Structural and thermodynamic basis of the inhibition of Leishmania major farnesyl diphosphate synthase by nitrogen-containing bisphosphonates

Farnesyl diphosphate synthase (FPPS) is an essential enzyme involved in the biosynthesis of sterols (cholesterol in humans and ergosterol in yeasts, fungi and trypanosomatid parasites) as well as in protein prenylation. It is inhibited by bisphos-phonates, a class of drugs used in humans to treat diverse bone-related diseases. The development of bisphosphonates as antiparasitic compounds targeting ergosterol biosynthesis has become an important route for therapeutic intervention. Here, the X-ray crystallographic structures of complexes of FPPS from Leishmania major (the causative agent of cutaneous leishmaniasis) with three bisphosphonates determined at resolutions of 1.8, 1.9 and 2.3 Å are reported. Two of the inhibitors, 1-(2-hydroxy-2,2- diphosphonoethyl)-3-phenylpyridinium (300B) and 3-butyl-1-(2,2-diphosphonoethyl) pyridinium (476A), co-crystallize with the homoallylic substrate isopentenyl diphosphate (IPP) and three Ca²⁺ ions. A third inhibitor, 3-fluoro-1-(2-hydroxy-2,2-diphosphonoethyl)pyridinium (46I), was found to bind two Mg²⁺ ions but not IPP. Calorimetric studies showed that binding of the inhibitors is entropically driven. Comparison of the structures of L. major FPPS (LmFPPS) and human FPPS provides new information for the design of bisphosphonates that will be more specific for inhibition of LmFPPS. The asymmetric structure of the LmFPPS-46I homodimer indicates that binding of the allylic substrate to both monomers of the dimer results in an asymmetric dimer with one open and one closed homoallylic site. It is proposed that IPP first binds to the open site, which then closes, opening the site on the other monomer, which closes after binding the second IPP, leading to the symmetric fully occupied FPPS dimer observed in other structures.