Protonation State and Hydration of Bisphosphonate Bound to Farnesyl Pyrophosphate Synthase

Farnesyl pyrophosphate synthase (FPPS)
catalyzes the condensation
of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate
to FPP and is known to be a molecular target of osteoporosis drugs,
such as risedronate (RIS), which is a nitrogen-containing bisphosphonate.
The protonation states and hydration structure of RIS bound to FPPS
were determined by neutron protein crystallography, which allows direct
visualization of hydrogens and deuteriums. The structure analysis
revealed that the phosphate groups of RIS were fully deprotonated
with the abnormally decreased pKa, and
that the roles of E93 and D264 consisted of canceling the extra negative
charges upon the binding of ligands. Collectively, our neutron structures
provided insights into the physicochemical properties during the bisphosphonate
binding event.