TRIOSEPHOSPHATE ISOMERASE FROM PLASMODIUM-FALCIPARUM - THE CRYSTAL-STRUCTURE PROVIDES INSIGHTS INTO ANTIMALARIAL DRUG DESIGN

Background: Malaria caused by the parasite Plasmodium falciparum is a major public health concern. The parasite lacks a functional tricarbox ylic acid cycle, making glycolysis its sole energy source. Although pa rasite enzymes have been considered as potential antimalarial drug tar gets, little is known about their structural biology. Here we report t he crystal structure of triosephosphate isomerase (TIM) from P. falcip arum at 2.2 Angstrom resolution. Results: The crystal structure of P. falciparum TIM (PfTIM), expressed in Escherichia coli, was determined by the molecular replacement method using the structure of trypanosoma l TIM as the starting model. Comparison of the PfTIM structure with ot her TIM structures, particularly human TIM, revealed several differenc es, In most TIMs the residue at position 183 is a glutamate but in PtT IM it is a leucine, This leucine residue is completely exposed and tog ether with the surrounding positively charged patch, may be responsibl e for binding TIM to the erythrocyte membrane. Another interesting fea ture is the occurrence of a cysteine residue at the dimer interface of PfTIM (Cys13), in contrast to human TIM where this residue is a methi onine. Finally, residue 96 of human TIM (Ser96), which occurs near the active site, has been replaced by phenylalanine in PfTIM. Conclusions : Although the human and Plasmodium enzymes share 42% amino acid seque nce identity, several key differences suggest that PfTIM may turn out to be a potential drug target. We have identified a region which may b e responsible for binding PfTIM to cytoskeletal elements or the band 3 protein of erythrocytes; attachment to the erythrocyte membrane may s ubsequently lead to the extracellular exposure of parts of the protein . This feature may be important in view of a recent report that patien ts suffering from P. falciparum malaria mount an antibody response to TIM leading to prolonged hemolysis. A second approach to drug design m ay be provided by the mutation of the largely conserved residue (Ser96 ) to phenylalanine in PfTIM. This difference may be of importance in d esigning specific active-site inhibitors against the enzyme. Finally, specific inhibition of PfTIM subunit assembly might be possible by tar geting Cys13 at the dimer interface. The crystal structure of PfTIM pr ovides a framework for new therapeutic leads.