Stabilization of Active Site Dynamics Leads to Increased Activity with 3'-Azido-3'-deoxythymidine Monophosphate for...

Thymidylate kinases are essential enzymes with roles in DNA synthesis and repair and have been the target of drug development for antimalarials, antifungals, HIV treatment, and cancer therapeutics. Human thymidylate kinase (hTMPK) conversion of the anti-HIV prodrug 3'-azido-3'-deoxythymidine (AZT or zidovudine) monophosphate to diphosphate is the rate-limiting step in the activation of AZT. A point mutant (F105Y) has been previously reported with significantly increased activity for the monophosphate form of the drug [3'-azidothymidine-5'-monophosphate (AZTMP)]. Using solution nuclear magnetic resonance (NMR) techniques, we show that while the wild-type (WT) and F105Y hTMPK adopt the same structure in solution, significant changes in dynamics may explain their different activities toward TMP and AZTMP. ¹³C spin-relaxation measurements show that there is little change in dynamics on the ps to ns time scale. In contrast, methyl ¹H relaxation dispersion shows that AZTMP alters adenosine nucleotide handling in the WT protein but not in the mutant. Additionally, the F105Y mutant has reduced conformational flexibility, leading to an increase in affinity for the product ADP and a slower rate of phosphorylation of TMP. The dynamics at the catalytic center for F105Y bound to AZTMP are tuned to the same frequency as WT bound to TMP, which may explain the mutant's catalytic efficiency toward the prodrug.