Improving Nucleoside Diphosphate Kinase for Antiviral Nucleotide Analogs Activation*

Abstract

Antiviral nucleoside analog therapies rely on their incorporation by viral DNA polymerases/reverse transcriptase leading to chain termination. The analogs (3′-deoxy-3′-azidothymidine (AZT), 2′,3′-didehydro-2′,3′-dideoxythymidine (d4T), and other dideoxynucleosides) are sequentially converted into triphosphate by cellular kinases of the nucleoside salvage pathway and are often poor substrates of these enzymes. Nucleoside diphosphate (NDP) kinase phosphorylates the diphosphate derivatives of the analogs with an efficiency some 10⁴ lower than for its natural substrates. Kinetic and structural studies of Dictyosteliumand human NDP kinases show that the sugar 3′-OH, absent from all antiviral analogs, is required for catalysis. To improve the catalytic efficiency of NDP kinase on the analogs, we engineered several mutants with a protein OH group replacing the sugar 3′-OH. The substitution of Asn-115 in Ser and Leu-55 in His results in an NDP kinase mutant with an enhanced ability to phosphorylate antiviral derivatives. Transfection of the mutant enzyme in Escherichia coliresults in an increased sensitivity to AZT. An x-ray structure at 2.15-Å resolution of the Dictyostelium enzyme bearing the serine substitution in complex with theR _p-α-borano-triphosphate derivative of AZT shows that the enhanced activity reflects an improved geometry of binding and a favorable interaction of the 3′-azido group with the engineered serine.