dGMP Binding to Thymidylate Kinase from Plasmodium falciparum Shows Half-Site Binding and Induces Protein Dynamics at the Dimer Interface

Plasmodium falciparum thymidylate kinase (PfTMK) is an essential enzyme for the growth of the organism because of its critical role in the de novo synthesis of deoxythymidine 5'-diphosphate (TDP), a precursor for TTP that is required for DNA replication and repair. The kinetics, thermodynamic parameters, and substrate binding properties of PfTMK for TMP, dGMP, ADP, and ATP were measured and characterized by steady-state kinetics and a combination of isothermal titration calorimetry, tryptophan fluorescence titration, and NMR. Mutational studies were performed to investigate residues that contribute to the unique ability of PfTMK to also utilize dGMP as a substrate. Isothermal titration calorimetry experiments revealed that dGMP binding exhibits a unique half-site binding mechanism. The occlusion of the empty site in the dGMP complex is supported by molecular mechanics calculations. Relaxation dispersion experiments show that the dGMP and enzyme complex is more dynamic at the dimer interface than the TMP complex on the mu s-ms time scale. The unique properties of dGMP binding need to be considered in the design of guanosine-based PfTMK-specific inhibitors.