Enthalpy versus entropy-driven binding of bisphosphonates to farnesyl diphosphate synthase.
We report the results of an ITC (isothermal titration calorimetry) investigation of the binding of six bisphosphonates to the enzyme farnesyl diphosphate synthase (FPPS; EC 2.5.1.10) from Trypanosoma brucei. The bisphosphonates investigated were zoledronate, risedronate, ibandronate, pamidronate, 2-phenyl-1-hydroxyethane-1,1-bisphosphonate, and 1-(2,2-bisphosphonoethyl)-3-iodo pyridinium. At pH = 7.4, both risedronate and the phenylethane bisphosphonate bind in an enthalpy-driven manner (DeltaH approximately -9 to 10 kcal mol-1), but the other four bisphosphonates bind in an entropy-driven manner (DeltaS varying from 31.2 to 55.1 cal K-1 mol-1). However, at pH = 8.5, zoledronate binding switches from entropy to enthalpy-driven. The DeltaG results are highly correlated with FPPS inhibition results obtained using a radiochemical assay (R2 = 0.85, N = 11, P < 0.001). The DeltaH and DeltaS results are interpreted in terms of a model in which bisphosphonates with charged side chains have positive DeltaH values, due to the enthalpic cost of desolvation (due to strong ion-dipole interactions) and, likewise, a positive DeltaS, due to an increase in water entropy (both ligand and protein associated) on ligand binding to FPPS: the hydrophobic effect. For the neutral side chains (risedronate at pH 7.4, 8.5 and zoledronate at pH 8.5, as well as the phenylethane bisphosphonate), binding is overwhelmingly enthalpy-driven, with the enhanced activity of the basic side chain containing species being attributable to their becoming protonated in the active site. Given the large size of the bisphosphonate market and the potential importance of the development of these compounds for cancer immunotherapy and anti-parasitic chemotherapy, these results are of broad general interest in the context of the development of new, potent, and selective FPPS inhibitors.
Yin
Fenglin
F
Department of Biophysics, University of Illinois at Urbana-Champaign, 607 South Mathews Avenue, Urbana, Illinois 61801, USA.
Cao
Rong
R
Goddard
Amanda
A
Zhang
Yonghui
Y
Oldfield
Eric
E
D000818:Animals; D004164:Diphosphonates / Q000737:chemistry* / Q000378:metabolism; D012968:Etidronic Acid / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D051228:Geranyltranstransferase / Q000737:chemistry* / Q000378:metabolism; D057927:Hydrophobic and Hydrophilic Interactions; D000077557:Ibandronic Acid; D007700:Kinetics; D000068296:Risedronic Acid; D055672:Static Electricity; D013816:Thermodynamics; D014346:Trypanosoma brucei brucei / Q000201:enzymology
D004164:Diphosphonates; D051228:Geranyltranstransferase; D000068296:Risedronic Acid; D012968:Etidronic Acid; D000077557:Ibandronic Acid