Abstract
The proliferation of the malaria-causing parasite Plasmodium falciparum within the erythrocyte is concomitant with massive phosphatidylcholine and phosphatidylethanolamine biosynthesis. Based on pharmacological and genetic data, de novo biosynthesis pathways of both phospholipids appear essential for parasite survival. The present study characterizes P. falciparum choline kinase (PfCK) and ethanolamine kinase (PfEK), which catalyse the first enzymatic steps of these essential metabolic pathways. Recombinant PfCK and PfEK were expressed as His-tagged fusion proteins from over-expressing E. coli strains, then purified to homogeneity and characterized. Using mice-polyclonal antibodies against recombinant kinases, PfCK and PfEK were shown to be localized within the parasite cytoplasm. Protein expression levels increased during erythrocytic development. PfCK and PfEK appeared specific to their respective substrate and followed Michaelis-Menten kinetics. The Km value of PfCK for choline was 135.3 ± 15.5 μM. PfCK was also able to phosphorylate ethanolamine with a very low affinity. PfEK was found to be an ethanolamine-specific kinase (Km = 475.7 ± 80.2 μM for ethanolamine). The quaternary ammonium Hemicholinium-3 and an ethanolamine analog, 2-amino 1-butanol selectively inhibited PfCK or PfEK. In contrast, the bisthiazolium compound T3 designed as a choline analog and currently in clinical trials for antimalarial treatment, affected similarly PfCK and PfEK activities. Inhibition exerted by T3 was competitive for both PfCK and PfEK and correlated with the impairment of cellular phosphatidylcholine biosynthesis. Comparative analyses of sequences and structures between both kinase types gave insights into their specific inhibition profiles and into the dual capacity of T3 to inhibit both PfCK and PfEK.