9-(Phosphonoalkyl)guanine derivatives as substrates or inhibitors of guanylate kinase

Several 9-(phosphonoalkyl)guanines (Gua(CH 2) n CH 2-PO 3H 2; n = 4–6) and 9-(difluorophosphonoalkyl)guanines (Gua(CH 2) n CF 2PO 3H 2; n = 3–7) were studied as potential substrates and inhibitors of guanylate kinase. These compounds are inhibitors of the enzyme except 9-(5-phosphonopentyl)guanine ( n = 4) which is a substrate with an efficiency of phosphorylation of about 0.3% that of GMP, as estimated from the V max K m ratios. The phosphonate and difluorophosphonate derivatives with n = 5 produce optimal inhibition. These two compounds have similar affinity, both being competitive inhibitors with respect to GMP and noncompetitive inhibitors with respect to ATP. pH-dependence studies indicate that the dianionic rather than the monoanionic form of these compounds bind to the enzyme. The lack of phosphorylation of 9-(5,5-difluoro-5-phosphonopentyl)guanine by guanylate kinase is explained by the decreased nucleophilic character of the oxygen atoms of the phosphonate group rather than by inadequate binding to the GMP-binding site.