Biochemistry. 2013 Dec 2. [Epub ahead of print]
Discrimination of Potent Inhibitors of Toxoplasma gondii Enoyl-Acyl Carrier Protein Reductase by Thermal Shift Assay
Afanador GA, Muench SP, McPhillie M, Fomovska A, Schön A, Zhou Y, Cheng G, Stec J, Freundlich JS, Shieh HM, Anderson JW, Jacobus DP, Fidock DA, Kozikowski AP, Fishwick CW, Rice DW, Freire E, McLeod R, Prigge ST.
Many microbial pathogens rely on a type II fatty acid synthesis (FASII) pathway which is distinct from the type I pathway found in humans. Enoyl-Acyl Carrier Protein Reductase (ENR) is an essential FASII pathway enzyme and the target of a number of antimicrobial drug discovery efforts. The biocide triclosan is established as a potent inhibitor of ENR and has been the starting point for medicinal chemistry studies. We evaluated a series of triclosan analogs for their ability to inhibit the growth of Toxoplasma gondii, a pervasive human pathogen, and its ENR enzyme (TgENR). Several compounds were identified that inhibited TgENR at low nanomolar concentrations, but could not be further differentiated due to the limited dynamic range of the TgENR activity assay. Thus, we adapted a thermal shift assay (TSA) to directly measure the dissociation constant (Kd) of the most potent inhibitors identified in this study as well as inhibitors from previous studies. Furthermore, the TSA allowed us to determine the mode of action of these compounds in the presence of NADH or NAD+ cofactors. We found that all of the inhibitors bind to a TgENR/NAD+ complex, but that they differed in their dependence on NAD+ concentration. Ultimately, we were able to identify compounds which bind to the TgENR/NAD+ complex in the low femtomolar range. This shows how TSA data combined with enzyme inhibition, parasite growth inhibition data and ADMET predictions allow for better discrimination between potent ENR inhibitors for future medicine development.
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