Visualizing drug inhibitor binding interactions using microcrystal electron diffraction

Visualizing drug inhibitor binding interactions at the atomic level is important for both structure-based drug design and fragment-based screening methods. Rapid and uniform soaking with potentially less lattice defects make small macromolecular crystals attractive targets for studying ligand biding using 3D microcrystal electron diffraction (MicroED). However, so far no drug inhibitor binding interactions could unambiguously be resolved by electron diffraction. Here, we use MicroED to study the binding of a sulfonamide inhibitor to human carbonic anhydrase isoform II (HCA II). We show that MicroED data can efficiently be collected in-house on a conventional TEM from thin hydrated microcrystals after a brief soaking with the clinical drug inhibitor acetazolamide (AZM). The data are of high enough quality to unequivocally fit and resolve the inhibitor bound to the active site of the protein. We anticipate MicroED can play an important role in future drug discovery experiments, complementing existing methods in structural biology such as x-ray and neutron diffraction.