The substrate binding preferences of Plasmodium thymidylate kinase

Plasmodium falciparum thymidylate kinase (PfTMK) is a potential chemotherapeutic target as it can tolerate a range of substrates, which distinguishes it from other thymidylate kinases. An important step in drug development is to determine the interaction of ligands competing for their target sites in a proposed drug target. The estimated binding affinity of thymidylate (TMP) with PfTMK/deoxyguanylate complex was as low as 0.9×10(4) M(-1) with a very low exothermic signal of -3.9 kcal mol(-1). Furthermore, titration of PfTMK/TMP with deoxyguanylate (dGMP) showed a very small heat signal corresponding to nonspecific background heat. Titration of PfTMK with a 1:1 mixture of TMP and dGMP showed a binding affinity corresponding to the average of the binding affinity for individual reactions. Thus, dGMP was unable to displace TMP from its binding site, while TMP was able to partially displace dGMP from the its binding site accompanied by a weak exothermic signal and lowered affinity. Based on these results, we propose that pyrimidine based inhibitors will compete with TMP and may be able to more efficiently displace dGMP from binding sites compared with purine based compounds. In addition, the synthesis of purine based compounds as inhibitors of PfTMK will be highly selective for the parasitic enzyme, however, they need to be potent enough to displace TMP from its binding site.