Targeting Isoprenoid Biosynthesis in Plasmodium falciparum

The methylerythritol phosphate (MEP) pathway, which synthesizes isoprenoid precursors in malaria parasites, is distinct from the mammalian process. Previous chemical and genetic evidence have suggested that MEP pathway function is essential, and it has therefore been of great interest as a potential drug target. Here, we present chemical and genetic evidence that the third MEP pathway enzyme, IspD, is essential. Furthermore, growth inhibition by the Malaria Box compound, 1R,3S-MMV008138, in cultured P. falciparum results from enzymatic inhibition of IspD. Inhibition is competitive with the CTP substrate of IspD. Because structural studies were not feasible, the mode of inhibitor binding was predicted using a model based on regions of homology between PfIspD and E. coli IspD. Substrate utilization and inhibition studies of active site point mutants supported the prediction that 1R,3S-MMV008138 and the CTP substrate occupy overlapping, but distinct areas of the active site. Inhibitor binding is dominated by enthalpically favorable interactions, but amino acids predicted to act as potential hydrogen bonding partners did not appear to be important for inhibitor binding. PfIspD is much larger than bacterial homologs and contains several apparently unique domains, which may be involved in these interactions. Future studies on 1R,3S-MMV008138 analogs are expected to shed further light on determinants of PfIspD inhibition, progressing towards the development of novel antimalarial therapies