Mechanism of inhibition of botulinum neurotoxin type A light chain by two quinolinol compounds

Quinolinol-based compounds are a promising starting point for discovery of effective inhibitors of the clostridial neurotoxin, botulinum neurotoxin type A light chain (BoNT/A LC). Insights into the mechanism of inhibition by quinolinol compounds facilitate interpretation of docking data and inhibitor optimization. In this study, a fluorogenic substrate of BoNT/A, SNAPtide, was used to study the mechanism by which two new quinolinol compounds, MSU58 and MSU84, with IC₅₀ values of 3.3 μM and 5.8 μM, respectively, inhibit BoNT/A LC. Kinetic studies and model discrimination analysis showed both compounds to be competitive inhibitors of BoNT/A LC with inhibition constants (K_I) 3.2 μM and 6.2 μM for MSU58 and MSU84, respectively. These data indicate that the inhibitors bind in the BoNT/A LC active site and that inhibitor binding is mutually exclusive with the binding of the substrate. This is the first study to report the competitive inhibition of BoNT/A LC by quinolinol compounds. These data help define the inhibitor binding pocket and, along with structure activity relationship studies, provide immediate direction for further compound synthesis.