Kinetic analysis of a recombinantly expressed Plasmodium falciparum dihydrofolate synthase-folylpolyglutamate synthase

Abstract

Malaria is a life-threatening parasitic disease that causes at least 300 million acute illnesses annually, of which at least one million infected humans die, mainly children under the age of 5 years. This overwhelming burden is due to the most virulent causative agent, Plasmodium falciparum, as a result of its prevalence in sub-Saharan Africa, as well as its resistance to nearly all anti-malarials in use. There is thus an urgent need to discover and characterise new novel parasitic targets for chemotherapeutic intervention. Folate metabolism is the target of several anti-malarials such as pyrimethamine and sulfadoxine. These drugs cause a decrease in parasite growth since Plasmodia have a high rate of replication and demand for nucleotides during DNA synthesis. The parasite is almost totally resistant to the current antifolates. Further insights into the folate pathway and its drugs are essential for the understanding of the resistance mechanism and to identify/characterise new drug targets for inhibition. Three possible new drug targets were identified and characterised in the folate pathway (Lee C.S. et al., 2001). One of these targets is the bifunctional enzyme, dihydrofolate-synthase folylpolyglutamate synthase (DHFS-FPGS). The bifunctionality and activity of the -dhfs-fpgs gene in Plasmodium was confirmed by functional complementation in yeast and bacteria and shown to be unique to Plasmodia and bacteria. This gene indicated only a 15-17% homology to other organisms; DHFS activity is usually only present in prokaryotes but not in humans or other eukaryotes (Salcedo E. et al., 2001). Although part of a bifunctional protein and having closely related catalytic functions, the DHFS and FPGS activities have distinct roles to play in both the de novo and salvage pathways of folate metabolism. These characteristics indicate DHFS-FPGS as a potentially good drug target since a single inhibitor is likely to have a drastic effect on both routes and consequently arrest DNA synthesis in the malaria parasite. This could prove to be a very effective and novel antimalarial strategy. Comparative expression studies of synthetic and native DHFS-FPGS presented here indicated that the highest quantity of protein is expressed from the synthetic gene. However, results indicated that most of the recombinant protein expressed in various E. coli cell lines, produced insoluble protein aggregates. Various strategies were employed in an attempt to improve recombinant soluble expression including auto-induction of T7 promoter activity. However, this did not result in an increased percentage of soluble protein expression even though improved total protein expression was observed. The inclusion of chaperone proteins resulted in a minor change in soluble expression. Activity assays of the DHFS-FPGS from these two methods indicated that active protein was produced in a correctly folded manner. Due to the high amount of recombinant protein present in the inclusion bodies, various methods were investigated to isolate and refold the DHFS-FPGS protein. The use of a non-ionic and ionic detergent for refolding resulted in pure, solubilised, active protein. Activity assays of the refolded, soluble protein indicated that the protein is active. Preliminary kinetic analysis was unsuccessful and requires further investigations.