Fragment Screening Hit Draws Attention to a Novel Transient Pocket Adjacent to the Recognition Site of the tRNA Modifying Enzyme TGT

Fragment based lead discovery was applied to tRNA guanine transglycosylase, an enzyme modifying post transcriptionally tRNAs in Shigella, the causative agent of shigellosis. TGT inhibition prevents translation of Shigella s virulence factor VirF, hence reducing pathogenicity. One discovered fragment opens a transient subpocket in the preQ1 recognition site by pushing back an aspartate residue. This step is associated with reorganization of further amino acids structurally transforming a loop adjacent to the recognition site by duplicating the volume of the preQ1 recognition pocket. We synthesized 6 carboxamido , 6 hydrazido , and 4 guanidino benzimidazoles to target the opened pocket, including a dihydro imidazoquinazoline with a propyn 1 yl exit vector pointing into the transient pocket and displacing a conserved water network. MD simulations and hydration site analysis suggest water displacement to contribute favorably to ligand binding. A cysteine residue, exclusively present in bacterial TGTs, serves as gatekeeper of the transient subpocket. It becomes accessible upon pocket opening for selective covalent attachment of electrophilic ligands in eubacterial TGT