionic, probably with limited ability to penetrate cell membr anes.