Thermodynamic Evaluation of Ligand Binding in the Plant-like Phosphoethanolamine Methyltransferases of the Parasitic Nematode Haemonchus contortus

Nematodes are a major cause of disease and the discovery of new pathways not found in hosts is critical for development of therapeutic targets. Previous studies suggest that Caenorhabditis elegans synthesizes phosphocholine via two S-adenosylmethionine (AdoMet)-dependent phosphoethanolamine methyltransferases (PMT). Here we examine two PMT from the parasitic nematode Haemonchus contortus. Sequence analysis suggests that HcPMT1 contains a methyltransferase domain in the N-terminal half of the protein and that HcPMT2 encodes a C-terminal methyltransferase domain, as in the C. elegans proteins. Kinetic analysis demonstrates that HcPMT1 catalyzes the conversion of phosphoethanolamine to phosphomonomethylethanolamine (pMME) and that HcPMT2 methylates pMME to phosphodimethylethanolamine (pDME) and pDME to phosphocholine. The IC50 values for miltefosine, sinefungin, amodiaquine, diphenhydramine, and tacrine suggest differences in the active sites of these two enzymes. To examine the interaction of AdoMet and S-adenosylhomocysteine (AdoCys), isothermal titration calorimetry confirmed the presence of a single binding site in each enzyme. Binding of AdoMet and AdoCys is tight (K-d similar to 2-25 mu M) over a range of temperatures (5-25 degrees C) and NaCl concentrations (0.05-0.5 M). Heat capacity changes for AdoMet and AdoCys binding suggests that each HcPMT differs in interaction surface area. Nonlinear van't Hoff plots also indicate a possible conformational change upon AdoMet/AdoCys binding. Functional analysis of the PMT from a parasitic nematode provides new insights on inhibitor and AdoMet/AdoCys binding to these enzymes.